View source for Harmful algal bloom

{{Short description|Population explosion of organisms that can kill marine life}}
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[[File:Blue-gree algae bloom Lake Erie.png|thumb|[[Cyanobacteria]] (blue-green algae) bloom on [[Lake Erie]] (United States) in 2009. These kinds of algae can cause harmful algal bloom.]]

A '''harmful algal bloom''' ('''HAB'''), or '''excessive algae growth''', sometimes called a '''red tide''' in marine environments, is an [[algal bloom]] that causes negative impacts to other organisms by production of natural [[phycotoxin|algae-produced toxins]], [[anoxic waters|water deoxygenation]], mechanical damage to other organisms, or by other means. HABs are sometimes defined as only those algal blooms that produce toxins, and sometimes as any algal bloom that can result in severely lower [[oxygen saturation|oxygen levels]] in natural waters, killing organisms in [[marine habitats|marine]] or [[fresh water]]s.<ref name=":4">{{cite journal|title=Eutrophication and harmful algal blooms: A scientific consensus|author=J. Heisler |author2=P.M. Glibert |author3=J.M. Burkholder |author4=D.M. Anderson |author5=W. Cochlan |author6=W.C. Dennison b |author7=Q. Dortch |author8=C.J. Gobler |author9=C.A. Heil |author10=E. Humphries |author11=A. Lewitus |author12=R. Magnien |author13=H.G. Marshallm |author14=K. Sellner |author15=D.A. Stockwell |author16=D.K. Stoecker |author17=M. Suddleson |journal=Harmful Algae|volume=8|issue=1|year=2008|pages=3–13|doi=10.1016/j.hal.2008.08.006|pmid=28781587|pmc=5543702|bibcode=2008HAlga...8....3H }}</ref> Blooms can last from a few days to many months. After the bloom dies, the [[microorganism|microbes]] that decompose the dead algae use up more of the oxygen, generating a "[[dead zone (ecology)|dead zone]]" which can cause [[fish kill|fish die-offs]]. When these zones cover a large area for an extended period of time, neither fish nor plants are able to survive.

It is sometimes unclear what causes specific HABs as their occurrence in some locations appears to be entirely natural,<ref name=":2" /> while in others they appear to be a result of human activities.<ref name=":3" /> In certain locations there are links to particular drivers like nutrients, but HABs have also been occurring since before humans started to affect the environment. HABs are induced by [[eutrophication]], which is an [[nutrient pollution|overabundance of nutrients]] in the water. The two most common nutrients are fixed nitrogen ([[nitrate]]s, [[ammonia]], and [[urea]]) and [[phosphate]].<ref name=CDC>{{cite web |url=https://www.cdc.gov/habs/general.html |title=Harmful Algal Blooms |date=9 March 2021 |publisher=[[Centers for Disease Control and Prevention|CDC]]}}</ref> The excess nutrients are emitted by [[agricultural pollution|agriculture]], industrial pollution, excessive [[fertilizer]] use in urban/suburban areas, and associated [[urban runoff]]. Higher water temperature and low circulation also contribute.{{citation needed|date=August 2023}}

HABs can cause significant harm to animals, the environment and economies. They have been increasing in size and frequency worldwide, a fact that many experts attribute to global [[climate change]]. The U.S. [[National Oceanic and Atmospheric Administration]] (NOAA) predicts more harmful blooms in the [[Pacific Ocean]].<ref>{{cite news |last=Harvey |first=Chelsea |date=2016-09-29 |title=The Pacific blob caused an "unprecedented" toxic algal bloom — and there's more to come |url=https://www.washingtonpost.com/news/energy-environment/wp/2016/09/29/warm-oceans-caused-last-years-toxic-blob-and-more-algae-blooms-may-be-in-store |newspaper=Washington Post}}</ref> Potential remedies include chemical treatment, additional reservoirs, sensors and monitoring devices, reducing nutrient runoff, research and management as well as monitoring and reporting.<ref>{{Cite journal |last1=Sukenik |first1=Assaf |last2=Kaplan |first2=Aaron |date=9 July 2021 |title=Cyanobacterial Harmful Algal Blooms in Aquatic Ecosystems: A Comprehensive Outlook on Current and Emerging Mitigation and Control Approaches |journal=Microorganisms |volume=9 |issue=7 |pages=1472 |doi=10.3390/microorganisms9071472 |doi-access=free |pmid=34361909 |pmc=8306311 |issn=2076-2607}}</ref>

Terrestrial runoff, containing fertilizer, sewage and livestock wastes, transports abundant nutrients to the seawater and stimulates bloom events. Natural causes, such as river floods or [[upwelling]] of nutrients from the [[sea floor]], often following massive storms, provide nutrients and trigger bloom events as well. Increasing coastal developments and aquaculture also contribute to the occurrence of coastal HABs.<ref name=":62">{{cite journal |last1=Anderson |first1=Donald M. |last2=Glibert |first2=Patricia M. |author-link2=Patricia Glibert|last3=Burkholder |first3=Joann M. |author-link3=JoAnn Burkholder|title=Harmful algal blooms and eutrophication: Nutrient sources, composition, and consequences |journal=Estuaries |date=August 2002 |volume=25 |issue=4 |pages=704–726 |doi=10.1007/BF02804901 |s2cid=44207554 |url=https://link.springer.com/article/10.1007/BF02804901 |access-date=7 April 2021|url-access=subscription }}</ref><ref name=":63">{{cite web |last1=Hall |first1=Danielle |title=What Exactly Is a Red Tide? |url=https://ocean.si.edu/ocean-life/plants-algae/what-exactly-red-tide |website=Smithsonian |access-date=7 April 2021}}</ref>  Effects of HABs can worsen locally due to wind driven [[Langmuir circulation]] and their [[Langmuir circulation#Biological effects|biological effects]].{{TOC limit|3}}
{{Plankton sidebar|bloom}}

== Description and identification ==
[[File:Banter See 1348.jpg|thumb|[[Cyanobacteria]] algae on the coast of northern Germany]]
HABs from [[cyanobacteria]] (blue-green algae) can appear as a foam, scum, or mat on or just below the surface of water and can take on various colors depending on their pigments.<ref name=CDC/> Cyanobacteria blooms in freshwater lakes or rivers may appear bright green, often with surface streaks that look like floating paint.<ref name=California/> Cyanobacterial blooms are a global problem.<ref>{{Cite journal|last1=Harke|first1=Matthew J.|last2=Steffen|first2=Morgan M.|last3=Gobler|first3=Christopher J.|last4=Otten|first4=Timothy G.|last5=Wilhelm|first5=Steven W.|last6=Wood|first6=Susanna A.|last7=Paerl|first7=Hans W.|date=2016-04-01|title=A review of the global ecology, genomics, and biogeography of the toxic cyanobacterium, Microcystis spp.|journal=Harmful Algae|series=Global Expansion of Harmful Cyanobacterial Blooms: Diversity, ecology, causes, and controls|language=en|volume=54|pages=4–20|doi=10.1016/j.hal.2015.12.007|pmid=28073480|issn=1568-9883|doi-access=free|bibcode=2016HAlga..54....4H }}</ref>

Most blooms occur in warm waters with excessive nutrients.<ref name="CDC" /> The harmful effects from such blooms are due to the toxins they produce or from using up oxygen in the water which can lead to fish die-offs.<ref name="usatoday">[https://www.usatoday.com/story/news/nation-now/2015/08/07/algae-blooms-economy-west-coast-toxic-bloom/31222581/ "What you need to know about toxic algae blooms"], ''USA Today'', August 7, 2015</ref>  Not all [[algal bloom]]s produce toxins, however, with some only discoloring water, producing a smelly odor, or adding a bad taste to the water. Unfortunately, it is not possible to tell if a bloom is harmful from just appearances, since sampling and microscopic examination is required.<ref name=CDC/> In many cases microscopy is not sufficient to tell the difference between toxic and non-toxic populations. In these cases, tools can be employed to measure the toxin level or to determine if the toxin-production genes are present.<ref>{{Cite journal|last1=Rinta-Kanto|first1=J. M.|last2=Ouellette|first2=A. J. A.|last3=Boyer|first3=G. L.|last4=Twiss|first4=M. R.|last5=Bridgeman|first5=T. B.|last6=Wilhelm|first6=S. W.|date=June 2005|title=Quantification of Toxic Microcystis spp. during the 2003 and 2004 Blooms in Western Lake Erie using Quantitative Real-Time PCR|url=https://pubs.acs.org/doi/10.1021/es048249u|journal=Environmental Science & Technology|language=en|volume=39|issue=11|pages=4198–4205|doi=10.1021/es048249u|pmid=15984800|bibcode=2005EnST...39.4198R|issn=0013-936X|url-access=subscription}}</ref>

== Terminology ==
In a narrow definition, harmful algal blooms are only those blooms that release toxins that affect other species. On the other hand, any algal bloom can cause dead zones due to [[Hypoxia (environmental)|low oxygen levels]], and could therefore be called "harmful" in that sense. The usage of the term "harmful algal blooms" in the media and scientific literature is varied. In a broader definition, all "organisms and events are considered to be HABs if they negatively impact human health or socioeconomic interests or are detrimental to aquatic systems".<ref name="Kudela 2017">{{cite journal |last1=Kudela |first1=Raphael |last2=Berdalet |first2=Elisa |last3=Enevoldsen |first3=Henrik |last4=Pitcher |first4=Grant |last5=Raine |first5=Robin |last6=Urban |first6=Ed |title=GEOHAB–The Global Ecology and Oceanography of Harmful Algal Blooms Program: Motivation, Goals, and Legacy |journal=Oceanography |date=1 March 2017 |volume=30 |issue=1 |pages=12–21 |doi=10.5670/oceanog.2017.106|doi-access=free |bibcode=2017Ocgpy..30a..12K |hdl=10261/151090 |hdl-access=free }}</ref> A harmful algal bloom is "a societal concept rather than a scientific definition".<ref name="Kudela 2017" />

A similarly broad definition of HABs was adopted by the [[United States Environmental Protection Agency|US Environmental Protection Agency]] in 2008 who stated that HABs include "potentially toxic (auxotrophic, heterotrophic) species and high-biomass producers that can cause hypoxia and anoxia and indiscriminate mortalities of marine life after reaching dense concentrations, whether or not toxins are produced".<ref name=":4" />

===Red tide===
Harmful algal bloom in coastal areas are also often referred to as "red tides".<ref name="Kudela 2017" />  The term "red tide" is derived from blooms of any of several species of [[dinoflagellate]], such as ''[[Karenia brevis]]''.<ref>{{cite web |title=Harmful Algal Blooms (HABs): Red Tide |url=https://www.cdc.gov/hab/redtide/default.htm |access-date=2 Oct 2011 |publisher=U.S. Centers for Disease Control and Prevention}}</ref> However, the term is misleading since algal blooms can widely vary in color, and growth of algae is unrelated to the [[tide]]s. Not all red tides are produced by dinoflagellates. The [[mixotrophic]] ciliate ''[[Mesodinium rubrum]]'' produces non-toxic blooms coloured deep red by chloroplasts it obtains from the algae it eats.{{hsp}}<ref>{{cite journal|last1=Dierssen|first1=Heidi|last2=McManus|first2=George B.|last3=Chlus|first3=Adam|last4=Qiu|first4=Dajun|last5=Gao|first5=Bo-Cai|last6=Lin|first6=Senjie|year=2015|title=Space station image captures a red tide ciliate bloom at high spectral and spatial resolution|journal=Proceedings of the National Academy of Sciences|volume=112|issue=48|pages=14783–14787|bibcode=2015PNAS..11214783D|doi=10.1073/pnas.1512538112|pmc=4672822|pmid=26627232|doi-access=free}}</ref>

[[File:Karenia brevis Anatomy (1).png|thumb|The dinoflagellate labeled above is the microscopic alga'' [[Karenia brevis]]''. It is the cause of a HAB event in the Gulf of Mexico. The algae propel themselves using a longitudinal flagellum (A) and a transverse flagellum (B). The longitudinal flagellum lies in a groove-like structure called the cingulum (F). The [[dinoflagellate]] is separated into an upper portion called the epitheca (C) where the apical horn resides (E) and a lower portion called the hypotheca (D).]]

As a technical term, it is being replaced in favor of more precise terminology, including the generic term "harmful algal bloom" for harmful species, and "[[algal bloom]]" for benign species.{{citation needed|date=January 2023}}

==Types==
There are three main types of phytoplankton which can form into harmful algal blooms: [[cyanobacteria]], [[dinoflagellates]], and [[diatoms]]. All three are made up of microscopic floating organisms which, like plants, can create their own food from sunlight by means of [[photosynthesis]]. That ability makes the majority of them an essential part of the [[food web]] for small fish and other organisms.<ref name=Black>{{cite book |last1=Black |first1=Jacquelyn G. |last2=Black |first2=Laura J. 
|year=2012 |title=Microbiology: Principles and explorations |edition=8th |publisher=John Wiley & Sons }}</ref>{{rp|246}}

===Cyanobacteria===
Harmful algal blooms in freshwater lakes and rivers, or at [[estuaries]], where rivers flow into the ocean, are caused by cyanobacteria, which are commonly referred to as "blue-green algae",<ref name=Peebles>{{cite news |last=Peebles |first=Ernst B. |date=20 July 2016 |title=Why toxic algae blooms like Florida's are so dangerous to people and wildlife |website=[[Huffington Post]] |url=http://www.huffingtonpost.com/the-conversation-us/why-toxic-algae-blooms-li_b_11082734.html }}</ref> but are in fact [[Prokaryote|prokaryotic]] bacteria,<ref>{{cite journal |last1=Stanier |first1=R.Y. |last2=Bazine |first2=G.C. |date=October 1977 |title=Phototrophic prokaryotes: The cyanobacteria |journal=[[Annual Review of Microbiology]] |volume=31 |issue=1 |pages=225–274 |doi=10.1146/annurev.mi.31.100177.001301|pmid=410354|issn=0066-4227}}</ref> as opposed to algae which are [[eukaryote]]s.<ref>{{Cite journal |last1=Yoon |first1=Hwan Su|last2=Hackett |first2=Jeremiah D. |last3=Ciniglia |first3=Claudia|last4=Pinto |first4=Gabriele |last5=Bhattacharya |first5=Debashish |year=2004 |title=A molecular timeline for the origin of photosynthetic eukaryotes |journal=[[Molecular Biology and Evolution]] |volume=21 |issue=5 |pages=809–818 |doi=10.1093/molbev/msh075 |doi-access=free |pmid=14963099 |issn=1537-1719 }}</ref> Some cyanobacteria, including the widespread genus [[Microcystis|''Microsystis'']], can produce hazardous cyanotoxins such as [[microcystins]],<ref name=NOAA>{{cite web |title=Are all algal blooms harmful? |date=18 April 2016 |website=oceanservice.noaa.gov |publisher=[[NOAA]] |url=http://oceanservice.noaa.gov/facts/habharm.html }}</ref> which are [[hepatotoxins]] that harm the liver of mammals.<ref name=Utah /> Other types of cyanobacteria can also produce hepatotoxins, as well as neurotoxins, cytotoxins, and endotoxins.<ref>{{cite journal |last1=Bláha |first1=Luděk|last2=Babica |first2=Pavel |last3=Maršálek |first3=Blahoslav |date=January 2009 |title=Toxins produced in cyanobacterial water blooms – toxicity and risks |journal= Interdisciplinary Toxicology  |volume=2 |issue=2 |pages=36–41 |doi=10.2478/v10102-009-0006-2 |pmid=21217843 |issn=1337-9569 |pmc=2984099 }}</ref> [[Water purification]] plants may be unable to remove these toxins, leading to increasingly common localised advisories against drinking tap water, as happened in Toledo, Ohio in August&nbsp;2014.<ref name=Toledo/>

In August&nbsp;2021, there were 47&nbsp;lakes confirmed to have algal blooms in New York State alone.<ref>{{cite web |title=Summer days are waning, but algal blooms are growing |date=2021-08-30 |df=dmy-all |department=Commentary & opinion |via=wamc.org |publisher=[[WAMC]] / Northeast Public Radio |url=https://www.wamc.org/commentary-opinion/2021-08-30/summer-days-are-waning-but-algal-blooms-are-growing |access-date=2021-09-22 |language=en}}</ref><ref>{{cite web |title=ArcGIS Web Application |website=nysdec.maps.arcgis.com |url=https://nysdec.maps.arcgis.com/apps/webappviewer/index.html?id=ae91142c812a4ab997ba739ed9723e6e |access-date=2021-09-22 }}</ref> In September&nbsp;2021, Spokane County's Environmental Programs issued a HAB alert for Newman Lake following tests showing potentially harmful toxicity levels for cyanobacteria,<ref>{{cite press release |first=Kelli |last=Hawkins |date=17 September 2021 |title=Harmful algae alert for Newman Lake |website=SRHD.org |publisher=Spokane Regional Health District |place=Spokane, WA |url=https://srhd.org/news/2021/harmful-algae-alert-for-newman-lake |access-date=2021-09-22 |lang=en-US}}</ref> while in the same month record-high levels of microcystins were reported leading to an extended 'Do Not Drink' advisory for 280&nbsp;households at [[Clear Lake (California)|Clear Lake]], California's second-largest freshwater lake.<ref>{{cite news |last=Walton |first=Brett |date=2021-09-22 |df=dmy-all |title=Toxin levels spike, prompting drinking water emergency in northern California |website=Circle of Blue |url=https://www.circleofblue.org/2021/world/toxin-levels-spike-prompting-drinking-water-emergency-in-northern-california/ |access-date=2021-09-22 |language=en-US}}</ref> Water conditions in Florida, meanwhile, continue to deteriorate under increasing nutrient inflows, causing severe HAB events in both freshwater and marine areas.<ref>{{cite web |title=Red tide and algae blooms: Florida waters in crisis |date=2021-09-20 |website=abcactionnews.com |publisher=[[WFTS]] |url=https://www.abcactionnews.com/news/full-circle/red-tide-and-algae-blooms-florida-waters-in-crisis |access-date=2021-09-22 |language=en-US }}</ref>

HABs also cause harm by blocking the sunlight used by plants and algae to photosynthesise, or by depleting the dissolved oxygen needed by fish and other aquatic animals, which can lead to fish die-offs.<ref name=usatoday/>  When such oxygen-depleted water covers a large area for an extended period of time, it can become [[Hypoxia (environmental)|hypoxic]] or even anoxic; these areas are commonly called [[dead zone (ecology)|dead zones]]. These dead zones can be the result of numerous different factors ranging from natural phenomenon to deliberate human intervention, and are not just limited to large bodies of fresh water as found in the great lakes, but are also prone to bodies of salt water as well.<ref>{{cite web |title='Dead zone' is a more common term for hypoxia, which refers to a reduced level of oxygen in the water |publisher=[[US Department of Commerce]] |series=[[National Oceanic and Atmospheric Administration]] |website=oceanservice.noaa.gov|language=EN-US |url=https://oceanservice.noaa.gov/facts/deadzone.html |access-date=2017-10-22}}</ref>

=== Dual-stage life systems of algal species ===
Many of the species that form harmful algae blooms will undergo a dual-stage life system. These species will alternate between a [[Benthic zone|benthic]] resting stage and a [[Pelagic zone|pelagic]] vegetative state. The benthic resting stage corresponds to when these species are resting near the ocean floor. In this stage, the species cells are waiting for optimal conditions so that they can move towards the surface. These species will then transition from the benthic resting stage into the pelagic vegetative state - where they are more active and found near the water body surface. In the pelagic vegetative state, these cells are able to grow and multiply. It is within the pelagic vegetative state that a bloom is able to occur - as the cells rapidly reproduce and take over the upper regions of the body of water. The transition between these two life stages can have multiple effects on the algae bloom (such as rapid termination of the HAB as cells convert from the pelagic state to the benthic state). Many of the algal species that undergo this dual-stage life cycle are capable of rapid vertical migration. This migration is required for the movement from the benthic area of bodies of water to the pelagic zone. These species require immense amounts of energy as they pass through the various [[thermocline]]s, [[halocline]]s, and [[pycnocline]]s that are associated with the bodies of water in which these cells exist.<ref>{{cite journal |last1=Tobin |first1=Elizabeth D. |last2=Grünbaum |first2=Daniel |last3=Patterson |first3=Johnathan |last4=Cattolico |first4=Rose Ann |date=2013-10-04 |df=dmy-all |title=Behavioral and physiological changes during benthic-pelagic transition in the harmful alga, ''Heterosigma akashiwo'': Potential for rapid bloom formation |journal=[[PLoS One]] |volume=8|issue=10 |page=e76663 |bibcode=2013PLoSO...876663T |doi=10.1371/journal.pone.0076663 |doi-access=free |issn=1932-6203 |pmc=3790758 |pmid=24124586 |language=en }}</ref>

=== Diatoms and dinoflagellates (in marine coastal areas) ===
[[File:La-Jolla-Red-Tide.780.jpg|thumb|A harmful algal bloom event off the coast of [[San Diego, California]]]]The other types of algae are [[diatom]]s and [[dinoflagellate]]s, found primarily in marine environments, such as ocean coastlines or bays, where they can also form algal blooms. Coastal HABs are a natural phenomenon,<ref name=Red>{{cite web |title=FAQs about red tides |department=Water / Envvironmental concerns |series=Texas Parks & Wildlife Department |publisher=[[Government of Texas]] |place=Austin, TX |url=http://tpwd.texas.gov/landwater/water/environconcerns/hab/redtide/faq.phtml }}</ref><ref>{{cite journal |last=Turkoglu |first=Muhammet |date=2013-08-01 |df=dmy-all |title=Red tides of the dinoflagellate ''Noctiluca scintillans'' associated with eutrophication in the Sea of Marmara (the Dardanelles, Turkey) |journal=Oceanologia |language=en |volume=55 |issue=3 |pages=709–732 |doi=10.5697/oc.55-3.709 |doi-access=free |issn=0078-3234 |bibcode=2013Ocga...55..709T }}</ref> although in many instances, particularly when they form close to coastlines or in estuaries, it has been shown that they are exacerbated by human-induced [[eutrophication]] and / or climate change.<ref>{{cite journal |last1=Chen |first1=Xiu-hua |last2=Zhu |first2=Liang-sheng |last3=Zhang |first3=Hong-sheng |year=2007 |title=Numerical simulation of summer circulation in the East China Sea and its application in estimating the sources of red tides in the Yangtze river estuary and adjacent sea areas |journal=Journal of Hydrodynamics |volume=19 |issue=3 |pages=272–281 |doi=10.1016/s1001-6058(07)60059-6 |bibcode=2007JHyDy..19..272C |s2cid=119393454 |issn=1001-6058 }}</ref><ref>{{cite journal |last1=Velikova |first1=Violeta |last2=Moncheva |first2=Snejana |last3=Petrova |first3=Daniela |year=1999 |title=Phytoplankton dynamics and red tides (1987–1997) in the Bulgarian Black Sea |journal=Water Science and Technology |volume=39 |issue=8 |pages=27–36 |doi=10.2166/wst.1999.0378 |bibcode=1999WSTec..39Q..27V |issn=0273-1223 }}</ref><ref>{{Cite journal |last1=Song |first1=Yantao |last2=Wang |first2=Ping |last3=Li |first3=Guangdi |last4=Zhou |first4=Daowei |year=2014 |title=Relationships between functional diversity and ecosystem functioning: A review |journal=Acta Ecologica Sinica |volume=34 |issue=2 |pages=85–91 |doi=10.1016/j.chnaes.2014.01.001 |bibcode=2014AcEcS..34...85S |issn=1872-2032 }}</ref><ref>{{cite journal |last1=Phlips |first1=Edward J. |last2=Badylak |first2=Susan|last3=Lasi |first3=Margaret A. |last4=Chamberlain|first4=Robert|last5=Green |first5=Whitney C.|last6=Hall|first6=Lauren M.|last7=Hart |first7=Jane A. |last8=Lockwood |first8=Jean C.|last9=Miller|first9=Janice D. |last10=Morris |first10=Lori J. |last11=Steward |first11=Joel S. |display-authors=6 |year=2015 |title=From red tides to green and brown tides: Bloom dynamics in a restricted subtropical lagoon under shifting climatic conditions |journal=Estuaries and Coasts |language=en |volume=38 |issue=3 |pages=886–904 |doi=10.1007/s12237-014-9874-6 |bibcode=2015EstCo..38..886P |s2cid=83480080 |issn=1559-2731 }}</ref> They can occur when warmer water, lower salinity, and nutrients reach certain levels, which then stimulates their growth.<ref>{{Cite journal |last=Shi |first=X. |author2=Zou, Y. |author3=Zhang, Y. |author4=Ding, G. |author5=Xiao, Y. |author6=Lin, S. |author7=Chen, J. |title=Salinity decline promotes growth and harmful blooms of a toxic alga by diverting carbon flow |journal=Global Change Biology |volume=30 |issue=6 |page=e17348 |year=2024 |doi=10.1111/gcb.17348 |pmid=38822656|bibcode=2024GCBio..30E7348S }}</ref> Most HAB algae are dinoflagellates.<ref name=monterey>{{cite report |last=Ryan |first=John |date=15 August 2008 |title=Red tide & HAB studies in Monterey Bay |series=Monterey Bay Sanctuary Advisory Council Meeting |url=http://montereybay.noaa.gov/sac/2008/081508/081508ryan_ppt.pdf |archive-url=https://web.archive.org/web/20160804000641/http://montereybay.noaa.gov/sac/2008/081508/081508ryan_ppt.pdf |archive-date=2016-08-04 }}</ref> They are visible in water at a concentration of 1,000 algae cells/ml, while in dense blooms they can measure over 200,000/ml.<ref>{{cite press release |title=Intense, widespread algal blooms reported in Chesapeake Bay |date=1 September 2015 |website=[[Science Daily]] |url=https://www.sciencedaily.com/releases/2015/09/150901144353.htm }}</ref>


Diatoms produce [[domoic acid]], another neurotoxin, which can cause seizures in higher vertebrates and birds as it concentrates up the food chain. Domoic acid readily accumulates in the bodies of [[shellfish]], [[sardines]], and [[anchovies]], which if then eaten by [[sea lions]], [[otters]], [[cetaceans]],  birds or people, can affect the nervous system causing serious injury or death.<ref name=marine>{{cite web |title=Domoic acid toxicity |series=Top research projects |publisher=[[The Marine Mammal Center]] |place=Marin Headlands, CA |url=http://www.marinemammalcenter.org/science/top-research-projects/domoic-acid-toxicity.html }}</ref> In the summer of 2015, the state governments closed important shellfish fisheries in [[Washington (state)|Washington]], [[Oregon]], and [[California]] because of high concentrations of domoic acid in shellfish.<ref name=shellfish>{{cite press release |title=NOAA Fisheries mobilizes to gauge unprecedented West Coast toxic algal bloom |date=June 2015 |publisher=[[Northwest Fisheries Science Center]] |url=https://www.nwfsc.noaa.gov/news/features/west_coast_algal_bloom/index.cfm }}</ref>

In the marine environment, single-celled, microscopic, plant-like organisms naturally occur in the well-lit surface layer of any body of water. These organisms, referred to as [[phytoplankton]] or [[microalgae]], form the base of the food web upon which nearly all other marine organisms depend. Of the 5000+ species of marine phytoplankton that exist worldwide, about 2% are known to be harmful or toxic.<ref>{{cite journal |last=Landsberg |first=J.H. |year=2002 |title=The effects of harmful algal blooms on aquatic organisms |journal= Reviews in Fisheries Science |volume=10 |issue=2 |pages=113–390 |doi=10.1080/20026491051695 |bibcode=2002RvFS...10..113L |s2cid=86185142 }}</ref> Blooms of harmful algae can have large and varied impacts on marine ecosystems, depending on the species involved, the environment where they are found, and the mechanism by which they exert negative effects.<ref>Díaz, Patricio,A., and Rosa I. Figueroa. "Toxic Algal Bloom Recurrence in the Era of Global Change: Lessons from the Chilean Patagonian Fjords." ''Microorganisms''11.8 (2023): 1874. ''ProQuest.'' Web. 7 Mar. 2025.</ref>

== List of common HAB genera ==
{{div col}}
* ''[[Gonyaulax]]''
* ''[[Karenia (dinoflagellate)|Karenia]]''
* ''[[Gymnodinium]]''
* ''[[Dinophysis]]''
* ''[[Noctiluca]]''
* ''[[Chattonella]]''
* ''[[Ceratium]]''
* ''[[Amoebophrya]]''
* ''[[Alexandrium (dinoflagellate)|Alexandrium]]''
* ''[[Cochlodinium]]''
{{div col end}}

==Causes==
[[File:Nonvisible cyanobacteria HAB.png|thumb|300px| Harmful algal blooms do not have to be clearly visible. This shows a bloom with high cyanobacteria toxin levels (over 5 μ/l) yet the bloom is not easy to see.<ref>[https://cdn.coastalscience.noaa.gov/page-attachments/research/Harmful%20Algal%20Blooms%20Report%20FINAL%20August.2017.pdf Harmful Algal Blooms and Hypoxia in the Great Lakes Research Plan and Action Strategy: An Interagency Report] ''National Science and Technology Council'', August 2017, page 21.</ref>]]

It is sometimes unclear what causes specific HABs as their occurrence in some locations appears to be entirely natural,<ref name=":2">{{cite journal|last1=Adams|first1=N. G.|last2=Lesoing|first2=M.|last3=Trainer|first3=V. L.|year=2000|title=Environmental conditions associated with domoic acid in razor clams on the Washington coast|journal=J Shellfish Res|volume=19|pages=1007–1015}}</ref> while in others they appear to be a result of human activities.<ref name=":3">{{cite book|last1=Lam|first1=C. W. Y.|title=Red tides. biology, environmental science and toxicology|last2=Ho|first2=K. C.|publisher=Elsevier|year=1989|isbn=978-0-444-01343-9|editor1-last=Okaichi|editor1-first=T.|location=New York|pages=49–52|chapter=Red tides in Tolo Harbor, Hong Kong|editor2-last=Anderson|editor2-first=D. M.|editor3-last=Nemoto|editor3-first=T.}}</ref> Furthermore, there are many different species of algae that can form HABs, each with different environmental requirements for optimal growth. The frequency and severity of HABs in some parts of the world have been linked to increased nutrient loading from human activities. In other areas, HABs are a predictable seasonal occurrence resulting from coastal upwelling, a natural result of the movement of certain ocean currents.<ref>{{cite journal|last1=Trainer|first1=V. L.|last2=Adams|first2=N. G.|last3=Bill|first3=B. D.|last4=Stehr|first4=C. M.|last5=Wekell|first5=J. C.|last6=Moeller|first6=P.|last7=Busman|first7=M.|last8=Woodruff|first8=D.|year=2000|title=Domoic acid production near California coastal upwelling zones, June 1998|journal=Limnol Oceanogr|volume=45|issue=8|pages=1818–1833|bibcode=2000LimOc..45.1818T|doi=10.4319/lo.2000.45.8.1818|doi-access=free|s2cid=54007265}}</ref>

The growth of marine phytoplankton (both non-toxic and toxic) is generally limited by the availability of nitrates and phosphates, which can be abundant in coastal upwelling zones as well as in agricultural run-off. The type of nitrates and phosphates available in the system are also a factor, since phytoplankton can grow at different rates depending on the relative abundance of these substances (e.g. [[ammonia]], [[urea]], nitrate ion).<ref name= Moore/>

A variety of other nutrient sources can also play an important role in affecting algal bloom formation, including iron, silica or carbon. Coastal [[water pollution]] produced by humans (including iron fertilization) and systematic increase in [[Global warming|sea water temperature]] have also been suggested as possible contributing factors in HABs.<ref name= Moore>{{cite journal|last=Moore|first=S.|display-authors=etal|year=2011|title=Impacts of climate variability and future climate change on harmful algal blooms and human health|journal=Proceedings of the Centers for Oceans and Human Health Investigators Meeting|volume=7|issue=Suppl 2 |pages=S4|doi=10.1186/1476-069X-7-S2-S4|pmc=2586717|pmid=19025675 |doi-access=free }}</ref>

Among the causes of algal blooms are:<ref name="Janssen-et-al-2019">{{cite journal |last1=Janssen |first1=Annette BG |last2=Janse |first2=Jan H |last3=Beusen |first3=Arthur HW |last4=Chang |first4=Manqi |last5=Harrison |first5=John A |last6=Huttunen |first6=Inese |last7=Kong |first7=Xiangzhen |last8=Rost |first8=Jasmijn |last9=Teurlincx |first9=Sven |last10=Troost |first10=Tineke A |last11=van Wijk |first11=Dianneke |year=2019 |title=How to model algal blooms in any lake on earth |journal=[[Current Opinion in Environmental Sustainability]] |publisher=[[Elsevier]] |volume=36 |pages=1–10 |doi=10.1016/j.cosust.2018.09.001 |issn=1877-3435 |last12=Mooij |first12=Wolf M |bibcode=2019COES...36....1J |s2cid=158187643|doi-access=free |hdl=10138/341512 |hdl-access=free }}</ref>
* Excess nutrients—[[phosphorus]] and [[nitrates]]—from fertilizers or [[sewage]] that are discharged to water bodies (also called [[nutrient pollution]])<ref name="EPA-Climate">{{cite web |url=https://www.epa.gov/nutrientpollution/climate-change-and-harmful-algal-blooms |title=Climate Change and Harmful Algal Blooms |author=<!--Not stated--> |date=2017-03-09 |website=Nutrient Pollution |publisher=U.S. Environmental Protection Agency (EPA) |location=Washington, D.C.}}</ref><ref name="Peebles" /><ref name=":0">{{Cite journal |last1=Amin |first1=Md Nurul |last2=Kroeze |first2=Carolien |last3=Strokal |first3=Maryna |date=2017 |title=Human waste: An underestimated source of nutrient pollution in coastal seas of Bangladesh, India and Pakistan |journal=Marine Pollution Bulletin |volume=118 |issue=1–2 |pages=131–140 |doi=10.1016/j.marpolbul.2017.02.045 |pmid=28238487 |bibcode=2017MarPB.118..131A |issn=0025-326X }}</ref>
* [[climate change]]<ref name="EPA-Climate" />
* [[thermal pollution]] from power plants and factories
* low water levels in inland waterways and lakes, which reduces water flow and increases water temperatures<ref name="Utah">[http://kutv.com/news/local/neurotoxic-algae-bloom-shuts-down-utah-lake "Neurotoxic algae bloom that shuts down Utah Lake can affect brain, liver"] ''KUTV'', July 15, 2016</ref><ref name="Russian">[http://www.pressdemocrat.com/news/5741904-181/russian-river-to-be-closely?artslide=0 "Russian River to be closely monitored this summer to guard against harmful algae blooms"], ''Press Democrat'' June 23, 2016</ref>
* invasive filter feeders—especially [[Zebra mussel]]s, ''Dreissena polymorpha''—which preferentially eat non-toxic algae, competitors to harmful algae<ref name="Harke-et-al-2016">{{cite journal | last1=Harke | first1=Matthew J. | last2=Steffen | first2=Morgan M. | last3=Gobler | first3=Christopher J. | last4=Otten | first4=Timothy G. | last5=Wilhelm | first5=Steven W. | last6=Wood | first6=Susanna A. | last7=Paerl | first7=Hans W. | title=A review of the global ecology, genomics, and biogeography of the toxic cyanobacterium, ''Microcystis'' spp. | journal=Harmful Algae | publisher=[[Elsevier]] | volume=54 | year=2016 | issn=1568-9883 | doi=10.1016/j.hal.2015.12.007 | pages=4–20| pmid=28073480 | doi-access=free | bibcode=2016HAlga..54....4H }}</ref>

=== Nutrients ===
{{Further|Eutrophication}}

Nutrients enter freshwater or marine environments as [[surface runoff]] from [[agricultural pollution]] and [[urban runoff]] from fertilized lawns, golf courses and other landscaped properties; and from [[sewage treatment plants]] that lack nutrient control systems.<ref name="EPA-sources">{{cite web |url=https://www.epa.gov/nutrientpollution/sources-and-solutions |title=Sources and Solutions |author=<!--Not stated--> |date=2017-03-10 |website=Nutrient Pollution |publisher=EPA}}</ref> Additional nutrients are introduced from atmospheric pollution.<ref name="Miller">Miller, G. Tyler Jr., ''Environmental Science'', Thomas Learning (2003) pp. 355{{endash}}357</ref> Coastal areas worldwide, especially wetlands and estuaries, coral reefs and swamps, are prone to being overloaded with those nutrients.<ref name="Miller" /> Most of the large cities along the [[Mediterranean Sea]], for example, discharge all of their sewage into the sea untreated.<ref name="Miller" /> The same is true for most coastal developing countries, while in parts of the developing world, as much as 70% of wastewater from large cities may re-enter water systems without being treated.<ref>{{Cite journal|date=2010-01-01|title=Eutrophication and nutrient release in urban areas of sub-Saharan Africa — A review|url=https://www.sciencedirect.com/science/article/abs/pii/S0048969709009632|journal=Science of the Total Environment|language=en|volume=408|issue=3|pages=447–455|doi=10.1016/j.scitotenv.2009.10.020|issn=0048-9697|last1=Nyenje|first1=P.M.|last2=Foppen|first2=J.W.|last3=Uhlenbrook|first3=S.|last4=Kulabako|first4=R.|last5=Muwanga|first5=A.|pmid=19889445|bibcode=2010ScTEn.408..447N|url-access=subscription}}</ref>

Residual nutrients in treated wastewater can also accumulate in downstream source water areas<ref>{{Cite journal|date=2013-05-15|title=A comparison of additional treatment processes to limit particle accumulation and microbial growth during drinking water distribution|url=https://www.sciencedirect.com/science/article/abs/pii/S0043135413001395|journal=Water Research|language=en|volume=47|issue=8|pages=2719–2728|doi=10.1016/j.watres.2013.02.035|issn=0043-1354|last1=Liu|first1=G.|last2=Lut|first2=M.C.|last3=Verberk|first3=J.Q.J.C.|last4=Van Dijk|first4=J.C.|pmid=23510692|bibcode=2013WatRe..47.2719L |url-access=subscription}}</ref> and fuel eutrophication, which leads progressively to a cyanobacteria-dominated system characterized by seasonal HABs. As more wastewater treatment infrastructure is built, more treated wastewater is returned to the natural water system, leading to a significant increase in these residual nutrients.{{citation needed|date=January 2023}}

Residual nutrients combine with nutrients from other sources to increase the sediment nutrient stockpile that is the driving force behind phase shifts to entrenched eutrophic conditions.{{citation needed|date=January 2023}}

This contributes to the ongoing degradation of dams, lakes, rivers, and reservoirs - source water areas that are starting to become known as ecological infrastructure,<ref>{{Cite web|title=• 'Ecological infrastructure' and 'green infrastructure'|url=http://biodiversityadvisor.sanbi.org/industry-and-conservation/biodiversity-in-the-urban-economy/understand/success-stories-2/|access-date=2021-09-22|website=biodiversityadvisor.sanbi.org|archive-date=2021-09-09|archive-url=https://web.archive.org/web/20210909181200/http://biodiversityadvisor.sanbi.org/industry-and-conservation/biodiversity-in-the-urban-economy/understand/success-stories-2/|url-status=dead}}</ref> placing increasing pressure on wastewater treatment works and water purification plants. Such pressures, in turn, intensify seasonal HABs.{{citation needed|date=January 2023}}

=== Climate change ===
{{Further|Effects of climate change on oceans}}
[[Climate change]] contributes to warmer waters which makes conditions more favorable for algae growth in more regions and farther north.<ref>[http://www.straitstimes.com/world/united-states/algal-blooms-likely-to-flourish-as-temperatures-climb "Algal blooms 'likely to flourish as temperatures climb'"], ''Straits Times'', July 20, 2016</ref><ref name="EPA-Climate" /> In general, still, warm, shallow water, combined with high-nutrient conditions in lakes or rivers, increases the risk of harmful algal blooms.<ref name="Russian" /> Warming of summer surface temperatures of lakes, which rose by 0.34&nbsp;°C decade per decade between 1985 and 2009 due to global warming, also will likely increase algal blooming by 20% over the next century.<ref>{{cite journal |last1=O'Reiley |display-authors=etal |year=2015 |title=Rapid and highly variable warming of lake surface waters around the globe |journal=[[Geophysical Research Letters]] |volume=42 |issue=24 |pages=10,773–10,781 |bibcode=2015GeoRL..4210773O |doi=10.1002/2015GL066235 |doi-access=free|hdl=10138/208854 |hdl-access=free }}</ref>

Although the drivers of harmful algal blooms are poorly understood, they do appear to have increased in range expansion and frequency in coastal areas since the 1980s.<ref name="SROCC_SPM_20190925">IPCC, 2019: [https://www.ipcc.ch/site/assets/uploads/sites/3/2022/03/01_SROCC_SPM_FINAL.pdf Summary for Policymakers]. In: [https://www.ipcc.ch/srocc/ IPCC Special Report on the Ocean and Cryosphere in a Changing Climate] [H.-O. Pörtner, D.C. Roberts, V. Masson-Delmotte, P. Zhai, M. Tignor, E. Poloczanska, K. Mintenbeck, A. Alegría, M.  Nicolai, A. Okem, J. Petzold, B. Rama, N.M. Weyer (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA. {{doi|10.1017/9781009157964.001}}.</ref>{{rp|16}} The is the result of human induced factors such as increased nutrient inputs ([[nutrient pollution]]) and climate change (in particular the warming of water temperatures).<ref name="SROCC_SPM_20190925" />{{rp|16}} The parameters that affect the formation of HABs are [[ocean warming]], marine heatwaves, [[Ocean deoxygenation|oxygen loss]], eutrophication and [[water pollution]].<ref>Caretta, M.A., A. Mukherji, M. Arfanuzzaman, R.A. Betts, A. Gelfan, Y. Hirabayashi, T.K. Lissner, J. Liu, E. Lopez Gunn, R. Morgan, S. Mwanga, and S. Supratid, 2022: [https://www.ipcc.ch/report/ar6/wg2/downloads/report/IPCC_AR6_WGII_Chapter04.pdf Chapter 4: Water]. In: [https://www.ipcc.ch/report/ar6/wg2/ Climate Change 2022: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change] [H.-O. Pörtner, D.C. Roberts, M. Tignor, E.S. Poloczanska, K. Mintenbeck, A. Alegría, M. Craig, S. Langsdorf, S. Löschke, V. Möller, A. Okem, B. Rama (eds.)]. Cambridge University Press, Cambridge, UK and New York, NY, USA, pp. 551–712, {{doi|10.1017/9781009325844.006}}</ref>{{rp|582}}

=== Causes or contributing factors of coastal HABs ===
[[File:Red tide.jpg|thumb|right|Coastal harmful algal bloom event]]HABs contain dense concentrations of organisms and appear as discolored water, often reddish-brown in color. It is a natural phenomenon, but the exact cause or combination of factors that result in a HAB event are not necessarily known.<ref name="Tx pwd">{{cite web |title=Red Tide FAQ |url=http://www.tpwd.state.tx.us/landwater/water/environconcerns/hab/redtide/faq.phtml |access-date=2018-08-15 |publisher=Texas Parks and Wildlife Department |location=Austin, TX}}</ref> However, three key natural factors are thought to play an important role in a bloom - salinity, temperature, and wind. HABs cause economic harm, so outbreaks are carefully monitored. For example, the [[Florida Fish and Wildlife Conservation Commission]]  provides an up-to-date status report on HABs in Florida.<ref>{{cite web |title=Red Tide Current Status Statewide Information |url=http://research.myfwc.com/features/view_article.asp?id=9670  |archive-url=https://web.archive.org/web/20090822163316/http://research.myfwc.com/features/view_article.asp?id=9670 |archive-date=2009-08-22 |access-date=2009-08-23 |publisher=Florida Fish and Wildlife Research Institute}}</ref> The [[Texas Parks and Wildlife Department]] also provides a status report.<ref>{{cite web |title=Red Tide Index |url=http://www.tpwd.state.tx.us/landwater/water/environconcerns/hab/redtide/ |access-date=2018-08-15 |publisher=Texas Parks and Wildlife Department}}</ref> While no  particular cause of HABs has been found, many different factors can contribute to their presence. These factors can include [[water pollution]], which originates from sources such as human [[sewage]] and [[Agricultural pollution|agricultural runoff]].<ref name="West, L. 2016">{{cite web |author=West, L. |date=2016 |title=Red Tide: Causes and Effects |url=http://environment.about.com/od/redtidesfaq/f/red_tide_causes.htm |work=About News |access-date=2022-04-10 |archive-date=2017-03-02 |archive-url=https://web.archive.org/web/20170302113127/http://environment.about.com/od/redtidesfaq/f/red_tide_causes.htm |url-status=dead }}</ref>

The occurrence of HABs in some locations appears to be entirely natural (algal blooms are a seasonal occurrence resulting from coastal upwelling, a natural result of the movement of certain ocean currents)<ref>{{cite journal |last1=Trainer |first1=VL |last2=Adams |first2=NG |last3=Bill |first3=BD |last4=Stehr |first4=CM |last5=Wekell |first5=JC |last6=Moeller |first6=P |last7=Busman |first7=M |last8=Woodruff |first8=D |year=2000 |title=Domoic acid production near California coastal upwelling zones, June (1998) |journal=Limnol Oceanogr |volume=45 |issue=8 |pages=1818–1833 |bibcode=2000LimOc..45.1818T |doi=10.4319/lo.2000.45.8.1818 |doi-access=free |s2cid=54007265}}</ref><ref>{{cite journal |last1=Adams |first1=NG |last2=Lesoing |first2=M |last3=Trainer |first3=VL |year=2000 |title=Environmental conditions associated with domoic acid in razor clams on the Washington coast |journal=J Shellfish Res |volume=19 |pages=1007–1015}}</ref> while in others they appear to be a result of increased [[nutrient pollution]] from human activities.<ref>Lam CWY, Ho KC (1989) Red tides in Tolo Harbor, Hong Kong. In: Okaichi T, Anderson DM, Nemoto T (eds) Red tides. Biology, environmental science and toxicology. Elsevier, New York, pp 49–52.</ref> The growth of marine phytoplankton is generally limited by the availability of [[nitrates]] and [[phosphates]], which can be abundant in agricultural run-off as well as coastal upwelling zones. Other factors such as iron-rich dust influx from large desert areas such as the Sahara Desert are thought to play a major role in causing HAB events.<ref>{{cite journal |last1=Walsh |display-authors=et al |year=2006 |title=Red tides in the Gulf of Mexico: Where, when, and why? |url=http://works.bepress.com/kendall_carder/7 |journal=Journal of Geophysical Research |volume=111 |issue=C11003 |pages=1–46 |bibcode=2006JGRC..11111003W |doi=10.1029/2004JC002813 |pmc=2856968 |pmid=20411040}}</ref> Some algal blooms on the Pacific Coast have also been linked to occurrences of large-scale climatic oscillations such as [[El Niño]] events.{{citation needed|date=January 2023}}

=== Other causes ===
Other factors such as iron-rich dust influx from large desert areas such as the [[Sahara]] are thought to play a role in causing HABs.<ref>{{cite journal |last=Walsh |display-authors=etal |year=2006 |title=Red tides in the Gulf of Mexico: Where, when, and why? |journal=Journal of Geophysical Research |volume=111 |issue=C11003 |pages=1–46 |bibcode=2006JGRC..11111003W |doi=10.1029/2004JC002813 |pmc=2856968 |pmid=20411040}}</ref> Some algal blooms on the [[Pacific coast]] have also been linked to natural occurrences of large-scale climatic oscillations such as [[El Niño]] events. HABs are also linked to heavy rainfall.<ref name="Morse 1006–10252">{{Cite journal |last1=Morse |first1=Ryan E. |last2=Shen |first2=Jian |last3=Blanco-Garcia |first3=Jose L. |last4=Hunley |first4=William S. |last5=Fentress |first5=Scott |last6=Wiggins |first6=Mike |last7=Mulholland |first7=Margaret R. |author-link7=Margaret Mulholland |date=1 September 2011 |title=Environmental and Physical Controls on the Formation and Transport of Blooms of the Dinoflagellate Cochlodinium polykrikoides Margalef in the Lower Chesapeake Bay and Its Tributaries |journal=Estuaries and Coasts |volume=34 |issue=5 |pages=1006–1025 |doi=10.1007/s12237-011-9398-2 |bibcode=2011EstCo..34.1006M |issn=1559-2723 |s2cid=84945112}}</ref> Although HABs in the Gulf of Mexico were witnessed in the early 1500s by explorer [[Cabeza de Vaca]],<ref>Cabeza de Vaca, Álvar Núnez. La Relación (1542). Translated by Martin A. dunsworth and José B. Fernández. Arte Público Press, Houston, Texas (1993)</ref> it is unclear what initiates these blooms and how large a role n[[Human impact on the environment|anthropogenic]] and natural factors play in their development.{{citation needed|date=August 2023}}

== Number and sizes ==
The number of reported harmful algal blooms (cyanobacterial) has been increasing throughout the world.<ref name="usgs2">{{cite report |title=Field Manual of Wildlife Diseases |date=1999 |publisher=National Wildlife Health Center, United States Geological Survey (USGS) |location=Madison, WI |page=263 |isbn=0-607-88096-1 |id=1999-001 |access-date=2016-07-24 |archive-url=https://web.archive.org/web/20120905040143/http://www.nwhc.usgs.gov/publications/field_manual/chapter_36.pdf |archive-date=2012-09-05  |editor1-last=Friend |chapter-url=http://www.nwhc.usgs.gov/publications/field_manual/chapter_36.pdf |editor1-first=Milton |chapter=Ch. 36. Algal toxins |editor2-first=J. Christian |editor2-last=Franson}}</ref>  It is unclear whether the apparent increase in frequency and severity of HABs in various parts of the world is in fact a real increase or is due to increased observation effort and advances in species identification technology.<ref>{{cite journal |last1=Sellner |first1=K.G. |last2=Doucette G.J. |first2=Doucette |last3=G.J. |first3=Kirkpatrick |year=2003 |title=Harmful Algal blooms: causes, impacts and detection |journal=Journal of Industrial Microbiology and Biotechnology |volume=30 |issue=7 |pages=383–406 |doi=10.1007/s10295-003-0074-9 |pmid=12898390 |s2cid=6454310|doi-access=free }}</ref><ref>{{cite journal |last=Van Dolah |first=F.M. |year=2000 |title=Marine Algal Toxins: Origins, Health Effects, and Their Increased Occurrence |url=http://www.ehponline.org/docs/2000/suppl-1/133-141vandolah/abstract.html  |journal=Environmental Health Perspectives |volume=108 |issue=suppl.1 |pages=133–141 |doi=10.1289/ehp.00108s1133 |jstor=3454638 |pmc=1637787 |pmid=10698729 |bibcode=2000EnvHP.108S.133V |archive-url=https://web.archive.org/web/20090120150050/http://www.ehponline.org/docs/2000/suppl-1/133-141vandolah/abstract.html |archive-date=20 January 2009}}</ref>

In 2008, the U.S. government prepared a report on the problem, "Harmful Algal Bloom Management and Response: Assessment and Plan".<ref name="US">[https://obamawhitehouse.archives.gov/sites/default/files/microsites/ostp/jsost_hab0908.pdf "Harmful Algal Bloom Management and Response: Assessment and Plan"] {{webarchive|url=https://web.archive.org/web/20170124175600/https://obamawhitehouse.archives.gov/sites/default/files/microsites/ostp/jsost_hab0908.pdf|date=2017-01-24}}, Office of Science and Technology Policy, Sept. 2008</ref> The report recognized the seriousness of the problem:

{{blockquote|It is widely believed that the frequency and geographic distribution of HABs have been increasing worldwide. All U.S. coastal states have experienced HABs over the last decade, and new species have emerged in some locations that were not previously known to cause problems. HAB frequency is also thought to be increasing in freshwater systems.<ref name=US/>}}

Researchers have reported the growth of HABs in Europe, Africa and Australia. Those have included blooms on some of the [[African Great Lakes]], such as [[Lake Victoria]], the second largest freshwater lake in the world.<ref name="Circle" /> [[India]] has been reporting an increase in the number of blooms each year.<ref>[http://www.thehindu.com/sci-tech/boom-in-harmful-algal-blooms/article963739.ece "Boom in harmful algal blooms"], ''The Hindu'', Dec 20, 2010</ref> In 1977 [[Hong Kong]] reported its first coastal HAB. By 1987 they were getting an average of 35 per year.<ref name="Brown" /> Additionally, there have been reports of harmful algal blooms throughout popular Canadian lakes such as Beaver Lake and Quamichan Lake. These blooms were responsible for the deaths of a few animals and led to swimming advisories.<ref>{{Cite news |title=Toxic blue-green algae ends swimming at popular Victoria lake |language=en |work=CBC News |url=http://www.cbc.ca/news/canada/british-columbia/blue-green-algae-beaver-lake-1.4214729 |access-date=2017-10-23}}</ref>

Global warming and pollution is causing algal blooms to form in places previously considered "impossible" or rare for them to exist, such as under the ice sheets in the [[Arctic]],<ref>[http://news.stanford.edu/news/2012/june/arctic-algal-blooms-060712.html "NASA mission, led by Stanford biologist, finds massive algal blooms under Arctic sea ice"], ''Stanford News'', June 7, 2012</ref> in [[Antarctica]],<ref>[http://www.livescience.com/18915-antarctic-algae-bloom-nasa.html "Behemoth Antarctic Algae Bloom Seen from Space"], ''Life Science'', March 7, 2012</ref> the [[Himalayan Mountains]],<ref>[http://www.smh.com.au/environment/pollution-neglect-and-too-much-love-killing-once-idyllic-himalayan-lake-20111104-1mzw0.html "Pollution, neglect and too much love killing once idyllic Himalayan lake"], ''The Sydney Morning Herald'', Nov. 5, 2011</ref> the [[Rocky Mountains]],<ref>[http://jordanramis.com/water-law-blog/addressing-algal-blooms-in-rocky-mountain-national-park/ "Addressing Algal Blooms in Rocky Mountain National Park"], Jordan Ramis, August 6, 2015</ref> and in the [[Sierra Nevada (U.S.)|Sierra Nevada Mountains]].<ref>[https://www.ucdmc.ucdavis.edu/emergency/wilderness/Sierra_Algae-pdf10-14-09.pdf " Algae in Sierra Nevada Mountain Wilderness Areas: Potential Health Hazards"], ''Journal of Mountain Medicine and Ecology'', University of California, Davis, Fall 2009</ref>

In the U.S., every coastal state has had harmful algal blooms over the last decade and new species have emerged in new locations that were not previously known to have caused problems. Inland, major rivers have seen an increase in their size and frequency. In 2015 the [[Ohio River]] had a bloom which stretched an "unprecedented" {{convert|650|mi|km}} into adjoining states and tested positive for toxins, which created drinking water and recreation problems.<ref>[http://www.dispatch.com/content/stories/local/2015/10/03/algae-along-the-ohio.html "Toxic algae bloom now stretches 650 miles along Ohio river"] {{Webarchive|url=https://web.archive.org/web/20160809034951/http://www.dispatch.com/content/stories/local/2015/10/03/algae-along-the-ohio.html |date=2016-08-09 }}, ''The Columbus Dispatch'', Oct. 3, 2015</ref> A portion of Utah's [[Jordan River (Utah)|Jordan River]] was closed due to toxic algal bloom in 2016.<ref name="Utah2" />

Off the west coast of [[South Africa]], HABs caused by ''[[Alexandrium catanella]]'' occur every spring. These blooms of organisms cause severe disruptions in [[Fishery|fisheries]] of these waters as the toxins in the phytoplankton cause filter-feeding [[shellfish]] in affected waters to become poisonous for human consumption.<ref>{{cite web |title=Red Tide Fact Sheet - Red Tide (Paralytic Shellfish Poisoning) |url=http://www.mass.gov/?pageID=eohhs2modulechunk&L=4&L0=Home&L1=Provider&L2=Guidance+for+Businesses&L3=Food+Safety&sid=Eeohhs2&b=terminalcontent&f=dph_environmental_foodsafety_p_red_tide&csid=Eeohhs2 |url-status=live |archive-url=https://web.archive.org/web/20090826001901/http://www.mass.gov/?pageID=eohhs2modulechunk&L=4&L0=Home&L1=Provider&L2=Guidance+for+Businesses&L3=Food+Safety&sid=Eeohhs2&b=terminalcontent&f=dph_environmental_foodsafety_p_red_tide&csid=Eeohhs2 |archive-date=26 August 2009 |access-date=2009-08-23 |publisher=mass.gov}}</ref>

== Harmful effects ==
{{Further|Eutrophication#Effects}}
As algal blooms grow, they deplete the oxygen in the water and block sunlight from reaching fish and plants. Such blooms can last from a few days to many months.<ref name="Utah2" /> With less light, plants beneath the bloom can die and fish can starve. Furthermore, the dense population of a bloom reduces oxygen saturation during the night by respiration.  And when the algae eventually die off, the microbes which decompose the dead algae use up even more oxygen, which in turn causes more fish to die or leave the area. When oxygen continues to be depleted by blooms it can lead to [[Hypoxia (environmental)|hypoxic]] [[dead zone (ecology)|dead zones]], where neither fish nor plants are able to survive.<ref>[https://www.science.org/doi/10.1126/science.1156401 "Spreading Dead Zones and Consequences for Marine Ecosystems"], ''Science'', August 15, 2008</ref> These dead zones in the case of the Chesapeake Bay, where they are a normal occurrence, are also suspected of being a major source of [[methane]].<ref>{{cite news |last=Dietrich |first=Tamara |date=2016-07-19 |title=Study: Chesapeake Bay a bigger methane generator than previously thought |url=http://www.dailypress.com/news/science/dp-nws-chesapeake-bay-methane-20160719-story.html |work=Daily Press |location=Newport News, VA}}</ref>

Scientists have found that HABs were a prominent feature of previous mass [[extinction event]]s, including the [[Permian–Triassic extinction event|End-Permian Extinction]].<ref>{{cite journal |last1=Mays |first1=Chris |last2=McLoughlin |first2=Stephen |display-authors=etal. |date=September 17, 2021 |title=Lethal microbial blooms delayed freshwater ecosystem recovery following the end-Permian extinction |journal=[[Nature Communications]] |volume=12 |issue=5511 |page=5511 |doi=10.1038/s41467-021-25711-3|pmid=34535650 |pmc=8448769 |bibcode=2021NatCo..12.5511M |doi-access=free }}</ref>

===Human health===
Tests have shown some toxins near blooms can be in the air and thereby be inhaled, which could affect health.<ref>[https://weather.com/science/environment/news/florida-martin-county-algae-toxic-air-particles-marina-rio "Tests Reveal Florida's Toxic Algae is Threatening Not Only The Water Quality but Also the Air"], ''Weather.com'', July 28, 2016</ref> This occurs due to the aerosolization of the toxins which are then transported by the wind. Not all algal blooms release harmful toxins into the atmosphere, because it is dependent on both the species and environmental conditions. Some species of microalgae/bacteria release toxins through cell lysis induced by physical stresses such as wave action. Other species release toxins through lysis induced by cellular processes, viral stresses, or chemical stresses in the water column. The toxins are mainly aerosolized into spray aerosols caused by wave action. The surface movement of water creates bubbles in the water, and when these bubbles reach the surface they pop. When the bubbles pop they eject water droplets which contain lysed cell material from harmful algal blooms (HAB's). Once ejected the aerosolized toxins are transported by the wind.<ref>{{cite journal |last1=Lim |first1=CC |title=Harmful algal bloom aerosols and human health |date=2023 |volume=93 |journal=eBioMedicine |doi=10.1016/j.ebiom.2023.104604 |pmid=37164781 |pmc=10363441 }}</ref> A 2017 study found that sea spray aerosol can be transported up to 1000&nbsp;km by the wind.<ref>{{cite journal |last1=Bondy |first1=A.L. |title=Inland sea spray aerosol transport and incomplete chloride depletion: varying degrees of reactive processing observed during SOAS. |journal=Environ Sci Technol |date=2017 |volume=51 |issue=17 |pages=9533–9542 |doi=10.1021/acs.est.7b02085 |pmid=28732168 |bibcode=2017EnST...51.9533B |url=https://pubmed.ncbi.nlm.nih.gov/28732168/ |access-date=13 February 2025}}</ref> [[File:Aerosolization of toxins from HABs.png|thumb|Process of HAB toxin aerosolization, wind transport, and human exposure.]] It is important to stay informed of local policies and response measures that are in place for HAB's even if you do not live directly on the coast because of the distances that these toxins can be transported.

====Food====
Eating fish or shellfish from lakes with a bloom nearby is not recommended.<ref name="California" />  Potent toxins are accumulated in shellfish that feed on the algae. If the shellfish are consumed, various types of poisoning may result. These include [[amnesic shellfish poisoning]] (ASP), [[diarrhetic shellfish poisoning]], [[neurotoxic shellfish poisoning]], and [[paralytic shellfish poisoning]].<ref name=":02" /> A 2002 study has shown that algal toxins may be the cause for as many as 60,000 intoxication cases in the world each year.<ref name=":02">{{Cite journal |last=Hoagland P., Anderson D.M., Kaoru Y., White A.W.|date=August 2002 |title=The Economic Effects of Harmful Algal Blooms in the United States: Estimates, Assessment Issues, and Information Needs |journal=Estuaries |volume=4b |issue=2 |pages=307–312 |jstor=258443}}</ref>

In 1987 a new illness emerged: [[amnesic shellfish poisoning]] (ASP). People who had eaten [[mussel]]s from [[Prince Edward Island]] were found to have ASP. The illness was caused by [[domoic acid]], produced by a diatom found in the area where the mussels were cultivated.<ref>{{Cite journal |last=Backer |first=Lorraine C. |date=Fall 2017 |title=Harmful Algal Blooms. At the interface between coast oceanography and human health |url=https://www.researchgate.net/publication/276189630 |journal=Oceanography |volume=19 |issue=2 |page=96}}</ref> A 2013 study found that toxic [[paralytic shellfish poisoning]] in the [[Philippines]] during HABs has caused at least 120 deaths over a few decades.<ref>[http://ojs.wpro.who.int/ojs/index.php/wpsar/article/view/302/485 "Lethal paralytic shellfish poisoning from consumption of green mussel broth, Western Samar, Philippines, August 2013"], ''World Health Organization'', Issue #2, April–June 2015</ref> After a 2014 HAB incident in [[Monterey Bay]], California, health officials warned people not to eat certain parts of anchovy, sardines, or crab caught in the bay.<ref name="KSBW">{{cite news |url=http://www.ksbw.com/news/toxic-algae-blooms-killing-sea-birds-threaten-humans/25729808 |title=Toxic algae blooms killing sea birds, threaten humans |work=KSBW TV News |date=April 30, 2014}}</ref>  In 2015 most shellfish fisheries in Washington, Oregon and California were shut down because of high concentrations of toxic domoic acid in shellfish.<ref name="shellfish" /> People have been warned that inhaling vapors from waves or wind during a HAB event may cause asthma attacks or lead to other respiratory ailments.<ref>{{cite journal |pmc=2683400 |pmid=17218574 |doi=10.1378/chest.06-1830 |volume=131 |issue=1 |title=Aerosolized red-tide toxins (brevetoxins) and asthma | journal=Chest |pages=187–94 |vauthors=Fleming LE, Kirkpatrick B, Backer LC, Bean JA, Wanner A, Reich A, Zaias J, Cheng YS, Pierce R, Naar J, Abraham WM, Baden DG|year=2007}}</ref>

In 2018 agricultural officials in [[Utah]] worried that even crops could become contaminated if irrigated with toxic water, although they admit that they can't measure contamination accurately because of so many variables in farming. They issued warnings to residents, however, out of caution.<ref name="KSL">{{cite news |url=https://www.ksl.com/?sid=40725118&nid=148 |title=Effects of algal blooms continue to spread throughout Wasatch Front |work=KSL TV News |date=July 19, 2016}}</ref>

====Drinking water====
[[File:Toxic Algae Bloom in Lake Erie.jpg|thumb|Satellite image of [[Lake Erie]] during a harmful algal bloom in 2011]]
Persons are generally warned not to enter or drink water from algal blooms, or let their pets swim in the water since many pets have died from algal blooms.<ref name="Russian" /> In at least one case, people began getting sick before warnings were issued.<ref>[http://www.sgvtribune.com/environment-and-nature/20160718/people-got-sick-at-pyramid-lake-before-the-state-reported-toxic-algae-bloom-could-it-have-been-avoided "People got sick at Pyramid Lake before the state reported toxic algae bloom. Could it have been avoided?"], ''San Gabriel Valley Tribune'', July 18, 2016</ref> There is no treatment available for animals, including livestock cattle, if they drink from algal blooms where such toxins are present. Pets are advised to be kept away from algal blooms to avoid contact.<ref>[http://www.marysvilleonline.net/news/article_49f895a6-4932-11e6-84b9-87884f9cfc1a.html?mode=print "Blue algae problems persist"], ''Marysville Online'', July 13, 2016</ref>

In some locations visitors have been warned not to even touch the water.<ref name="California" /> Boaters have been told that toxins in the water can be inhaled from the spray from wind or waves.<ref name="Peebles" /><ref name="California" /> Ocean beaches,<ref name="Florida" /> lakes<ref name="Utah" /> and rivers have been closed due to algal blooms.<ref name="Utah2">[http://www.heraldextra.com/news/local/utah-county-portion-of-jordan-river-closed-due-to-toxic/article_3a5a7e25-504c-5821-b9c8-ce288f5c4e5e.html "Utah County portion of Jordan River closed due to toxic algal bloom"] {{Webarchive|url=https://web.archive.org/web/20160724120618/http://www.heraldextra.com/news/local/utah-county-portion-of-jordan-river-closed-due-to-toxic/article_3a5a7e25-504c-5821-b9c8-ce288f5c4e5e.html |date=2016-07-24 }}, ''Daily Herald'', July 21, 2016</ref> After a dog died in 2015 from swimming in a bloom in California's [[Russian River (California)|Russian River]], officials likewise posted warnings for parts of the river.<ref>[http://www.pressdemocrat.com/news/4425912-181/dog-dies-in-russian-river?artslide=0 "Dog dies on Russian River, tests positive for toxic algae"] {{Webarchive|url=https://web.archive.org/web/20160817003023/http://www.pressdemocrat.com/news/4425912-181/dog-dies-in-russian-river?artslide=0 |date=2016-08-17 }}, Sept. 3, 2015</ref> Boiling the water at home before drinking does not remove the toxins.<ref name="California" />

In August 2014 the city of [[Toledo, Ohio]], advised its 500,000 residents to not drink tap water as the high toxin level from an algal bloom in western [[Lake Erie]] had affected their water treatment plant's ability to treat the water to a safe level.<ref name="Toledo">[http://www.ecowatch.com/toxic-algae-bloom-leaves-500-000-without-drinking-water-in-ohio-1881940537.html "Toxic Algae Bloom Leaves 500,000 Without Drinking Water in Ohio"], ''Ecowatch'', August 3, 2014</ref> The emergency required using bottled water for all normal uses except showering, which seriously affected public services and commercial businesses. The bloom returned in 2015<ref>[https://www.motherjones.com/tom-philpott/2015/08/giant-toxic-algae-bloom-haunts-toledo "The Big-Ag-Fueled Algae Bloom That Won't Leave Toledo's Water Supply Alone"], ''Mother Jones'', August 5, 2016</ref> and was forecast again for the summer of 2016.<ref>[http://www.ecowatch.com/lake-eries-toxic-algae-bloom-forecast-for-summer-2016-1891172391.html "Lake Erie's Toxic Algae Bloom Forecast for Summer 2016"], ''EcoWatch'', June 13, 2016</ref>

In 2004, a bloom in Kisumu Bay, which is the drinking water source for 500,000 people in [[Kisumu]], [[Kenya]], suffered from similar water contamination.<ref name="Circle">[http://www.circleofblue.org/2014/world/draft-world-stands-algae-dead-zones-ruin-water/ "World Stands By As Algae and Dead Zones Ruin Water"], ''Circle of Blue'', Sept. 25, 2014</ref> In China, water was cut off to residents in 2007 due to an algal bloom in its third largest lake, which forced 2 million people to use bottled water.<ref>[https://www.nbcnews.com/id/wbna18959222 "Algae smother Chinese lake, millions panic"], NBC News, May 31, 2007</ref><ref>{{cite news |last=Kahn |first=Joseph |date=2007-10-14 |title=In China, a Lake's Champion Imperils Himself |url=https://www.nytimes.com/2007/10/14/world/asia/14china.html?_r=0 | work=New York Times}}</ref> A smaller water shut-down in China affected 15,000 residents two years later at a different location.<ref>[https://web.archive.org/web/20090812010232/http://news.xinhuanet.com/english/2009-08/08/content_11847923.htm "Algal bloom in Central China reservoir affects drinking water of 15,000"], ''Chinaview'', July 8, 2009</ref> Australia in 2016 also had to cut off water to farmers.<ref>[http://www.theage.com.au/victoria/bluegreen-algal-bloom-chokes-murray-cuts-water-to-farmers-20160308-gndihv.html "Blue-green algal bloom chokes Murray, cuts water to farmers"], ''The Age'', March 9, 2016</ref>

Alan Steinman of [[Grand Valley State University]] has explained that among the major causes for the algal blooms in general, and Lake Erie specifically, is because blue-green algae thrive with high nutrients, along with warm and calm water. Lake Erie is more prone to blooms because it has a high nutrient level and is shallow, which causes it to warm up more quickly during the summer.<ref>Video interview: [https://www.youtube.com/watch?v=E2PsjhiZl2Q&t=38s Dr. Alan Steinman on Algal Blooms in Lake Erie] 13 min.</ref>

Symptoms from drinking toxic water can show up within a few hours after exposure. They can include nausea, vomiting, and diarrhea, or trigger headaches and gastrointestinal problems.<ref name="Utah" /> Although rare, liver toxicity can cause death.<ref name="Utah" /> Those symptoms can then lead to dehydration, another major concern. In high concentrations, the toxins in the algal waters when simply touched can cause skin rashes, irritate the eyes, nose, mouth or throat.<ref name="California" /> Those with suspected symptoms are told to call a doctor if symptoms persist or they can't hold down fluids after 24 hours.{{citation needed|date=January 2023}}

In studies at the population level bloom coverage has been significantly related to the risk of non-alcoholic [[liver disease]] death.<ref>{{cite journal|author1=Feng Zhang|author2=Jiyoung Lee|author3=Song Liang|author4=CK Shum|date=2015|title=Cyanobacteria blooms and non-alcoholic liver disease: evidence from a county level ecological study in the United States|journal=Environ Health|volume=14|issue=1 |page=41|doi=10.1186/s12940-015-0026-7|pmc=4428243|pmid=25948281 |doi-access=free |bibcode=2015EnvHe..14...41Z }}</ref>

====Neurological disorders====
Toxic algae blooms are thought to play a role in humans developing degenerative neurological disorders such as [[amyotrophic lateral sclerosis]] and [[Parkinson's disease]].<ref>{{cite news |last1=Konkel |first1=Lindsey |title=Are Algae Blooms Linked to Lou Gherig's Disease |url=https://www.scientificamerican.com/article/are-algae-blooms-linked-to-lou-gehrig-s-disease/ |access-date=18 August 2021 |publisher=Scientific American |date=December 11, 2014}}</ref>

Less than one percent of algal blooms produce hazardous toxins, such as [[microcystins]].<ref name="NOAA" /> Although blue-green or other algae do not usually pose a direct threat to health, the [[toxins]] (poisons) which they produce are considered dangerous to humans, land animals, sea mammals,  birds<ref name="Utah2" /> and fish when the toxins are ingested.<ref name="NOAA" /> The toxins are [[neurotoxins]] which destroy nerve tissue which can affect the nervous system, brain, and liver, and can lead to death.<ref name="Utah" />

==== Effects on humans from harmful algal blooms in marine environments ====
Humans are affected by the HAB species by ingesting improperly harvested shellfish, breathing in aerosolized brevetoxins (i.e. PbTx or ''Ptychodiscus'' toxins) and in some cases skin contact.<ref name=":6">{{cite journal |last1=Backer|first1=Lorraine C |last2=Fleming|first2=Lora E |last3=Rowan|first3=Alan |last4=Cheng|first4=Yung-Sung |last5=Benson|first5=Janet |last6=Pierce|first6=Richard H |last7=Zaias|first7=Julia |last8=Bean|first8=Judy |last9=Bossart|first9=Gregory D |date=March 2003 |title=Recreational exposure to aerosolized brevetoxins during Florida red tide events |journal=Harmful Algae |volume=2 |issue= 1|pages=19–28 |doi=10.1016/s1568-9883(03)00005-2 |bibcode=2003HAlga...2...19B |issn=1568-9883}}</ref> The brevetoxins bind to [[voltage-gated sodium channel]]s, important structures of cell membranes. Binding results in persistent activation of nerve cells, which interferes with neural transmission leading to health problems. These toxins are created within the unicellular organism, or as a metabolic product.<ref name="doi.org">{{Cite journal |doi=10.1007/s10646-008-0241-x|pmid=18758951|pmc=2683401|title=Harmful algal toxins of the Florida red tide (Karenia brevis): Natural chemical stressors in South Florida coastal ecosystems|journal=Ecotoxicology|volume=17|issue=7|pages=623–631|year=2008|last1=Pierce|first1=R. H.|last2=Henry|first2=M. S.|bibcode=2008Ecotx..17..623P }}</ref> The two major types of brevetoxin compounds have similar but distinct backbone structures. PbTx-2 is the primary intracellular brevetoxin produced by ''K. brevis'' blooms. However, over time, the PbTx-2 brevetoxin can be converted to PbTx-3 through metabolic changes.<ref name="doi.org" /> Researchers found that PbTx-2 has been the primary intracellular brevetoxin that converts over time into PbTx-3.<ref>Pierce, R.H., M. S. Henry. "Harmful algal toxins of the Florida red tide (Karenia brevis): natural chemical stressors in South Florida coastal ecosystems." ''Ecotoxicology'' 2008; 623–631.</ref>

In the U.S., the seafood consumed by humans is tested regularly for toxins by the USDA to ensure safe consumption. Such testing is common in other nations. However, improper harvesting of shellfish can cause [[paralytic shellfish poisoning]] and neurotoxic shellfish poisoning in humans.<ref name=":9" /><ref name=":8" /> Some symptoms include drowsiness, diarrhea, nausea, loss of motor control, tingling, numbing or aching of extremities, incoherence, and respiratory paralysis.<ref name="PMID10698729">{{Cite journal |last1=Van Dolah |first1=F. M. |year=2000 |title=Marine algal toxins: Origins, health effects, and their increased occurrence |journal=Environmental Health Perspectives |volume=108 |issue=Suppl 1 |pages=133–141 |doi=10.1289/ehp.00108s1133 |jstor=3454638 |pmc=1637787 |pmid=10698729|bibcode=2000EnvHP.108S.133V }}</ref> Reports of skin irritation after swimming in the ocean during a HAB are common.<ref name=":10" />

When the HAB cells rupture, they release extracellular brevetoxins into the environment. Some of those stay in the ocean, while other particles get aerosolized. During onshore winds, brevetoxins can become aerosolized by bubble-mediated transport, causing respiratory irritation, [[bronchoconstriction]], coughing, and wheezing, among other symptoms.<ref name=":10">Backer et al., Lorraine C., Laura E. Flemming, Alan Rowan. "Recreational exposure to aerosolized brevetoxins during Florida red tide events." Harmful Algae 2 (2003): 19–28. 6 March 2018.</ref>

It is recommended to avoid contact with wind-blown aerosolized toxin. Some individuals report a decrease in respiratory function after only 1 hour of exposure to a ''K. brevis'' red-tide beach and these symptoms may last for days.<ref>Fleming LE, Kirkpatrick B, Backer LC, et al. Initial evaluation of the effects of aerosolized Florida red tide toxins (brevetoxins) in persons with asthma. ''Environ Health Perspect.'' 2005;113:650–657.</ref> People with severe or persistent respiratory conditions (such as chronic lung disease or asthma) may experience stronger adverse reactions.{{medical citation needed|date=July 2023}}

The [[National Oceanic and Atmospheric Administration]]'s [[National Ocean Service]] provides a public conditions report identifying possible respiratory irritation impacts in areas affected by HABs.<ref name=":444">{{cite web|url=http://www.tidesandcurrents.noaa.gov/hab/|title=Harmful Algal Bloom Operational Forecast System|publisher=www.tidesandcurrents.noaa.gov/hab/|access-date=2012-02-14|archive-date=2012-02-17|archive-url=https://web.archive.org/web/20120217061645/http://tidesandcurrents.noaa.gov/hab/|url-status=dead}}</ref>

===Economic impact===
====Recreation and tourism====
The hazards which accompany harmful algal blooms have hindered visitors' enjoyment of beaches and lakes in places in the U.S. such as Florida,<ref name="Florida">[http://wlrn.org/post/summer-heat-could-worsen-algae-blooms-florida-waters "Summer Heat Could Worsen Algae Blooms In Florida Waters"], ''WLRN'', July 14, 2016</ref> California,<ref name="California">[http://www.latimes.com/local/lanow/la-me-ln-pyramid-lake-algae-bloom-20160714-snap-story.html "Summer conditions growing toxic algae blooms in two California lakes"], ''Los Angeles Times'', July 21, 2016</ref> Vermont,<ref>[http://www.necn.com/news/new-england/Vt-Beaches-Reopen-After-Algae-Blooms-Clear-386953991.html "Vt. Beaches Reopen After Algae Blooms Clear"], ''NECN'', July 15, 2016</ref> and Utah.<ref name="Utah2" /> Persons hoping to enjoy their vacations or days off have been kept away to the detriment of local economies. Lakes and rivers in [[North Dakota]], [[Minnesota]], Utah, California and Ohio have had signs posted warning about the potential of health risk.<ref>[http://www.floridatoday.com/story/news/local/environment/2016/07/22/algae-blooming-waterways-all-around-country/87262178/ "Algae is blooming in waterways all around the country"], ''Florida Today'', July 22, 2016</ref>

Similar blooms have become more common in Europe, with France among the countries reporting them. In the summer of 2009, beaches in northern [[Brittany]] became covered by tonnes of potentially lethal rotting green algae. A horse being ridden along the beach collapsed and died from fumes given off by the rotting algae.<ref>[https://www.theguardian.com/world/2009/aug/10/france-brittany-coast-seaweed-algae "Lethal algae take over beaches in northern France"], ''The Guardian'', U.K., August 10, 2009</ref>

The economic damage resulting from lost business has become a serious concern. According to one report in 2016, the four main economic impacts from harmful algal blooms come from damage to human health, fisheries, tourism and recreation, and the cost of monitoring and management of area where blooms appear.<ref name="JRC" /> EPA estimates that algal blooms impact 65 percent of the country's major estuaries, with an annual cost of $2.2 billion.<ref name="KSL" /> In the U.S. there are an estimated 166 coastal dead zones.<ref name="KSL" /> Because data collection has been more difficult and limited from sources outside the U.S., most of the estimates as of 2016 have been primarily for the U.S.<ref name="JRC">[https://ec.europa.eu/jrc/en/publication/algal-bloom-and-its-economic-impact "Algal bloom and its economic impact"], ''European Commission Joint Research Centre'', 2016</ref>

In port cities in the [[Shandong Province]] of eastern China, residents are no longer surprised when massive algal blooms arrive each year and inundate beaches. Prior to the Beijing Olympics in 2008, over 10,000 people worked to clear 20,000 tons of dead algae from beaches.<ref name="China">[http://www.ibtimes.co.uk/china-yellow-sea-turns-green-qingdao-beaches-are-covered-algae-photos-1509734 "China: Yellow Sea turns green as Qingdao beaches are covered in algae"], ''International Business Times'', July 7, 2015</ref> In 2013 another bloom in China, thought to be its largest ever,<ref>[http://phys.org/news/2013-07-china-largest-ever-algae-bloom.html "China hit by largest-ever algae bloom"], ''Phys.org'', July 4, 2013</ref> covered an area of 7,500 square miles,<ref name="China" /> and was followed by another in 2015 which blanketed an even greater 13,500 square miles. The blooms in China are thought to be caused by pollution from untreated agricultural and industrial discharges into rivers leading to the ocean.<ref>[http://www.businessinsider.com/china-has-an-algae-beach-pollution-problem-2015-7?r=UK&IR=T "Slimy green algae is taking over China's beaches for an alarming reason"], ''Business Insider'', July 13, 2015</ref>

====Fisheries industry====
As early as 1976 a short-term, relatively small, dead zone off the coasts of [[New York (state)|New York]] and [[New Jersey]] cost commercial and recreational fisheries over $500 million.<ref name="SciAm2">[http://www.scientificamerican.com/article/oceanic-dead-zones-spread/ "Oceanic Dead Zones Continue to Spread"], ''Scientific American'', August 15, 2008</ref> In 1998 a HAB in Hong Kong killed over $10 million in high-value fish.<ref name="Brown">Brown, Lester; McGinn, Anne Platt. ''Vital Signs 1999–2000: The Environmental Trends that Are Shaping Our Future'', Routledge (1999 pp. 198–199)</ref>

In 2009, the economic impact for the state of [[Washington (state)|Washington's]] coastal counties dependent on its fishing industry was estimated to be $22 million.<ref>{{cite news |last=Miner |first=Colin |date=2009-11-27 |title=Assessing Algal Blooms' Economic Impact |url=https://green.blogs.nytimes.com/2009/11/27/assessing-algal-blooms-economic-impact/ |work=New York Times |series=Green: Energy, the Environment and the Bottom Line (blog)}}</ref> In 2016, the U.S. seafood industry expected future lost revenue could amount to $900 million annually.<ref name="JRC" />

NOAA has provided a few cost estimates for various blooms over the past few years:<ref name="NOAA3">{{cite web |url=http://www.noaa.gov/what-is-harmful-algal-bloom |title=What is a harmful algal bloom? |author=<!--Not stated--> |date=2016-04-27 |publisher=U.S. National Oceanic and Atmospheric Administration (NOAA) |location=Washington, D.C.}}</ref> $10.3 million in 2011 due to a HAB at Texas oyster landings; $2.4 million lost income by tribal commerce from 2015 fishery closures in the pacific northwest; $40 million from Washington state's loss of tourism from the same fishery closure.

Along with damage to businesses, the toll from human sickness results in lost wages and damaged health. The costs of medical treatment, investigation by health agencies through water sampling and testing, and the posting of warning signs at effected locations is also costly.<ref name="WHOI">[https://www.whoi.edu/fileserver.do?id=24159&pt=10&p=19132 "Estimated Annual Economic Impacts from Harmful Algal Blooms (HABs) in the United States"], Woods Hole Oceanographic Institution, September 2000</ref>

The closures applied to areas where this algae bloom occurs has a big negative impact of the fishing industries, add to that the high fish mortality that follows, the increase in price due to the shortage of fish available and decrease in the demand for seafood due to the fear of contamination by toxins.<ref>{{Cite book|title=Algae bloom and its economic impact |last=Sanseverino |first=Isabella |publisher=JRC publications |year=2016 |isbn=978-92-79-58101-4 |location=Europe |pages=23, 26, 27}}</ref> This causes a big economic loss for the industry.

Economic costs are estimated to rise. In June 2015, for instance, the largest known toxic HAB forced the shutdown of the west coast shellfish industry, the first time that has ever happened. One Seattle NOAA expert commented, "This is unprecedented in terms of the extent and magnitude of this harmful algal bloom and the warm water conditions we're seeing offshore...."<ref name="Oregon">[http://www.oregonlive.com/pacific-northwest-news/index.ssf/2015/06/west_coast_shellfish_industrie.html "Biggest-ever toxic algal bloom hits West Coast, shutting down shellfish industries"], ''Oregon Live'', June 16, 2015</ref> The bloom covered a range from [[Santa Barbara, California]] northward to [[Alaska]].<ref>[http://www.cbsnews.com/news/toxic-algae-bloom-in-pacific-ocean-could-be-largest-ever/ "Toxic algae bloom in Pacific Ocean could be largest ever"], ''CBS News'', June 17, 2015</ref>

The negative impact on fish can be even more severe when they are confined to pens, as they are in fish farms. In 2007 a fish farm in [[British Columbia]] lost 260 tons of salmon as a result of blooms,<ref>[http://www.farmedanddangerous.org/salmon-farming-problems/environmental-impacts/algae-blooms/ Algae Blooms in fish farming], Farmed and Dangerous.org</ref> and in 2016 a farm in Chile lost 23 million salmon after [[Red tide crisis in Chiloé|an algal bloom]].<ref>[http://thinkprogress.org/climate/2016/03/10/3758642/chile-algal-bloom-salmon/ "One Of The U.S.'s Top Salmon Providers Just Lost Millions Of Salmon"], ''Climate Progress'', March 10, 2016</ref>

===Environmental impact===

==== Dead zones ====
The presence of harmful algae bloom's can lead to [[Hypoxia (environmental)|hypoxia]] or [[Anoxic waters|anoxia]] in a body of water. The depletion of oxygen within a body of water can lead to the creation of a [[Dead zone (ecology)|dead zone]]. Dead zones occur when a body of water has become unsuitable for organism survival in that location. HAB's cause dead zones by consuming oxygen in these bodies of water - leaving minimal oxygen available to other marine organisms. When the HAB's die, their bodies will sink to the bottom of the body of water - as the decaying of their bodies (through bacteria) is what causes the consumption of oxygen. Once oxygen levels get so low, the HAB's have placed the body of water in hypoxia - and these low oxygen levels will cause marine organisms to seek out better suited locations for their survival.<ref>{{Cite journal|last=Joyce|first=S.|date=2000-03-01|title=The dead zones: oxygen-starved coastal waters.|journal=Environmental Health Perspectives|volume=108|issue=3|pages=A120–A125|doi=10.1289/ehp.108-a120|pmc=1637951|pmid=10706539}}</ref>

Blooms can harm the environment even without producing toxins by depleting oxygen from the water when growing and while decaying after they die. Blooms can also block sunlight to organisms living beneath it. A record-breaking number and size of blooms have formed in the Pacific coast, in Lake Erie, in the Chesapeake Bay and in the Gulf of Mexico, where a number of dead zones were created as a result.<ref name="Huffington">[http://www.huffingtonpost.com/entry/florida-algal-blooms_us_577d70cee4b0c590f7e7e3d2 "Toxic Algal Blooms Aren't Just Florida's Problem. And They're On The Rise."], ''Huffington Post'', July 7, 2016</ref> In the 1960s the number of dead zones worldwide was 49; the number rose to over 400 by 2008.<ref name="SciAm2" />

Among the largest dead zones were those in northern Europe's [[Baltic Sea]] and the Gulf of Mexico, which affects a $2.8 billion U.S. fish industry.<ref name="Circle" /> Unfortunately, dead zones rarely recover and usually grow in size.<ref name="SciAm2" /> One of the few dead zones to ever recover was in the [[Black Sea]], which returned to normal fairly quickly after the collapse of the Soviet Union in the 1990s due to a resulting reduction in fertilizer use.<ref name="SciAm2" />

{{multiple image
| align             = left
| direction         = horizontal
| image1            = Scientists discover under-ice phytoplankton bloom 1.ogv
| width1            = 300
| alt1              =
| caption1          = The US Coast Guard Cutter Healy ferried scientists to 26 study sites in the Arctic, where blooms ranged in concentration from high (red) to low (purple).
| image2            = Scientists discover under-ice phytoplankton bloom 2.ogv
| width2            = 300
| alt2              =
| caption2          = Researcher David Mayer of Clark University lowers a video camera below the ice to observe a dense bloom of phytoplankton.
}}
{{clear}}

====Fish die-offs====
Massive fish die-offs have been caused by HABs.<ref name="WHOI-2">[https://www.whoi.edu/science/B/redtide/foodweb/fishkills.html "Fish Kills due to Harmful Algal Blooms"], ''Woods Hole Oceanographic Institute''</ref> In 2016, 23 million salmon which were being farmed in Chile died from a toxic algae bloom.<ref name="salmon">[http://www.ecowatch.com/23-million-salmon-dead-due-to-toxic-algal-bloom-in-chile-1882188276.html "23 Million Salmon Dead Due to Toxic Algal Bloom in Chile"], ''EcoWatch'', March 10, 2016</ref> To get rid of the dead fish, the ones fit for consumption were made into fishmeal and the rest were dumped 60 miles offshore to avoid risks to human health.<ref name="salmon" /> The economic cost of that die-off is estimated to have been $800 million.<ref name="salmon" /> Environmental expert [[Lester Brown]] has written that the farming of salmon and shrimp in offshore ponds concentrates waste, which contributes to [[eutrophication]] and the creation of dead zones.<ref>Brown, Lester R. [http://www.earth-policy.org/images/uploads/book_files/pb4book.pdf ''Plan B 4.0: Mobilizing to Save Civilization''] {{Webarchive|url=https://web.archive.org/web/20141226005303/http://www.earth-policy.org/images/uploads/book_files/pb4book.pdf |date=2014-12-26 }}, Earth Policy Institute, p. 227</ref>

Other countries have reported similar impacts, with cities such as [[Rio de Janeiro]], Brazil seeing major fish die-offs from blooms becoming a common occurrence.<ref name="Brazil">[http://america.aljazeera.com/articles/2015/4/21/brazil-removes-50-tons-of-dead-fish-from-olympic-waters.html "Brazil removes 50 tons of dead fish from Olympic waters"], ''Aljazeera'', April 21, 2015</ref> In early 2015, Rio collected an estimated 50 tons of dead fish from the lagoon where water events in the 2016 Olympics were planned to take place.<ref name="Brazil" />

The [[Monterey Bay]] has suffered from harmful algal blooms, most recently in 2015: "Periodic blooms of toxin-producing ''[[Pseudo-nitzschia]]'' diatoms have been documented for over 25 years in Monterey Bay and elsewhere along the U.S. west coast. During large blooms, the [[Bioaccumulation|toxin accumulates]] in shellfish and small fish such as anchovies and sardines that feed on algae, forcing the closure of some fisheries and poisoning marine mammals and birds that feed on contaminated fish."<ref>Tim Stephens, [http://news.ucsc.edu/2015/05/algal-bloom.html Large bloom of toxic algae under way in Monterey Bay and beyond], [[UC Santa Cruz]] (June 2, 2015).</ref> Similar fish die-offs from toxic algae or lack of oxygen have been seen in Russia,<ref>[https://translate.google.com/translate?sl=auto&tl=en&js=y&prev=_t&hl=en&ie=UTF-8&u=http%3A%2F%2Fsobitie.com.ua%2Fnovosti%2Fv-kamenskom-mestnye-vlasti-nanyali-podryadchikov-ubirat-gory-mertvoy-ryby-s-plyazhey&edit-text=&act=url "Kamensk local authorities have hired contractors to clean up mountains of dead fish from the beaches"], July 18, 2016</ref> Colombia,<ref>[https://translate.google.com/translate?sl=auto&tl=en&js=y&prev=_t&hl=en&ie=UTF-8&u=http%3A%2F%2Fwww.wradio.com.co%2Fnoticias%2Fregionales%2Freportan-mortandad-de-peces-en-los-palafitos%2F20160717%2Fnota%2F3191232.aspx&edit-text=&act=url "Fish kills reported in the Palafitos"], W Radio, July 17, 2016</ref> Vietnam,<ref>[http://english.vietnamnet.vn/fms/society/160721/thanh-hoa--locals-wear-masks-as-smell-from-dead-fish-overpowering.html "Thanh Hoa: Locals wear masks as smell from dead fish overpowering"], Vietnam.net, July 19, 2016</ref> China,<ref>[https://translate.google.com/translate?sl=auto&tl=en&js=y&prev=_t&hl=en&ie=UTF-8&u=http%3A%2F%2Fjs.ifeng.com%2Fa%2F20160706%2F4725768_0.shtml&edit-text=&act=url "Hongze Lake Suqian great, full of dead fish breeding area"], Modern Express Network, July 6, 2016</ref> Canada,<ref>[http://www.digitaljournal.com/news/environment/tens-of-thousands-of-fish-dead-in-quebec-s-yamaska-river/article/469215 "Massive fish kill in Quebec's Yamaska River puzzle scientists"], ''Digital Journal'', July 4, 2016</ref> Turkey,<ref>[http://www.hurriyetdailynews.com/scores-of-starfish-wash-ashore-in-turkeys-northwest.aspx?pageID=238&nID=101009&NewsCatID=341 "Scores of starfish wash ashore in Turkey's northwest"], ''Hurriyet Daily News'', June 28, 2016</ref> Indonesia,<ref>[http://www.radarcirebon.com/90-ton-ikan-waduk-darma-mati-masal-ini-penyebabnya.html "90 Tons of Fish Die Darma Masal"] {{Webarchive|url=https://web.archive.org/web/20160816150743/http://www.radarcirebon.com/90-ton-ikan-waduk-darma-mati-masal-ini-penyebabnya.html |date=2016-08-16 }}, Radar Cirebon, June 2, 2016</ref> and France.<ref>[https://translate.google.com/translate?sl=auto&tl=en&js=y&prev=_t&hl=en&ie=UTF-8&u=http%3A%2F%2Fwww.francetvinfo.fr%2Fmeteo%2Fmaine-et-loire-des-milliers-de-poissons-asphyxies-avec-la-decrue_1506061.html%23xtor%3DAL-67-&edit-text=&act=url "Maine-et-Loire: Thousands of fish suffocated with the decline"], ''France TV'', June 18, 2016</ref>

====Land animal deaths====
Land animals, including livestock and pets have been affected. Dogs have died from the toxins after swimming in algal blooms.<ref>[http://thinkprogress.org/climate/2015/02/02/3617937/california-algae-bloom/ "Unseasonal Toxic Algae Bloom In California Lake Kills Three Dogs"], ''Climate Progress'', Feb. 2, 2015</ref> Warnings have come from government agencies in the state of Ohio, which noted that many dogs and livestock deaths resulted from HAB exposure in the U.S. and other countries. They also noted in a 2003 report that during the previous 30 years, they have seen more frequent and longer-lasting harmful algal blooms."<ref name="Ohio">[http://epa.ohio.gov/portals/35/inland_lakes/OhioVeterinaryAssociationArticle40910%20.pdf "Harmful Algal Blooms Can Be Deadly to Pets and Livestock"], ''Ohio Environmental Protection Agency''</ref> In 50 countries and 27 states that year there were reports of human and animal illnesses linked to algal toxins.<ref name="Ohio" /> In Australia, the department of agriculture warned farmers that the toxins from a HAB had the "potential to kill large numbers of livestock very quickly."<ref>[http://agriculture.vic.gov.au/agriculture/pests-diseases-and-weeds/animal-diseases/beef-and-dairy-cows/blue-green-algal-poisoning-of-stock "Blue-green algal poisoning of stock"] {{Webarchive|url=https://web.archive.org/web/20180620104157/http://agriculture.vic.gov.au/agriculture/pests-diseases-and-weeds/animal-diseases/beef-and-dairy-cows/blue-green-algal-poisoning-of-stock |date=2018-06-20 }}, ''Agriculture Victoria''</ref>

[[File:Dead whale NOAA.jpg|thumb|left|Whales can be killed by harmful algal blooms.]]
Marine mammals have also been seriously harmed, as over 50 percent of unusual marine mammal deaths are caused by harmful algal blooms.<ref>{{cite web |url=http://celebrating200years.noaa.gov/foundations/coastal_research/dead_whale_500.html |title=Over 50 percent of unusual marine mammal mortality events are caused by harmful algal blooms. |author=<!--Not stated--> |date=2007-02-13 |website=Coastal Ecosystem Research |publisher=NOAA}}</ref> In 1999, over 65 [[bottlenose dolphins]] died during a coastal HAB in Florida.<ref>{{cite web |url=http://oceanservice.noaa.gov/websites/retiredsites/sotc_pdf/hab.pdf |title=Harmful Algal Blooms |author=<!--Not stated--> |date=2000-02-15 |website=State of the Coastal Environment |publisher=NOAA |access-date=2016-07-24 |archive-date=2011-07-27 |archive-url=https://web.archive.org/web/20110727002805/http://oceanservice.noaa.gov/websites/retiredsites/sotc_pdf/hab.pdf |url-status=dead }}</ref> In 2013 a HAB in southwest Florida killed a record number of [[Manatee]].<ref>[http://www.accuweather.com/en/outdoor-articles/beach-marine/red-tide-algae-bloom-kills-rec/8049099 "Red Tide Algae Bloom Kills Record Number of Manatee"] {{Webarchive|url=https://web.archive.org/web/20160816185353/http://www.accuweather.com/en/outdoor-articles/beach-marine/red-tide-algae-bloom-kills-rec/8049099 |date=2016-08-16 }}, ''Accuweather'', March 13, 2013</ref> Whales have also died in large numbers. During the period from 2005 to 2014, Argentina reported an average 65 baby whales dying which experts have linked to algal blooms. A whale expert there expects the whale population to be reduced significantly.<ref>[http://www.ecowatch.com/algal-blooms-linked-to-largest-die-off-of-great-whales-ever-recorded-1882115945.html "Algal Blooms Linked to Largest Die-Off of Great Whales Ever Recorded"], ''EcoWatch'', Oct. 29, 2015</ref> In 2003 off [[Cape Cod]] in the North Atlantic, at least 12 humpback whales died from toxic algae from a HAB.<ref>[http://www.nature.com/news/2003/030804/full/news030804-1.html "Toxic algae suspected in whale death"], ''Nature'', August 4, 2003</ref> In 2015 Alaska and British Columbia reported many [[humpback whales]] had likely died from HAB toxins, with 30 having washed ashore in Alaska. "Our leading theory at this point is that the harmful algal bloom has contributed to the deaths," said a NOAA spokesperson.<ref>[http://www.nigerian-news.com/30-dead-whales-wash-ashore-in-alaska-scientists-commence-investigations/ "30 Dead Whales Wash Ashore In Alaska; Scientists Commence Investigations"], ''Nigerian News'', August 24, 2015</ref><ref>[https://www.inverse.com/article/6223-toxic-algae-blooms-could-be-causing-whale-deaths "Toxic Algae Could Be Killing Dozens of Whales"], ''Inverse'', Sept. 16, 2015</ref>

Birds have died after eating dead fish contaminated with toxic algae. Rotting and decaying fish are eaten by birds such as [[pelicans]], [[seagulls]], [[cormorant]]s, and possibly marine or land mammals, which then become poisoned.<ref name="WHOI-2" /> The nervous systems of dead birds were examined and had failed from the toxin's effect.<ref name="KSBW" /> On the Oregon and Washington coast, a thousand [[scoter]]s, or sea ducks, were also killed in 2009. "This is huge," said a university professor.<ref name="Oregon2">[http://www.oregonlive.com/news/index.ssf/2009/10/foam_from_ocean_algae_bloom_ki.html "Foam from ocean algae bloom killing thousands of birds"], ''Oregon Live'', October 22, 2009</ref> As dying or dead birds washed up on the shore, wildlife agencies went into "an emergency crisis mode."<ref name="Oregon2" />

It has even been suggested that harmful algal blooms are responsible for the deaths of animals found in fossil troves,<ref>{{cite journal |author=Gramling C. |year=2017 |title=Toxic algae may be culprit in mysterious dinosaur deaths |journal=Science |volume=357 |issue=6354 |page=857 |pmid=28860363 |doi=10.1126/science.357.6354.857-a|bibcode=2017Sci...357..857G }}</ref> such as the dozens of [[cetacean]] skeletons found at [[Cerro Ballena]].<ref>{{cite journal|last1=Pyenson|first1=N. D.|last2=Gutsein|first2=C. S.|last3=Parham|first3=J. F.|last4=Le Roux|first4=J. P.|last5=Chavarria|first5=C. C.|last6=Little|first6=H.|last7=Metallo|first7=A.|last8=Rossi|first8=V.|last9=Valenzuela-Toro|first9=A. M.|last10=Velez-Juarbe|first10=J.|last11=Santelli|first11=C. M.|last12=Rogers|first12=D. R.|last13=Cozzuol|first13=M. A|last14=Suárez|first14=M. E.|date=2014|title=Repeated mass strandings of Miocene marine mammals from Atacama Region of Chile point to sudden death at sea|journal=Proceedings of the Royal Society B: Biological Sciences|volume=281|issue=1781|doi=10.1098/rspb.2013.3316|doi-access=free|pmc=3953850|pmid=24573855}} [https://royalsocietypublishing.org/action/downloadSupplement?doi=10.1098%2Frspb.2013.3316&file=rspb20133316supp1.pdf Supplemental Material]</ref>

=== Effects on marine ecosystems ===
Harmful algal blooms in marine ecosystems have been observed to cause adverse effects to a wide variety of aquatic organisms, most notably marine mammals, sea turtles, seabirds and finfish. The impacts of HAB toxins on these groups can include harmful changes to their developmental, immunological, neurological, or reproductive capacities. The most conspicuous effects of HABs on marine wildlife are large-scale mortality events associated with toxin-producing blooms. For example, a [[mass mortality event]] of 107 [[bottlenose dolphins]] occurred along the Florida panhandle in the spring of 2004 due to ingestion of contaminated [[menhaden]] with high levels of [[brevetoxin]].<ref name="Flewelling LJ 2005">{{cite journal|last=Flewelling|first=L. J.|display-authors=etal|year=2005|title=Red tides and marine mammal mortalities|journal=[[Nature (journal)|Nature]]|volume=435|issue=7043|pages=755–756|bibcode=2005Natur.435..755F|doi=10.1038/nature435755a|pmc=2659475|pmid=15944690}}</ref> Manatee mortalities have also been attributed to brevetoxin but unlike dolphins, the main toxin vector was endemic seagrass species (''Thalassia testudinum'') in which high concentrations of brevetoxins were detected and subsequently found as a main component of the stomach contents of manatees.<ref name="Flewelling LJ 2005" />

Additional marine mammal species, like the highly endangered [[North Atlantic right whale]], have been exposed to neurotoxins by preying on highly contaminated [[zooplankton]].<ref name="Durbin E 2002">Durbin E et al. (2002) North Atlantic right whale, ''Eubalaena glacialis'', exposed to paralytic shellfish poisoning (PSP) toxins by a zooplankton vector, ''Calanus finmarchicus''. Harmful Algae I, : 243–251 (2002)</ref> With the summertime habitat of this species overlapping with seasonal blooms of the toxic dinoflagellate ''Alexandrium fundyense'', and subsequent copepod grazing, foraging right whales will ingest large concentrations of these contaminated [[copepods]]. Ingestion of such contaminated prey can affect respiratory capabilities, feeding behavior, and ultimately the reproductive condition of the population.<ref name="Durbin E 2002" />

Immune system responses have been affected by brevetoxin exposure in another critically endangered species, the [[loggerhead sea turtle]]. Brevetoxin exposure, from inhalation of aerosolized toxins and ingestion of contaminated prey, can have clinical signs of increased lethargy and muscle weakness in loggerhead sea turtles causing these animals to wash ashore in a decreased metabolic state with increases of immune system responses upon blood analysis.<ref>{{cite journal|last=Walsh|first=C. J.|display-authors=etal|year=2010|title=Effects of brevetoxin exposure on the immune system of loggerhead sea turtles|journal=Aquatic Toxicology|volume=97|issue=4|pages=293–303|doi=10.1016/j.aquatox.2009.12.014|pmid=20060602}}</ref>

Examples of common harmful effects of HABs include:

# the production of neurotoxins which cause mass mortalities in fish, seabirds, sea turtles, and marine mammals
# human illness or death from consumption of seafood contaminated by toxic algae<ref>{{cite web|title=Red Tide FAQ - Is it safe to eat oysters during a red tide?|url=http://www.tpwd.state.tx.us/landwater/water/environconcerns/hab/redtide/faq.phtml#q9|access-date=2009-08-23|publisher=Tpwd.state.tx.us}}</ref>
# mechanical damage to other organisms, such as disruption of epithelial gill tissues in fish, resulting in asphyxiation
# oxygen depletion of the water column (hypoxia or [[Anoxic waters|anoxia]]) from cellular respiration and bacterial degradation

[[File:Gulf dead zone.jpg|thumb|Dead zone in the southern U.S.]]

==== Marine life exposure ====
HABs occur naturally off coasts all over the world. Marine dinoflagellates produce ichthyotoxins. Where HABs occur, dead fish wash up on shore for up to two weeks after a HAB has been through the area. In addition to killing fish, the toxic algae contaminate shellfish. Some [[mollusks]] are not susceptible to the toxin, and store it in their fatty tissues. By consuming the organisms responsible for HABs, shellfish can accumulate and retain [[saxitoxin]] produced by these organisms. Saxitoxin blocks [[sodium channel]]s and ingestion can cause [[paralysis]] within 30 minutes.<ref name=":8">{{cite web|url=http://www.tpwd.state.tx.us/landwater/water/environconcerns/hab/redtide/faq.phtml#q9|title=Red Tide FAQ – Is it safe to eat oysters during a red tide?|publisher=www.tpwd.state.tx.us|access-date=2009-08-23}}
</ref>

In addition to directly harming marine animals and vegetation loss, harmful algal blooms can also lead to [[ocean acidification]], which occurs when the amount of carbon dioxide in the water is increased to unnatural levels. Ocean acidification slows the growth of certain species of fish and shellfish, and even prevents shell formation in certain species of mollusks. These subtle, small changes can add up over time to cause chain reactions and devastating effects on whole marine ecosystems.<ref>US Department of Commerce, N. O. and A. A. (NOAA). (2019, April 2). What is Eutrophication? NOAA's National Ocean Service. Retrieved July 9, 2022, from https://oceanservice.noaa.gov/facts/eutrophication.html</ref>
Other animals that eat exposed shellfish are susceptible to the neurotoxin, which may lead to neurotoxic shellfish poisoning<ref name=":9">{{cite journal |first1=Sharon M. |last1=Watkins |first2=Andrew |last2=Reich |first3=Lora E. |last3=Fleming |first4=Roberta |last4=Hammond |title=Neurotoxic Shellfish Poisoning |journal=Marine Drugs |year=2008 |volume=6 |issue=3 |pages=431–455 |doi=10.3390/md20080021 |pmid=19005578 |pmc=2579735|doi-access=free }}</ref> and sometimes even death. Most mollusks and clams filter feed, which results in higher concentrations of the toxin than just drinking the water.<ref name=":111">Brand et al., Larry E., Lisa Campbell, Eileen Bresnan. "Karenia: The biology and ecology of a toxic genus." Harmful Algae 14 (2012): 156–178. 6 March 2018.</ref> [[Scaup]], for example, are [[diving ducks]] whose diet mainly consists of mollusks. When scaup eat the filter-feeding shellfish that have accumulated high levels of the HAB toxin, their population becomes a prime target for poisoning. However, even birds that do not eat mollusks can be affected by simply eating dead fish on the beach or drinking the water.<ref name=":22">Forrester et al., Donald J., Jack M. Gaskin, Franklin H. White. "AN EPIZOOTIC OF WATERFOWL IN FLORIDA." Journal of Wildlife Diseases 13 (1997): 160–167.</ref>

The toxins released by the blooms can kill marine animals including [[dolphin]]s, sea turtles, birds, and [[manatee]]s.<ref name=":7">{{cite web | url=https://start1.org/red-tide/effects/ | title=Red Tide & Red Algae Effects | date=2015 }}</ref><ref>{{cite web | url=http://www.floridahealth.gov/environmental-health/aquatic-toxins/_documents/red-tide-rack-card-2014-two-up-897kb.pdf | title=Top 10 Red Tide Facts |publisher=Florida Department of Health | date=2016 }}</ref> The Florida Manatee, a subspecies of the West Indian Manatee, is a species often impacted by red tide blooms. Florida manatees are often exposed to the poisonous red-tide toxins either by consumption or inhalation. There are many small barnacles, crustaceans, and other [https://edis.ifas.ufl.edu/publication/SG188 epiphytes] that grow on the blades of seagrass. These tiny creatures filter particles from the water around them and use these particles as their main food source. During red tide blooms, they also filter the toxic red tide cells from the water, which then becomes concentrated inside them. Although these toxins do not harm epiphytes, they are extremely poisonous to marine creatures who consume (or accidentally consume) the exposed epiphytes, such as manatees. When manatees unknowingly consume exposed epiphytes while grazing on sea grass, the toxins are subsequently released from the epiphytes and ingested by the manatees. In addition to consumption, manatees may also become exposed to air-borne Brevetoxins released from harmful red-tide cells when passing through algal blooms.<ref>Hall-Scharf , B., & Ubeda , A. J. (2019, October 3). SG188/SG188: How red tides impact manatees. Retrieved July 10, 2022, from https://edis.ifas.ufl.edu/publication/SG188</ref>
Manatees also have an immunoresponse to HABs and their toxins that can make them even more susceptible to other stressors. Due to this susceptibility, manatees can die from either the immediate, or the after effects of the HAB.<ref>{{Cite journal|last1=Landsberg|first1=J.H.|last2=Flewelling|first2=L.J.|last3=Naar|first3=J.|date=March 2009|title=Karenia brevis red tides, brevetoxins in the food web, and impacts on natural resources: Decadal advancements|journal=Harmful Algae|volume=8|issue=4|pages=598–607|doi=10.1016/j.hal.2008.11.010|bibcode=2009HAlga...8..598L |issn=1568-9883}}</ref> In addition to causing manatee mortalities, red-tide exposure also causes severe [https://www.researchgate.net/publication/271274562_Sublethal_red_tide_toxin_exposure_in_free-ranging_manatees_Trichechus_manatus_affects_the_immune_system_through_reduced_lymphocyte_proliferation_responses_inflammation_and_oxidative_stress sublethal health problems among Florida manatee populations]. Studies have shown that red-tide exposure among free-ranging Florida manatees has been shown to negatively impact immune functioning by causing increased inflammation, a reduction in lymphocyte proliferation responses, and oxidative stress.<ref>{{ cite Walsh, C. J., Butawan, M., Yordy, J., Ball, R., Flewelling, L., de Wit, M., & Bonde, R. K. (2015). Sublethal red tide toxin exposure in free-ranging manatees (Trichechus manatus) affects the immune system through reduced lymphocyte proliferation responses, inflammation, and oxidative stress. Aquatic Toxicology, 161, 73-84 }}</ref>
Fish such as Atlantic herring, American pollock, winter flounder, Atlantic salmon, and cod were dosed orally with these toxins in an experiment, and within minutes the subjects started to exhibit a loss of equilibrium and began to swim in an irregular, jerking pattern, followed by paralysis and shallow, arrhythmic breathing and eventually death, after about an hour.<ref name=":33">White, A. W. "Sensitivity of Marine Fishes to Toxins from the Red-Tide Dinoflagellate Gonyaulax excavata and Implications for Fish Kills." Marine Biology 65 (1981): 255–260. 6 March 2018.</ref> HABs have been shown to have a negative effect also in the memory functions of sea lions.<ref>{{Cite journal|last1=Cook|first1=P.F.|last2=Reichmuth|first2=C.|date=2015|title=Algal toxin impairs sea lion memory and hippocampal connectivity, with implications for strandings|url=https://www.science.org/doi/10.1126/science.aac5675|journal=Science|volume=350|issue=6267|pages=1545–1547|doi=10.1126/science.aac5675|pmid=26668068|bibcode=2015Sci...350.1545C|s2cid=22981507|url-access=subscription}}</ref>

== Potential remedies ==
{{Further|eutrophication#Reversal and remediation}}

===Reducing nutrient runoff===
[[File:Runoff of soil & fertilizer.jpg|thumb|Soil and fertilizer runoff from a farm after heavy rains]]
Since many algal blooms are caused by a major influx of nutrient-rich runoff into a water body, programs to treat wastewater, reduce the overuse of fertilizers in agriculture and reducing the bulk flow of runoff can be effective for reducing severe algal blooms at river mouths, [[Estuary|estuaries]], and the ocean directly in front of the river's mouth.

The nitrates and phosphorus in fertilizers cause algal blooms when they run off into lakes and rivers after heavy rains. Modifications in farming methods have been suggested, such as only using fertilizer in a targeted way at the appropriate time exactly where it can do the most good for crops to reduce potential runoff.<ref name="SciAm">Biello, David. [http://www.scientificamerican.com/article/deadly-algae-are-everywhere-thanks-to-agriculture/ "Deadly Algae Are Everywhere, Thanks to Agriculture"], ''Scientific American'', August 8, 2014</ref> A method used successfully is [[drip irrigation]], which instead of widely dispersing fertilizers on fields, drip-irrigates plant roots through a network of tubes and emitters, leaving no traces of fertilizer to be washed away.<ref>Siegel, Seth M. ''Let There Be Water: Israel's Solution for a Water-Starved World'', Macmillan (2015) p. 66</ref> Drip irrigation also prevents the formation of algal blooms in [[reservoirs]] for drinking water while saving up to 50% of water typically used by agriculture.<ref>[http://www.oecdobserver.org/news/fullstory.php/aid/4819/Israel:_Innovations_overcoming_water_scarcity.html "Israel: Innovations overcoming water scarcity"], ''OECD Observer'', April 2015</ref><ref>[http://www.sandiegouniontribune.com/news/2015/jun/16/israel-drought-water-reclamation/ "How Israel survived its devastating drought"], ''San Diego Union-Tribune'', June 16, 2015</ref>

There have also been proposals to create buffer zones of foliage and wetlands to help filter out the phosphorus before it reaches water.<ref name="SciAm" /> Other experts have suggested using conservation tillage, changing crop rotations, and restoring wetlands.<ref name="SciAm" /> It is possible for some dead zones to shrink within a year under proper management.<ref name="Larsen">Larsen, Janet. [http://www.earth-policy.org/plan_b_updates/2004/update41 "Dead Zones Increasing in the World's Coastal Waters"], Earth Policy Institute, June 16, 2004</ref>

There have been a few success stories in controlling chemicals. After Norway's lobster fishery collapsed in 1986 due to low oxygen levels, for instance, the government in neighboring Denmark took action and reduced phosphorus output by 80 percent which brought oxygen levels closer to normal.<ref name="Larsen" /> Similarly, dead zones in the [[Black Sea]] and along the [[Danube River]] recovered after phosphorus applications by farmers were reduced by 60%.<ref name="Larsen" />

Nutrients can be permanently removed from wetlands harvesting wetland plants, reducing nutrient influx into surrounding bodies of water.<ref>{{Cite journal |last1=Jeke |first1=Nicholson N. |last2=Zvomuya |first2=Francis |last3=Cicek |first3=Nazim |last4=Ross |first4=Lisette |last5=Badiou |first5=Pascal |date=September 2015 |title=Biomass, Nutrient, and Trace Element Accumulation and Partitioning in Cattail (Typha latifolia L.) during Wetland Phytoremediation of Municipal Biosolids |journal=Journal of Environmental Quality |volume=44 |issue=5 |pages=1541–1549 |doi=10.2134/jeq2015.02.0064 |issn=0047-2425 |pmid=26436271|doi-access=free |bibcode=2015JEnvQ..44.1541J }}</ref><ref>{{Cite journal |last1=Cicek |first1=N. |last2=Lambert |first2=S. |last3=Venema |first3=H.D. |last4=Snelgrove |first4=K.R. |last5=Bibeau |first5=E.L. |last6=Grosshans |first6=R. |date=June 2006 |title=Nutrient removal and bio-energy production from Netley-Libau Marsh at Lake Winnipeg through annual biomass harvesting |journal=Biomass and Bioenergy |volume=30 |issue=6 |pages=529–536 |doi=10.1016/j.biombioe.2005.12.009 |bibcode=2006BmBe...30..529C |issn=0961-9534}}</ref> Research is ongoing to determine the efficacy of floating mats of cattails in removing nutrients from surface waters too deep to sustain the growth of wetland plants.<ref>{{Cite web |date=2015-10-01 |title=The Floating Bioplatforms of IISD-ELA |url=https://www.iisd.org/ela/blog/research-highlights/floating-bioplatforms/ |access-date=2020-07-08 |website=IISD Experimental Lakes Area}}</ref>

In the U.S., surface runoff is the largest source of nutrients added to rivers and lakes, but is mostly unregulated under the federal [[Clean Water Act]].<ref name="EPA 319 report">{{cite report |url=https://www.epa.gov/nps/319-grant-reports-and-project-summaries |title=National Nonpoint Source Program: A catalyst for water quality improvements |date=October 2016 |publisher=EPA |id=EPA 841-R-16-009}}</ref>{{rp|10}} <ref name="NPDES basic">{{cite web |author=<!--Not stated--> |date=2018-07-25 |title=NPDES Permit Basics |url=https://www.epa.gov/npdes/npdes-permit-basics |website=National Pollutant Discharge Elimination System |publisher=EPA |quote=By law, agricultural stormwater discharges and return flows from irrigated agriculture are not 'point sources.'}}</ref><ref>{{cite web |last=Kozacek |first=Code |date=2016-07-20 |title=Algal Blooms Are No Accident For Florida Everglades and Estuaries |url=https://www.circleofblue.org/2016/united-states/algal-blooms-no-accident-florida-everglades-estuaries/ |website=Circle of Blue |location=Traverse City, Michigan}}</ref> Locally developed initiatives to reduce [[nutrient pollution]] are underway in various areas of the country, such as the [[Great Lakes]] region and the [[Chesapeake Bay]].<ref name="EPA-Great Lakes">{{cite web |author=<!--Not stated--> |date=2016-08-29 |title=Great Lakes Water Quality Agreement |url=https://www.epa.gov/glwqa |publisher=EPA}}</ref><ref name="EPA-Chesapeake TMDL">{{cite web |author=<!--Not stated--> |date=2017-02-09 |title=Chesapeake Bay Total Maximum Daily Load (TMDL) |url=https://www.epa.gov/chesapeake-bay-tmdl |publisher=EPA}}</ref> To help reduce algal blooms in [[Lake Erie]], the State of [[Ohio]] presented a plan in 2016 to reduce phosphorus runoff.<ref>[http://www.dispatch.com/content/stories/local/2016/05/26/states-plan-to-restore-lake-erie-stops-short-of-mandating-farming-changes.html# "Ohio plan to restore Lake Erie won't mandate farming changes"] {{webarchive|url=https://web.archive.org/web/20160801131937/http://www.dispatch.com/content/stories/local/2016/05/26/states-plan-to-restore-lake-erie-stops-short-of-mandating-farming-changes.html|date=2016-08-01}}, ''The Columbus Dispatch'', July 27, 2016</ref>

=== Chemical treatment ===
Although a number of [[algaecide]]s have been effective in killing algae, they have been used mostly in small bodies of water. For large algal blooms, however, adding algaecides such as silver nitrate or copper sulfate can have worse effects, such as killing fish outright and harming other wildlife.<ref name="Greensburg">[http://www.greensburgdailynews.com/news/local_news/what-is-creeping-into-our-lakes/article_d77a16fa-ef97-5f5c-bb18-ae7b1acca0fa.html "What is creeping into our lakes?"], ''Greensburg Daily News'', August 16, 2016</ref> Cyanobacteria can also develop resistance to copper-containing algaecides, requiring a larger quantity of the chemical to be effective for HAB management, but introducing a greater risk to other species in the region.<ref name="Zerrifi-et-al-2018">{{Cite journal |last1=Zerrifi |first1=Soukaina El Amrani |last2=El Khalloufi |first2=Fatima |last3=Oudra |first3=Brahim |last4=Vasconcelos |first4=Vitor |date=2018-02-09 |title=Seaweed Bioactive Compounds against Pathogens and Microalgae: Potential Uses on Pharmacology and Harmful Algae Bloom Control |journal=Marine Drugs |volume=16 |issue=2 |page=55 |doi=10.3390/md16020055 |issn=1660-3397 |pmc=5852483 |pmid=29425153 |doi-access=free}}</ref> The negative effects can therefore be worse than letting the algae die off naturally.<ref name="Greensburg" /><ref name="OSU">[http://oregonstate.edu/ua/ncs/archives/2015/jun/toxic-algal-blooms-behind-klamath-river-dams-create-health-risks-far-downstream "Toxic algal blooms behind Klamath River dams create health risks far downstream"], ''Oregon State University News'', June 16, 2015</ref>[[File:Efficacy of Aluminum Modified Clays.png|thumb|The left graph shows the efficacy of aluminum chloride modified clay (AC-MC), aluminum sulfide modified clay (AS-MC), polyaluminum modified clay (PAC-MC) and standard untreated clay in deionized water for removing ''[[Aureococcus anophagefferens]]'', a bloom causing algae. The right graph shows the same clays tested in seawater.<ref name="Liu-et-al-2016">{{Cite journal |last1=Liu |first1=Yang |last2=Cao |first2=Xihua |last3=Yu |first3=Zhiming |last4=Song |first4=Xiuxian |last5=Qiu |first5=Lixia |date=2016-02-15 |title=Controlling harmful algae blooms using aluminum-modified clay |journal=Marine Pollution Bulletin |volume=103 |issue=1–2 |pages=211–219 |doi=10.1016/j.marpolbul.2015.12.017 |issn=1879-3363 |pmid=26763322|bibcode=2016MarPB.103..211L }}</ref>]]

In 2019, [[Chippewa Lake]] in Northeast Ohio became the first lake in the U.S. to successfully test a new chemical treatment. The chemical formula killed all of the toxic algae in the lake within a single day. The formula has already been used in China, South Africa and Israel.<ref>[https://www.news5cleveland.com/news/local-news/oh-medina/chippewa-lake-becomes-first-testing-site-of-new-algae-bloom-technology-produced-by-israeli-company "Chippewa Lake becomes first testing site of new algae bloom technology produced by Israeli company"], ABC News, Cleveland, OH August 27, 2019</ref>

In February 2020, [[Roodeplaat Dam]] in [[Gauteng]] Province, [[South Africa]] was treated with a new [[Algaecide|algicide]] formulation against a severe bloom of ''[[Microcystis]]'' sp. This formulation allows the granular product to float and slow-release its active ingredient, [[sodium percarbonate]], that releases [[hydrogen peroxide]] (H<sub>2</sub>O<sub>2</sub>), on the water surface. Consequently, the effective concentrations are limited, vertically, to the surface of the water; and spatially to areas where [[cyanobacteria]] are abundant. This provide the aquatic organisms a "safe haven" in untreated areas and avoids the adverse effects associated with the use of standard [[Algaecide|algicides]].<ref>{{Cite web |date=2020-04-27 |title=Israeli Company Successfully Treats Roodeplaat Dam of Toxic Algae Blooms |url=https://www.industryleadersmagazine.com/israeli-company-successfully-treats-roodeplaat-dam-of-toxic-algae-blooms/ |access-date=2020-04-27 |website=Industry Leaders Magazine}}</ref>

Bioactive compounds isolated from terrestrial and aquatic plants, particularly seaweeds, have seen results as a more environmentally friendly control for HABs. Molecules found in seaweeds such as [[Corallina]], [[Sargassum]], and [[Saccharina japonica]] have shown to inhibit some bloom-forming microalgae. In addition to their anti-microalgal effects, the bioactive molecules found in these seaweeds also have antibacterial, antifungal, and antioxidant properties.<ref name="Zerrifi-et-al-2018" />

==== Removal of HABs using aluminum-modified clay ====
Other chemicals are being tested for their efficacy for removing cyanobacteria during blooms. Modified clays, such as aluminum chloride modified clay (AC-MC), aluminum sulfide modified clay (AS-MC) and polyaluminum chloride modified clay (PAC-MC) have shown positive results ''in vitro'' for the removal of [[Aureococcus]] by trapping the microalgae in the sediment of clay, removing it from the top layer of water where harmful blooms can occur.<ref name="Liu-et-al-2016" />

Many efforts have been made in an attempt to control HAB's so that the harm that they cause can be kept at a minimum. Studies into the use of clay to control HAB's have proven that this method may be an effective way to reduce the negative effects caused by HAB's. The addition of [[Aluminium chloride|aluminum chloride]], [[Aluminium sulfate|aluminum sulfate]], or [[Polyaluminium chloride|polyaluminum chloride]] to clay can modify the clay surface and increase its efficiency in the removal of HAB's from a body of water. The addition of aluminum-containing compounds causes the clay particles to achieve a positive charge, with these particles then undergoing [[flocculation]] with the harmful algae cells. The algae cells then group together: becoming a [[sediment]] instead of a [[Suspension (chemistry)|suspension]]. The process of flocculation will limit the bloom growth and reduce the impact in which the bloom can have on an area.<ref>{{Cite journal |last1=Liu |first1=Yang |last2=Cao |first2=Xihua |last3=Yu |first3=Zhiming |last4=Song |first4=Xiuxian |last5=Qiu |first5=Lixia |date=2016-02-15 |title=Controlling harmful algae blooms using aluminum-modified clay |url=https://www.sciencedirect.com/science/article/pii/S0025326X15302204 |journal=Marine Pollution Bulletin |language=en |volume=103 |issue=1 |pages=211–219 |doi=10.1016/j.marpolbul.2015.12.017 |issn=0025-326X |pmid=26763322|bibcode=2016MarPB.103..211L |url-access=subscription }}</ref>

In the Netherlands, successful algae and phosphate removal from surface water has been obtained by pumping affected water through a hydrodynamic separator. The treated water is then free from algae and contains a significant lower amount of phosphate since the removed algae cells contain a lot of phosphate. The treated water also gets a lower turbidity. Future projects will study the positive effects on the ecology and marine life as it is expected plant life will be restored and a reduction in bottom dwelling fish will automatically reduce the turbidity of the cleaned water. The removed algae and phosphate may find its way not as waste but as infeed to bio digesters.

=== Additional reservoirs ===
Other experts have proposed building reservoirs to prevent the movement of algae downstream. However, that can lead to the growth of algae within the reservoir, which become sediment traps with a resultant buildup of nutrients.<ref name="Greensburg" /> Some researchers found that intensive blooms in reservoirs were the primary source of toxic algae observed downstream, but the movement of algae has so far been less studied, although it is considered a likely cause of algae transport.<ref name="OSU" /><ref>{{cite web |author=<!--Not stated--> |date=2017-03-02 |title=Control and Treatment |url=https://www.epa.gov/nutrient-policy-data/control-and-treatment |website=Nutrient Policy and Data |publisher=EPA}}</ref>

=== Restoring shellfish populations ===
The decline of filter-feeding shellfish populations, such as [[oysters]], likely contribute to HAB occurrence.<ref>{{cite web |last=Brumbaugh |first=R.D. |display-authors=et al |date=2006 |title=A Practitioners Guide to the Design & Monitoring of Shellfish Restoration Projects: An Ecosystem Approach. The Nature Conservancy, Arlington, Virginia |url=http://www.habitat.noaa.gov/pdf/tncnoaa_shellfish_hotlinks_final.pdf  |archive-url=https://web.archive.org/web/20160304123652/http://www.habitat.noaa.gov/pdf/tncnoaa_shellfish_hotlinks_final.pdf |archive-date=4 March 2016 |access-date=2017-03-18 |website=Habitat.noaa.gov}}</ref> As such, numerous research projects are assessing the potential of restored shellfish populations to reduce HAB occurrence.<ref>{{cite web |title=Shinnecock Bay Restoration Program |url=http://www.shinnecockbay.org/program/current.html |access-date=2017-03-18 |website=Shinnecockbay.org}}</ref><ref>{{cite web |title=Delaware Oyster Gardening and Restoration - A Cooperative Effort |url=http://darc.cms.udel.edu/ibog/YSI%20article.pdf  |archive-url=https://web.archive.org/web/20160304091603/http://darc.cms.udel.edu/ibog/YSI%20article.pdf |archive-date=4 March 2016 |access-date=2017-03-18 |website=Darc.cms.udel.edu}}</ref><ref>{{Cite web |title=The Mobile Bay Oyster Gardening Program |url=http://masgc.org/oyster/documents/2012OGmanual.pdf  |archive-url=https://web.archive.org/web/20130525040928/http://masgc.org/oyster/documents/2012OGmanual.pdf |archive-date=25 May 2013 |access-date=5 August 2017}}</ref>

===Improved monitoring===
{{multiple image
| image1            = Super Blooms.ogv
| width1            = 300
| alt1              =
| caption1          = Algal blooms forming and breaking up over time
| direction         = horizontal
}}

Other remedies include using improved monitoring methods, trying to improve predictability, and testing new potential methods of controlling HABs.<ref name="US" /> Some countries surrounding the Baltic Sea, which has the world's largest dead zone, have considered using massive geoengineering options, such as forcing air into bottom layers to aerate them.<ref name="SciAm2" />

Mathematical models are useful to predict future algal blooms.<ref name="Janssen-et-al-2019" />

==== Sensors and monitoring devices ====
A growing number of scientists agree that there is an urgent need to protect the public by being able to forecast harmful algal blooms.<ref name="NYT-Richtel">{{cite news |last=Richtel |first=Matt |date=2016-07-18 |title=A Dreaded Forecast for Our Times: Algae, and Lots of It |work=New York Times |url=https://www.nytimes.com/2016/07/19/science/a-dreaded-forecast-for-our-times-algae-and-lots-of-it.html?_r=1}}</ref> One way they hope to do that is with sophisticated sensors which can help warn about potential blooms.<ref>[http://ehp.niehs.nih.gov/122-a206/ "Keeping Tabs on HABs: New Tools for Detecting, Monitoring, and Preventing Harmful Algal Blooms"], ''Environmental Health Perspectives'', August 1, 2014</ref> The same types of sensors can also be used by water treatment facilities to help them prepare for higher toxic levels.<ref name="NYT-Richtel" /><ref>[http://ehp.niehs.nih.gov/122-a206/ "Keeping Tabs on HABs: New Tools for Detecting, Monitoring, and Preventing Harmful Algal Blooms"], ''Environmental Health Perspectives'', August 2014</ref>

The only sensors now in use are located in the Gulf of Mexico. In 2008 similar sensors in the Gulf forewarned of an increased level of toxins that led to a shutdown of shellfish harvesting in Texas along with a recall of mussels, clams, and oysters, possibly saving many lives. With an increase in the size and frequency of HABs, experts state the need for significantly more sensors located around the country.<ref name="NYT-Richtel" /> The same kinds of sensors can also be used to detect threats to drinking water from intentional contamination.<ref>{{cite web |author=<!--Not stated--> |date=2016-12-20 |title=Water System Security and Resilience in Homeland Security Research |url=https://www.epa.gov/homeland-security-research/water-system-security-and-resilience-homeland-security-research |publisher=EPA}}</ref>

Satellite and remote sensing technologies are growing in importance for monitoring, tracking, and detecting HABs.<ref>{{cite journal |last1=Anderson |first1=Donald M. |date=July 2009 |title=Approaches to monitoring, control, and management of harmful algal blooms (HABs) |journal=Ocean and Coastal Management |volume=52 |issue=7 |pages=342–347 |doi=10.1016/j.ocecoaman.2009.04.006 |pmc=2818325 |pmid=20161650|bibcode=2009OCM....52..342A }}</ref><ref>{{cite journal |last1=Wu |first1=Di |last2=Zhang |first2=Feiyang |last3=Liu |first3=Jia |date=March 9, 2019 |title=A review on drone-based harmful algae blooms monitoring |journal=Environmental Monitoring and Assessment |volume=191 |issue=4 |page=211 |doi=10.1007/s10661-019-7365-8 |pmid=30852736 |bibcode=2019EMnAs.191..211W |s2cid=73725756}}</ref><ref>{{cite journal |last1=Kim |first1=Jun Song |last2=Seo |first2=Il Won |last3=Baek |first3=Donghae |date=May 2018 |title=Modeling spatial variability of harmful algal bloom in regulated rivers using a depth-averaged 2D numerical model |journal=Journal of Hydro-environment Research |volume=20 |pages=63–76 |doi=10.1016/j.jher.2018.04.008 |bibcode=2018JHER...20...63K |s2cid=134289465}}</ref><ref>{{Cite journal |last1=Akbarnejad Nesheli |first1=Sara |last2=Quackenbush |first2=Lindi J. |last3=McCaffrey |first3=Lewis |date=January 2024 |title=Estimating Chlorophyll-a and Phycocyanin Concentrations in Inland Temperate Lakes across New York State Using Sentinel-2 Images: Application of Google Earth Engine for Efficient Satellite Image Processing |journal=Remote Sensing |language=en |volume=16 |issue=18 |pages=3504 |doi=10.3390/rs16183504 |doi-access=free |bibcode=2024RemS...16.3504A |issn=2072-4292}}</ref> Four U.S. federal agencies—EPA, the [[National Aeronautics and Space Administration]] (NASA), NOAA, and the [[U.S. Geological Survey]] (USGS)—are working on ways to detect and measure cyanobacteria blooms using satellite data.<ref>[http://www.algaeindustrymagazine.com/us-agencies-creating-algal-bloom-early-warning-system/ "US agencies creating algal bloom early warning system"], ''Algae Industry Magazine'', April 8, 2015</ref> The data may help develop early-warning indicators of cyanobacteria blooms by monitoring both local and national coverage.<ref name="satellite">[http://toxics.usgs.gov/highlights/2015-12-21-cyanobacteria_sensing.html "Remote Sensing Provides a National View of Cyanobacteria Blooms"] {{Webarchive|url=https://web.archive.org/web/20160722072709/http://toxics.usgs.gov/highlights/2015-12-21-cyanobacteria_sensing.html |date=2016-07-22 }}, USGS</ref> In 2016 automated early-warning monitoring systems were successfully tested, and for the first time proven to identify the rapid growth of algae and the subsequent depletion of oxygen in the water.<ref>[https://www.news.virginia.edu/content/scientists-develop-early-warning-system-toxic-algae-blooms "Scientists Develop Early-Warning System for Toxic Algae Blooms"] {{Webarchive|url=https://web.archive.org/web/20170107102300/https://www.news.virginia.edu/content/scientists-develop-early-warning-system-toxic-algae-blooms |date=2017-01-07 }}, ''UVA Today'', Univ. of Virginia, Jan. 4, 2017</ref>

==Examples==
=== Notable occurrences ===
* 1530: First alleged case off the [[Florida]] Gulf Coast is without foundation.<ref>{{Citation |url=https://www.fs.fed.us/rm/boise/AWAE/scientists/profiles/Magana/HistoryKarenia%20brevis.pdf |title=A historical assessment of Karenia brevis in the western Gulf of Mexico|date=2018-08-16|language=en-US}}</ref><!--There is no citation or evidence for a 1530 occurrence other than a Log of with a fish kill.--> According to Marine Lab at University of Miami, the first possible Red Tide in Florida was in 1844.  Earlier "signs" were from boats sorting fish on their way to home port dumping trash fish overboard. Thus "dead fish" reports along the coast were not Red Tide.<ref name="Log In or Sign Up to View">{{Cite web|url=https://lookaside.fbsbx.com/file/cedar42.pdf?token=AWzGTCtrYAfPEvfoG44qbiQtzi-bAEVoQ5UrOC_towT6KfAhMKFv6KvRkWihdDXgcb9KaStz-TTWm4YeNNww-mPEiE2E9Lk66Hx7M8SysYcFc0b0eLt38BZNjMcMIqwtzGka2F6ITpcD1Ejr1VxwKSwF|title=Log In or Sign Up to View|website=lookaside.fbsbx.com|language=en|access-date=2018-07-21}}</ref>
* 1793: The first recorded case occurring in [[British Columbia]], [[Canada]].<ref name="GoogleDocs">{{cite web|url=https://docs.google.com/viewer?a=v&q=cache:I3hBpDwi8iUJ:www.fishdept.sabah.gov.my/download/redtideInfo.doc+&hl=en&gl=my&pid=bl&srcid=ADGEESjseyHhHp5Pf_4VmBBRMV6HMrFzggjLej8UjSJpXq1jzbAXun7bd4NfcHlfMO_rrNYo2haICv3AR2JD8-U3AEnMMzFFGjx3NqoUKLqlXB3pA1gGBUPlxYhlNz8ljYF3kr2KeWKZ&sig=AHIEtbT7Jmn7OLth-Dq8saRzi_6ADGvF3g|title=PARALYTIC SHELLFISH POISONING (PSP)|publisher=Sabah Fish Department.com|access-date=2013-01-11}}</ref>
* 1840: No deaths of humans have been attributed to Florida red tide, but people may experience respiratory irritation (coughing, sneezing, and tearing) when the red tide organism (''Karenia brevis'') is present along a coast and winds blow its aerosolized toxins. Swimming is usually safe, but skin irritation and burning is possible in areas of high concentration of red tide.<ref>{{cite web|url=http://taylor.ifas.ufl.edu/marine_redtide.shtml|title=Marine & Natural Resources – Red Tide & Fish Kill Resources – Taylor County Extension Office|access-date=18 October 2016}}</ref>
* 1844:  First possible case off the Florida Gulf Coast according to Marine Lab University of Miami, probably by ships off shore, no known inhabitants of the coast reporting.<ref name="Log In or Sign Up to View" />
* 1901: ''[[Lingulodinium polyedrum]]'' produces brilliant displays of bioluminescence in warm coastal waters. Seen in [[Southern California]] regularly since at least 1901.<ref>{{cite web |last=Nelson |first=Bryan |date=2011-11-11 |title=What is causing the waves in California to glow? &#124; MNN - Mother Nature Network |url=http://www.mnn.com/earth-matters/wilderness-resources/stories/what-is-causing-the-waves-in-california-to-glow |access-date=2017-03-18 |publisher=MNN}}</ref>
* 1916: Massive fish kill along SW Florida coast. Noxious air thought to be seismic underwater explosion releasing chlorine gas.<ref>{{Cite web|url=https://chroniclingamerica.loc.gov/lccn/sn95047324/1916-12-07/ed-1/seq-1/|title=The Punta Gorda herald. [volume] (Punta Gorda, Fla.) 1893-1958, December 07, 1916, Image 1|first=National Endowment for the|last=Humanities|date=December 7, 1916|via=chroniclingamerica.loc.gov}}</ref>
* 1947: Southwest Florida: A massive bloom that lasts close a year almost destroys the commercial fishing industry and sponge beds. The resulting poisoned surf caused beaches to need to be evacuated.<ref>{{cite news |author1=SHT Staff Writer |title=Red-tide timeline |archive-date=11 April 2022|url=https://www.heraldtribune.com/story/news/2006/07/16/red-tide-timeline/28564944007/|archive-url= https://web.archive.org/web/20220411044046/https://www.heraldtribune.com/story/news/2006/07/16/red-tide-timeline/28564944007/|work=Sarasota Herald-Tribune |date=July 16, 2006}}</ref>
* 1972: A red tide was caused in New England by a toxic dinoflagellate ''Alexandrium (Gonyaulax) tamarense''. The red tides caused by the dinoflagellate ''[[Gonyaulax]]'' are serious because this organism produces saxitoxin and gonyautoxins which accumulate in shellfish and if ingested may lead to [[Paralytic shellfish poisoning|paralytic shellfish poisoning (PSP)]] and can lead to death.<ref>{{Cite web|url=http://www.utas.edu.au/docs/plant_science/HAB2000/|archive-url=https://web.archive.org/web/20081211004726/http://www.utas.edu.au/docs/plant_science/HAB2000/|title=HAB 2000<!-- Bot generated title -->|archive-date=December 11, 2008}}</ref>
* 1972 and 1973: Red tides killed two villagers west of Port Moresby. In March 1973 a red tide invaded Port Moresby Harbour and destroyed a Japanese pearl farm.<ref name="MacLean_74">{{cite web|url=http://www.spc.int/DigitalLibrary/Doc/FAME/Reports/MacLean_74_Poison.pdf|title=Shellfish Poisoning in the South Pacific|author=MacLean, J.L.|date=February 1974|publisher=South Pacific Commission}}</ref>
* In 1972, a red tide was caused in [[New England]] by a toxic dinoflagellate ''[[Alexandrium tamarense|Alexandrium (Gonyaulax) tamarense]]''.<ref>{{cite web|title=HAB 2000<!-- Bot generated title -->|url=http://www.utas.edu.au/docs/plant_science/HAB2000/|archive-url=https://web.archive.org/web/20081211004726/http://www.utas.edu.au/docs/plant_science/HAB2000/|archive-date=11 December 2008|website=utas.edu.au}}</ref>
* 1976: The first PSP case in [[Sabah]], [[East Malaysia|Malaysian Borneo]] where 202 victims were reported to be suffering and 7 deaths.<ref name="GoogleDocs" /><ref name="nst.com.my">{{cite news|url=http://www.nst.com.my/nation/general/red-tide-warning-1.196724|title=Red tide warning|access-date=2013-01-07|date=2013-01-06|newspaper=[[New Straits Times]]|archive-url=https://web.archive.org/web/20130107005300/http://www.nst.com.my/nation/general/red-tide-warning-1.196724|archive-date=2013-01-07}}</ref><ref name="dailyexpress.com.my">{{cite news|url=http://www.dailyexpress.com.my/news.cfm?NewsID=83846|title=2 Red Tide deaths in Sabah|access-date=2013-01-11|date=2013-01-06|newspaper=[[Daily Express (Sabah)|Daily Express]]}}</ref>
* 1987: A red algae bloom in [[Prince Edward Island]] caused over a million dollars in losses.<ref>{{Cite web|title=Red Tides|url=https://waves-vagues.dfo-mpo.gc.ca/Library/4060066x.pdf|access-date=October 3, 2020}}</ref>
* 1991: The largest algal bloom on record was the [[1991 Darling River cyanobacterial bloom]] in Australia, largely of ''[[Anabaena circinalis]]'', between October and December 1991 over {{convert|1000|km|mi|-1}} of the [[Barwon River (New South Wales)|Barwon]] and [[Darling River]]s.<ref>{{cite journal |last1=Bowling |first1=L.C. |last2=Baker |first2=P.D |year=1996 |title=Major Cyanobacterial Bloom in the Barwon-Darling River, Australia, in 1991, and Underlying Limnological Conditions |journal=Marine and Freshwater Research |volume=47 |issue=4 |pages=643–657 |doi=10.1071/MF9960643|bibcode=1996MFRes..47..643B }}</ref>
* 2005: The Canadian red tide was discovered to have come further south than it has in years prior by the ship (R/V) Oceanus,<ref>{{cite web|url=http://www.whoi.edu/page.do?pid=8158|title=R/V Oceanus Archived Information|access-date=18 October 2016}}</ref> closing shellfish beds in Maine and Massachusetts and alerting authorities as far south as [[Montauk, New York|Montauk]] ([[Long Island]], NY) to check their beds.<ref>{{cite web |url=http://www.nationalfisherman.com/month-content.asp?ItemID=1156&pcid=184&cid=196&archive=yes |author=Moore, Kirk |title=Northeast Oysters: The bigger danger, growers assert, would be the label of endangered |publisher=National Fisherman |access-date=2008-07-31  |archive-url=https://web.archive.org/web/20070808213003/http://www.nationalfisherman.com/month-content.asp?ItemID=1156&pcid=184&cid=196&archive=yes |archive-date=2007-08-08 }}</ref> Experts who discovered the reproductive cysts in the seabed warn of a possible spread to Long Island in the future, halting the area's fishing and shellfish industry and threatening the tourist trade, which constitutes a significant portion of the island's economy.
* In 2008 large blooms of the algae ''Cochlodinium polykrikoid'' were found along the Chesapeake Bay and nearby tributaries such as the James River, causing millions of dollars in damage and numerous beach closures.<ref name="Morse 1006–10252" />
* In 2009, [[Brittany|Brittany, France]] experienced recurring macroalgal blooms caused by the high amount of fertilizer discharging in the sea due to [[intensive pig farming]], causing lethal gas emissions that have led to one case of human unconsciousness and three animal deaths.<ref>{{cite news|last=Chrisafis|first=Angelique|date=10 August 2009|title=Lethal algae take over beaches in northern France|work=The Guardian|location=London|url=https://www.theguardian.com/world/2009/aug/10/france-brittany-coast-seaweed-algae}}</ref>
* In 2010, dissolved iron in the ash from the [[Eyjafjallajökull]] volcano triggered a plankton bloom in the [[North Atlantic]].<ref>{{cite news|date=10 April 2013|title=Iceland volcano ash cloud triggers plankton bloom|newspaper=BBC News|url=https://www.bbc.co.uk/news/science-environment-22045941}}</ref>
* 2011: Northern California<ref>{{cite news| url=http://www.sfgate.com/bayarea/article/Red-tide-killing-abalone-off-California-2309794.php | work=The San Francisco Chronicle | first=Peter | last=Fimrite | date=2011-09-17 | title=Red tide killing abalone off California}}</ref>
* 2011: [[Gulf of Mexico]]<ref>{{cite news| url=http://www.huffingtonpost.com/2011/10/18/texas-gulf-coast-algae-bloom_n_1016915.html | work=Huffington Post | title=Texas Gulf Coast Sees Largest Algae Bloom in Over A Decade | date=2011-10-18}}</ref>
* In 2013, an algal bloom was caused in [[Qingdao]], [[China]], by [[sea lettuce]].<ref>{{cite news|last=Jacobs|first=Andrew|date=5 July 2013|title=Huge Algae Bloom Afflicts Coastal Chinese City|newspaper=The New York Times|url=https://www.nytimes.com/2013/07/06/world/asia/huge-algae-bloom-afflicts-qingdao-china.html?_r=0}}</ref>
* 2013: In January, a red tide occurred again on the [[West Coast Division|West Coast]] [[South China Sea|Sea]] of [[Sabah]] in the [[East Malaysia|Malaysian Borneo]].<ref name="nst.com.my" /><ref name="thestar.com.my">{{cite news|url=http://thestar.com.my/news/story.asp?file=/2013/1/7/nation/12541940&sec=nation |title=Sabah issues red tide alert |access-date=2013-01-07 |author=MUGUNTAN VANAR |date=2013-01-07 |newspaper=[[The Star (Malaysia)|The Star Online]]  |archive-url=https://web.archive.org/web/20130108194051/http://thestar.com.my/news/story.asp?file=%2F2013%2F1%2F7%2Fnation%2F12541940&sec=nation |archive-date=2013-01-08 }}</ref> Two human fatalities were reported after they consumed [[shellfish]] contaminated with the [[Shellfish poisoning|red tide toxin]].<ref name="nst.com.my" /><ref name="dailyexpress.com.my" /><ref name="thestar.com.my" />
* 2013: In January, a red tide bloom appeared at Sarasota beach – mainly Siesta Key, Florida causing a fish kill that had a negative impact on tourists, and caused respiratory issues for beach-goers.<ref>{{cite web|url=http://www.heraldtribune.com/article/20130116/ARTICLE/130119704/2416/NEWS?p=1&tc=pg|title=UPDATE: Red tide, fish kill reported at Sarasota beaches|first=J. David|last=McSwane|access-date=18 October 2016|archive-date=12 October 2015|archive-url=https://web.archive.org/web/20151012181028/http://www.heraldtribune.com/article/20130116/ARTICLE/130119704/2416/NEWS?p=1&tc=pg|url-status=dead}}</ref>
* In 2014, ''[[Myrionecta rubra]]'' (previously known as ''Mesodinium rubrum''), a [[ciliate]] protist that ingests [[cryptomonad]] algae, caused a bloom in southeastern coast of [[Brazil]].<ref>{{cite web|date=2014-01-30|title=A Dark Bloom in the South Atlantic: Image of the Day|url=http://earthobservatory.nasa.gov/IOTD/view.php?id=82968&src=share|access-date=2017-03-18|website=Earthobservatory.nasa.gov}}</ref>
* In 2014, blue green algae caused a bloom in the western basin of [[Lake Erie]], poisoning the [[Toledo, Ohio]] water system connected to 500,000 people.<ref>{{cite news|last=Tanber|first=George|date=2014-08-02|title=Toxin leaves 500,000 in northwest Ohio without drinking water|work=Reuters|url=https://www.reuters.com/article/us-usa-water-ohio-idUSKBN0G20L120140802|url-status=live|access-date=2017-03-18|archive-url=https://web.archive.org/web/20150924203159/http://www.reuters.com/article/2014/08/02/us-usa-water-ohio-idUSKBN0G20L120140802|archive-date=24 September 2015}}</ref>
* 2014: In August, massive 'Florida red tide' {{convert|90|mi}} long and {{convert|60|mi}} wide.<ref name="Florida22">{{cite news | url=http://www.orlandosentinel.com/news/nationworld/la-sci-sn-florida-red-tide-algae-bloom-20140811,0,280402.story | title=Massive 'Florida red tide' is now 90 miles long and 60 miles wide | newspaper=Orlando Sentinel | date=11 August 2014 | access-date=30 September 2015 | author=Netburn, Deborah}}</ref>
* 2015: June, 12 persons hospitalized in the Philippine province of [[Bohol]] for red tide poisoning.<ref>{{cite web|url=http://www.rappler.com/nation/97465-red-tide-poisoning-tagbilaran-bohol|title=12 persons hospitalized in Bohol for red tide poisoning|date=June 25, 2015|author=Israel, Dale G.}}</ref>
* 2015: August, several beaches in the Netherlands between [[Katwijk]] and [[Scheveningen]] were plagued. Government institutions dissuaded swimmers from entering the water.<ref name="Netherlands">{{cite web | url=http://nos.nl/artikel/2050113-rijkswaterstaat-niet-zwemmen-tussen-katwijk-en-scheveningen.html | title=Rijkswaterstaat, do not swim between Katwijk and Scheveningen | publisher=Dutch Public Broadcasting, NOS | date=3 August 2015 | access-date=30 September 2015}}</ref>
* 2015: September, a red tide bloom occurred in the Gulf of Mexico, affecting [[Padre Island National Seashore]] along [[North Padre Island]] and [[South Padre Island]] in Texas.<ref name="Texas">{{cite web|url=https://tpwd.texas.gov/landwater/water/environconcerns/hab/redtide/status.phtml|title=Red Tide in Texas, Current Status|date=15 September 2015|publisher=Texas Parks and Wildlife|access-date=30 September 2015}}</ref>
* 2017 and 2018: ''K. brevis'' red tide algae with warnings not to swim, state of emergency declared, dead [[dolphin]] and [[manatee]], worsened by [[Caloosahatchee River]]. Peaked in the summer of 2018.  Toxic harmful algae bloom red tide in Southwest Florida.<ref>{{Cite news|url=http://www.winknews.com/2018/07/04/algae-red-tide-impacting-swfl-water-quality/|title=Algae, red tide impacting SWFL water quality|date=2018-07-04|work=WINK NEWS|access-date=2018-07-05|language=en-US}}</ref><ref>{{Cite news|url=http://news.wjct.org/post/toxic-algae-blooms-red-tide-and-need-permanent-solution|title=Toxic Algae Blooms, Red Tide, and the Need for a Permanent Solution|last=Glenn|first=Julie|access-date=2018-07-05|language=en}}</ref><ref>{{Cite web|url=http://myfwc.com/research/redtide/research/current/richardson/|archive-url=https://web.archive.org/web/20150419025551/http://myfwc.com/research/redtide/research/current/richardson/|url-status=dead|archive-date=April 19, 2015|title=What forms of nutrients can Karenia brevis use to grow and bloom?|website=myfwc.com|language=en|access-date=2018-08-14}}</ref> A rare harmful algal bloom along Florida's east coast of [[Palm Beach County, Florida|Palm Beach County]] occurred the weekend of September 30, 2018.<ref>{{cite web | url=https://www.miamiherald.com/news/article219331165.html | title=Red tide confirmed off Palm Beach in rare outbreak for Florida's east coast| website=[[Miami Herald]]}}</ref>
* In 2019, blue-green algae, or [[Cyanobacteria]] blooms,<ref>{{Cite web|last=US Department of Commerce|first=NOAA|title=Lake Erie Harmful Algal Bloom|url=https://www.weather.gov/cle/LakeErieHAB|archive-url=https://web.archive.org/web/20190812144803/https://www.weather.gov/cle/LakeErieHAB|archive-date=12 August 2019|access-date=2019-08-22|website=www.weather.gov}}</ref> were again problematic on Lake Erie. In early August 2019, satellite images depicted a bloom stretching up to 1,300 square kilometers, with the epicentre near [[Toledo, Ohio]].<ref name="uwindsor.ca">{{cite news|last=Sacheli|first=Sarah|date=8 August 2019|title=UWindsor researchers test the waters for harmful algae bloom|work=DailyNews|publisher=University of Windsor|url=http://www.uwindsor.ca/dailynews/2019-08-07/uwindsor-researchers-test-waters-harmful-algae-bloom|archive-url=https://web.archive.org/web/20190812145246/http://www.uwindsor.ca/dailynews/2019-08-07/uwindsor-researchers-test-waters-harmful-algae-bloom|archive-date=12 August 2019}}</ref> The largest Lake Erie bloom to date occurred in 2015, exceeding the severity index at 10.5 and in 2011 at a 10.<ref name="Windsor Star - Hill">{{cite news|last=Hill|first=Sharon|date=7 August 2019|title=Large Lake Erie algal bloom nearing Colchester tested for toxicity|work=Windsor Star|url=https://windsorstar.com/news/local-news/large-lake-erie-algal-bloom-nearing-colchester-tested-for-toxicity|access-date=22 August 2019|archive-url=https://web.archive.org/web/20190811140408/https://windsorstar.com/news/local-news/large-lake-erie-algal-bloom-nearing-colchester-tested-for-toxicity|archive-date=11 August 2019}}</ref> A large bloom does not necessarily mean the cyanobacteria ... will produce toxins", said Michael McKay, of the [[University of Windsor]]. Water quality testing was underway in August.<ref name="uwindsor.ca" /><ref name="Windsor Star - Hill" />
* In 2019, a bloom of [[Noctiluca scintillans|''Noctiluca'']] algae caused [[Milky seas effect|bioluminescent glow]] off the coast of Chennai, India. Similar blooms have been reported annually in the northern [[Arabian Sea]] since the early 2000s.<ref>{{Cite news|author=The Hindu Net Desk|date=2019-08-19|title=What caused the blue glow on Chennai beaches?|work=The Hindu|url=https://www.thehindu.com/news/cities/chennai/what-caused-the-blue-glow-on-chennai-beaches/article29134492.ece|access-date=22 August 2019|issn=0971-751X}}</ref>
* 2021: In July, a large red tide occurred on the [[Gulf Coast of the United States|Gulf Coast]] of Florida in and around [[Tampa Bay]]. The event has caused the death of millions of pounds of fish,<ref>{{Cite web|date=2021-07-23|title=Pinellas County already matches 2018 fishkill by cleaning up over 3 million pounds of dead fish|url=https://www.abcactionnews.com/news/region-pinellas/pinellas-county-already-matches-2018-fishkill-by-cleaning-up-over-3-million-pounds-of-dead-fish|access-date=2021-07-27|website=WFTS|language=en}}</ref> and led to the National Weather Service declaring a Beach Hazard.<ref>{{Cite web|title=National Weather Service issues beach hazard statement over red tide concerns|url=https://www.wtsp.com/article/news/red-tide/red-tide-pinellas-county-beach-hazard/67-be019fb0-449f-497f-8aef-09517915fb6d|access-date=2021-07-27|website=wtsp.com|date=23 July 2021 |language=en-US}}</ref>
* 2021: in October, the [[2021 North-East England shellfish die-off|mass deaths of shellfish]] (specifically crabs and lobster) on the beaches of Northern England, led to and algal bloom being blamed as the cause by the UK Government. However, those who work in the fishing industry in the area, and some academics, have stated that pyridine poisoning is the cause.<ref>{{cite news |title=North East coast shellfish deaths blamed on harmful algae |url=https://www.bbc.co.uk/news/uk-england-tees-60248563 |access-date=13 November 2022 |work=BBC News |date=3 February 2022}}</ref><ref>{{cite news |title=Update on investigation into the deaths of crabs and lobster in the North East |url=https://www.gov.uk/government/news/update-on-investigation-into-the-deaths-of-crabs-and-lobster-in-the-north-east |access-date=13 November 2022 |work=GOV.UK |date=3 February 2022 |language=en}}</ref>
* 2023: A blue-green algae bloom occurred in [[Lough Neagh]], [[Northern Ireland]], the largest fresh water lake in the [[UK]] and [[Ireland]] where 40% of Northern Ireland gets its tap water from.  It is caused by [[Northern Ireland]] experiencing both the wettest and hottest summer on record making conditions perfect for blue-green algae. Poor management of the Lough is being blamed. The bloom has killed dogs and wildlife, including swans.<ref>{{cite news |title=Lough Neagh: What does future hold for UK's largest freshwater lake? |url=https://www.bbc.co.uk/news/uk-northern-ireland-66751451|access-date=13 September 2023 |work=BBC News |date=10 September 2023}}</ref>

=== United States ===
In July 2016 [[Florida]] declared a [[state of emergency]] for four counties as a result of blooms. They were said to be "destroying" a number of businesses and affecting local economies, with many needing to shut down entirely.<ref>[http://www.natureworldnews.com/articles/25512/20160719/look-florida-toxic-algae-choking-economy-environment.htm "How Florida's Toxic Algae is Choking the Economy And The Environment"], ''Nature World News'', July 19, 2016</ref> Some beaches were closed, and hotels and restaurants suffered a drop in business. Tourist sporting activities such as fishing and boating were also affected.<ref>[https://www.nbcnews.com/business/consumer/florida-tourism-not-seeing-green-toxic-algae-chokes-business-n607106 "Florida Tourism Not Seeing Green as Toxic Algae Chokes Business"], ''NBC News'', July 11, 2016</ref><ref name="Florida2">[https://www.cnbc.com/2016/07/05/toxic-algae-driving-away-florida-beachgoers.html "Toxic algae driving away Florida beachgoers"], ''CNBC'', July 5, 2016</ref>

In 2019, the biggest [[Sargassum]] bloom ever seen created a crisis in the [[Tourism industry]] in [[North America]]. This event was likely caused by climate change and [[nutrient pollution]] from [[fertilizers]].<ref>{{Cite journal |last=Wang |first=Mengqiu |date=July 2019 |title=The great Atlantic Sargassum belt |journal=Science |volume=365 |issue=6448 |pages=83–87 |bibcode=2019Sci...365...83W |doi=10.1126/science.aaw7912 |pmid=31273122 |s2cid=195804245|doi-access=free }}</ref> Several Caribbean countries considered declaring a state of emergency due to the impact on tourism as a result of environmental damage and potentially toxic and harmful health effects.<ref>{{Cite news |last=Taylor |first=Alexandra |date=September 2019 |title=Sargassum is strangling tourism in the Caribbean. Can scientists find a use for it? |work=Chemical & Engineering News |url=https://cen.acs.org/environment/sustainability/Sargassum-strangling-tourism-Caribbean-scientists/97/i34 |access-date=April 21, 2021}}</ref>

==== On the U.S. coasts ====
The [[Gulf of Maine]] frequently experiences blooms of the [[dinoflagellate]] ''[[Alexandrium fundyense]]'', an organism that produces [[saxitoxin]], the neurotoxin responsible for [[paralytic shellfish poisoning]]. The well-known "Florida red tide" that occurs in the Gulf of Mexico is a HAB caused by ''[[Karenia brevis]]'', another dinoflagellate which produces brevetoxin, the neurotoxin responsible for [[neurotoxic shellfish poisoning]]. California coastal waters also experience seasonal blooms of ''[[Pseudo-nitzschia]]'', a [[diatom]] known to produce [[domoic acid]], the neurotoxin responsible for [[amnesic shellfish poisoning]]. [[File:Red tide＿2017.jpg|thumb|upright=1.2| Marine harmful algal bloom in a harbor, [[Japan]]]]
The term red tide is most often used in the US to refer to ''[[Karenia brevis]]'' blooms in the eastern [[Gulf of Mexico]], also called the Florida red tide. ''K. brevis'' is one of many different species of the genus ''Karenia'' found in the world's oceans.
<ref name="Fleming, L. E. 2011">{{cite journal |last1=Fleming |first1=L.E. |last2=Kirkpatrick |first2=B. |last3=Backer |first3=L.C. |last4=Walsh |first4=C.J. |last5=Nierenberg |first5=K. |last6=Clark |first6=J. |display-authors=etal |date=2011 |title=Review of Florida red tide and human health effects |journal=Harmful Algae |volume=10 |issue=2 |pages=224–233 |doi=10.1016/j.hal.2010.08.006 |pmc=3014608 |pmid=21218152|bibcode=2011HAlga..10..224F }}</ref>

Major advances have occurred  in the study of dinoflagellates and their genomics. Some include identification of the toxin-producing genes ([[Polyketide synthase|''PKS'']] genes), exploration of environmental changes (temperature, light/dark, etc.) have on gene expression, as well as an appreciation of the complexity of the ''Karenia'' genome.<ref name="Fleming, L. E. 2011" /> These blooms have been documented since the 1800s, and occur almost annually along Florida's coasts.<ref name="Fleming, L. E. 2011" />

There was increased research activity of harmful algae blooms (HABs) in the 1980s and 1990s. This was primarily driven by media attention from the discovery of new HAB organisms and the potential adverse health effects of their exposure to animals and humans.<ref>Abraham and Baden, 2006; Backer et al., 2003a, 2005a; Backer and Fleming, 2008; Fleming et al., 2001; Fleming et al., 2004; Okamoto and Fleming, 2005; Twiner et al., 2008; Zaias et al., 2010.</ref>{{Full citation needed|date=August 2018}} The Florida red tides have been observed to have spread as far as the eastern coast of Mexico.<ref name="Fleming, L. E. 2011" /> The density of these organisms during a bloom can exceed tens of millions of cells per litre of seawater, and often discolor the water a deep reddish-brown hue.

Red tide is also sometimes used to describe harmful algal blooms on the northeast coast of the United States, particularly in the [[Gulf of Maine]]. This type of bloom is caused by another species of dinoflagellate known as ''[[Alexandrium fundyense]]''. These blooms of organisms cause severe disruptions in [[fishery|fisheries]] of these waters, as the toxins in these organism cause filter-feeding shellfish in affected waters to become poisonous for human consumption due to saxitoxin.<ref>{{cite web |author=<!--Not stated--> |date=2015 |title=Red Tide (Paralytic Shellfish Poisoning) |url=https://www.mass.gov/files/documents/2017/10/13/red-tide-fact-sheet-english.pdf |publisher=Massachusetts Department of Public Health |location=Boston, MA}}</ref>

The related  ''[[Alexandrium monilatum]]'' is found in subtropical or tropical shallow seas and [[Estuary|estuaries]] in the western [[Atlantic Ocean]], the [[Caribbean Sea]], the Gulf of Mexico, and the eastern [[Pacific Ocean]].

==== Texas ====
Natural water reservoirs in [[Texas]] have been threatened by anthropogenic activities due to large petroleum refineries and oil wells (i.e. emission and wastewater discharge), massive agricultural activities (i.e. pesticide release) and mining extractions (i.e. toxic wastewater) as well as natural phenomena involving frequent HAB events. For the first time in 1985, the state of Texas documented the presence of the ''P. parvum'' (golden alga) bloom along the [[Pecos River]]. This phenomenon has affected 33 reservoirs in Texas along major river systems, including the Brazos, Canadian, Rio Grande, Colorado, and Red River, and has resulted in the death of more than 27 million fish and caused tens of millions of dollars in damage.<ref>{{Cite journal|last1=Sharifan|first1=Hamidreza|last2=Ma|first2=Xingmao|date=2017-08-31|title=Potential Photochemical Interactions of UV Filter Molecules with Multichlorinated Structure of Prymnesins in Harmful Algal Bloom Events|journal=Mini-Reviews in Organic Chemistry|volume=14|issue=5|doi=10.2174/1570193x14666170518124658}}</ref>

====Chesapeake Bay====
[[File:July 12, 2013 - Sassafras River, VA Algal Bloom (9321804011).jpg|thumb|An algal bloom on the [[Sassafras River]], a tributary of the Chesapeake Bay]]
The [[Chesapeake Bay]], the largest [[estuary]] in the U.S., has suffered from repeated large algal blooms for decades due to chemical runoff from multiple sources,<ref>[http://www.baltimoresun.com/health/bs-hs-algae-blooms-20150508-story.html "Scientists eye increase in harmful algae in Chesapeake"], ''The Baltimore Sun'', May 8, 2015</ref> including 9 large rivers and 141 smaller streams and creeks in parts of six states. In addition, the water is quite shallow and only 1% of the waste entering it gets flushed into the ocean.<ref name="Miller" />

By weight, 60% of the phosphates entering the bay in 2003 were from sewage treatment plants, while 60% of its nitrates came from fertilizer runoff, farm animal waste, and the atmosphere.<ref name="Miller" /> About 300 million pounds (140 Gg) of nitrates are added to the bay each year.<ref>[http://www.cbf.org/about-the-bay/issues/dead-zones/nitrogen-phosphorus "Too Much Nitrogen and Phosphorus Are Bad for the Bay"] {{webarchive|url=https://web.archive.org/web/20160728141032/http://www.cbf.org/about-the-bay/issues/dead-zones/nitrogen-phosphorus|date=2016-07-28}}, Chesapeake Bay Foundation, 2016</ref> The population increase in the bay [[drainage basin|watershed]], from 3.7 million people in 1940 to 18 million in 2015 is also a major factor,<ref name="Miller" /> as economic growth leads to the increased use of fertilizers and rising emissions of industrial waste.<ref>[http://www.scientificamerican.com/article/massive-nitrogen-pollution-accompanies-chinas-growth/ "Massive Nitrogen Pollution Accompanies China's Growth"], ''Scientific American'', Feb. 27, 2013</ref><ref>[http://e360.yale.edu/feature/on_lake_taihu_china_moves_to_battle_massive_algae_blooms/2429/ "On Lake Taihu, China Moves To Battle Massive Algae Blooms"], ''Environment 360'', Yale University, July 21, 2011</ref>

As of 2015, the six states and the local governments in the Chesapeake watershed have upgraded their sewage treatment plants to control nutrient discharges. The [[U.S. Environmental Protection Agency]] (EPA) estimates that sewage treatment plant improvements in the Chesapeake region between 1985 and 2015 have prevented the discharge of 900 million pounds (410 Gg) of nutrients, with nitrogen discharges reduced by 57% and phosphorus by 75%.<ref name="EPA-Chesapeake Pollution">{{cite web|author=<!--Not stated-->|date=2016-07-27|title=Wastewater Pollution Reduction in the Chesapeake Bay Watershed|url=https://www.epa.gov/chesapeake-bay-tmdl/wastewater-pollution-reduction-chesapeake-bay-watershed|publisher=EPA}}</ref> Agricultural and urban runoff pollution continue to be major sources of nutrients in the bay, and efforts to manage those problems are continuing throughout the {{convert|64000|sqmi|km2}} watershed.<ref>{{cite web|author=<!--Not stated-->|date=2016-09-29|title=Chesapeake Bay TMDL Fact Sheet|url=https://www.epa.gov/chesapeake-bay-tmdl/chesapeake-bay-tmdl-fact-sheet|publisher=EPA}}</ref>

====Lake Erie====
Recent algae blooms in Lake Erie have been fed primarily by agricultural runoff and have led to warnings for some people in Canada and Ohio not to drink their water.<ref>{{Cite web|date=2014-08-05|title=Toledo water crisis must spark serious environmental reforms, just as the burning Cuyahoga did, experts say|url=https://www.cleveland.com/open/index.ssf/2014/08/toledo_water_crisis_must_spark.html}}</ref><ref>{{Cite web|date=2014-10-04|title=Toxic Lake Erie algal blooms "reversible" experts say... But only if we stop the blame game and work together|url=https://www.mlive.com/environment/index.ssf/2014/10/toxic_lake_erie_algal_blooms_r.html}}</ref> The [[International Joint Commission]] has called on United States and Canada to drastically reduce phosphorus loads into Lake Erie to address the threat.<ref>{{cite report |author1=International Joint Commission |author2=Lake Erie Ecosystem Priority (LEEP) |author1-link=International Joint Commission |title=A balanced diet for Lake Erie: Reducing phosphorus loadings and harmful algal blooms |date=2014 |location=Washington, District of Columbia |isbn=978-1-927336-07-6 |url=https://legacyfiles.ijc.org/publications/2014%20IJC%20LEEP%20REPORT.pdf |archive-url=https://web.archive.org/web/20180810045000/http://www.ijc.org/files/publications/2014%20IJC%20LEEP%20REPORT.pdf |archive-date=August 10, 2018}}</ref><ref>{{Cite web |author1=International Joint Commission |url=http://www.ijc.org/en_/leep/report |title=Introducing the Report of the Lake Erie Ecosystem Priority (LEEP): highlights |date=2014 |access-date=2018-08-08 |archive-date=2018-08-06 |archive-url=https://web.archive.org/web/20180806151255/http://www.ijc.org/en_/leep/report |website=ijc.com }}</ref><ref>{{Cite web|date=March 2014|title=Joint U.S.-Canada Agency Calls for Big Phosphorus Reductions in Lake Erie|url=https://www.circleofblue.org/2014/world/joint-u-s-canada-agency-calls-big-phosphorus-reductions-lake-erie/}}</ref>

====Green Bay====
Green Bay has a dead zone caused by phosphorus pollution that appears to be getting worse.<ref>{{cite news |author1=((Associated Press)) |title=Scientists: 'Dead zone' showing up in waters of Green Bay |url=https://www.twincities.com/2013/08/15/scientists-dead-zone-showing-up-in-waters-of-green-bay/ |work=Twin Cities Pioneer Press |date=15 August 2013 |archive-url=https://web.archive.org/web/20200818085924/https://www.twincities.com/2013/08/15/scientists-dead-zone-showing-up-in-waters-of-green-bay/ |archive-date=August 18, 2020 |id=(UPDATED: November 7, 2015)}}</ref>

==== Okeechobee Waterway ====
[[File:Cropped lake okeechobee oli 2016184 lrg.jpg|thumb|Harmful algal bloom ([[cyanobacteria]]) on [[Lake Okeechobee]] in 2016 ]]
[[Lake Okeechobee]] is an ideal habitat for cyanobacteria because its shallow, sunny, and laden with nutrients from Florida's agriculture.<ref>{{Cite journal|last1=Havens|first1=Karl|last2=Li|first2=Bai-Lian|last3=Philips|first3=Edward|date=May 1998|title=Light availability as a possible regulator of cyanobacteria species composition in a shallow subtropical lake|url=https://www.researchgate.net/publication/249440655|journal=Freshwater Biology|volume=39|issue=3|pages=547–556|doi=10.1046/j.1365-2427.1998.00308.x|bibcode=1998FrBio..39..547H |via=ResearchGate}}</ref> The [[Okeechobee Waterway]] connects the lake to the [[Atlantic Ocean]] and the [[Gulf of Mexico]] through the [[St. Lucie River]] and the [[Caloosahatchee River|Caloosahatchee]] respectively. This means that harmful algal blooms are carried down the estuaries as water is released during the wet summer months. In July 2018 up to 90% of Lake Okeechobee was covered in algae.<ref>{{Cite web |last=Carlowicz |first=Michael |date=2022-07-15 |title=Blooming Lake Okeechobee |url=https://earthobservatory.nasa.gov/images/150093/blooming-lake-okeechobee |access-date=2024-10-24 |website=earthobservatory.nasa.gov |language=en}}</ref><ref>{{Cite news|last=Gomez|first=Melissa|date=9 July 2018|title=Algae bloom in florida prompts fears about harm to health and economy|work=The New York Times|url=https://www.nytimes.com/2018/07/09/us/algae-blooms-florida-nyt.html|access-date=30 August 2018}}</ref> Water draining from the lake filled the region with a noxious odor and caused respiratory problems in some humans during the following month.<ref>{{Cite web|last=Di Liberto|first=Tom|date=16 August 2018|title=Harmful algal blooms linger in parts of southern Florida in July and August 2018|url=https://www.climate.gov/news-features/event-tracker/harmful-algal-blooms-linger-parts-southern-florida-july-and-august-2018|archive-url=https://web.archive.org/web/20180816172344/https://www.climate.gov/news-features/event-tracker/harmful-algal-blooms-linger-parts-southern-florida-july-and-august-2018|url-status=dead|archive-date=August 16, 2018|access-date=30 August 2018|website=NOAA}}</ref> To make matters worse, harmful red tide blooms are historically common on Florida's coasts during these same summer months.<ref name=":1">{{Cite journal|last=Alcock|first=Frank|date=August 2007|title=An assessment of Florida red tide: causes, consequences, and management strategies.|url=https://dspace.mote.org/bitstream/2075/2911/1/Alcock-MPI_RedTide.pdf|journal=Mote Marine Technical Report|volume=1190|via=Mote Marine Laboratory}}</ref> Cyanobacteria in the rivers die as they reach saltwater but their nitrogen fixation feeds the red tide on the coast.<ref name=":1" /> Areas at the mouth of the estuaries such as [[Cape Coral, Florida|Cape Coral]] and [[Port St. Lucie, Florida|Port St. Lucie]] therefore experience the compounded effects of both types of harmful algal bloom. Cleanup crews hired by authorities in [[Lee County, Florida|Lee County]] - where the Caloosahatchee meets the Gulf of Mexico - removed more than 1700 tons of dead marine life in August 2018.<ref>{{Cite news|last=Murphy|first=Paul|date=23 August 2018|title=Florida's red tide has produced 2,000 tons of dead marine life and cost businesses more than $8 million|work=CNN|url=https://www.cnn.com/2018/08/22/us/red-tide-fishkill-costs-trnd/index.html|access-date=30 August 2018}}</ref>

=== Baltic Sea ===
In 2020, a large harmful algal bloom closed beaches in Poland and Finland, brought on by a combination of fertilizer runoff and extreme heat, posing a risk to flounder and mussel beds.<ref>{{cite news|date=August 2018|title=Toxic algae spreads in Baltic waters in biggest bloom in years|newspaper=Reuters|url=https://www.reuters.com/article/us-environment-baltic/toxic-algae-spreads-in-baltic-waters-in-biggest-bloom-in-years-idUSKBN1KM5RX|access-date=November 20, 2021}}</ref><ref>{{cite web|date=4 January 2020|title=Lethal algae blooms – an ecosystem out of balance|url=https://www.theguardian.com/environment/2020/jan/04/lethal-algae-blooms-an-ecosystem-out-of-balance|access-date=November 20, 2021|website=[[TheGuardian.com]]}}</ref> This is seen by the [[Baltic Sea Action Group]] as a threat to biodiversity and regional fishing stocks.<ref>{{cite web|title=EUTROPHICATION IN THE BALTIC SEA|url=https://www.bsag.fi/en/baltic-sea/eutrophication/|access-date=November 20, 2021}}</ref>

=== Coastal seas of Bangladesh, India, and Pakistan ===
[[Open defecation]] is common in south Asia, but human waste is an often overlooked source of nutrient pollution in marine pollution modeling. When nitrogen (N) and phosphorus (P) contributed by human waste was included in models for Bangladesh, India, and Pakistan, the estimated N and P inputs to bodies of water increased one to two orders of magnitude compared to previous models.<ref name=":0" /> River export of nutrients to coastal seas increases coastal [[eutrophication]] potential (ICEP). The ICEP of the [[Godavari River]] is three times higher when N and P inputs from human waste are included.

==See also==
* [[Brevetoxin]]
* [[Ciguatera]]
* [[Cyanobacterial bloom]]
* [[Cyanotoxin]]
* [[GEOHAB]] - an international research programme on the Global Ecology and Oceanography of Harmful algal blooms
* [[Milky seas effect]] – A phenomenon in which disturbed red algae dinoflagellates will make the water glow blue, at night
* ''[[Pfiesteria]]''
* [[Thin layers (oceanography)]]
* [[Water quality]]
* [[Water security]]

==References==
{{Reflist|colwidth=30em}}

==External links==
* [https://issha.org/ International Society for the Study of Harmful Algae (ISSHA)]
* [http://oceanservice.noaa.gov/hazards/hab/ FAQ about Harmful Algal Blooms] (NOAA)
* [http://habsos.noaa.gov/ Harmful Algal Blooms Observing System] (NOAA/HAB-OFS)
* [http://www.geohab.info GEOHAB: The International IOC-SCOR Research Programme on the Global Ecology and Oceanography of Harmful Algal Blooms]

{{Authority control}}

[[Category:Biological oceanography]]
[[Category:Aquatic ecology]]
[[Category:Fishing industry]]
[[Category:Water quality indicators]]
[[Category:Human impact on the environment]]
[[Category:Agriculture and the environment]]
[[Category:Climate change and the environment]]
[[Category:Water pollution]]
[[Category:Algal blooms]]
[[Category:Red tide]]
[[Category:Dinoflagellate biology]]
[[Category:Fisheries science]]