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. When oxygen continues to be depleted by blooms it can lead to
hypoxic dead zones, where neither fish nor plants are able to survive. Scientists have found that HABs were a prominent feature of previous mass
extinction events, including the
End-Permian Extinction.
Human health Tests have shown some toxins near blooms can be in the air and thereby be inhaled, which could affect health. 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. 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.
One Health Harmful Algal Bloom System (OHHABS) The One Health Harmful Algal Bloom System (OHHABS), launched by the U.S. Centers for Disease Control and Prevention (CDC) in 2016, is the only national public health surveillance system in the United States that systematically collects data on harmful algal blooms (HABs) and the human and animal illnesses they cause. State and territorial health departments, often in collaboration with environmental and animal health partners, voluntarily report HAB events in freshwater, marine, or brackish waters, along with associated illnesses in people, pets, livestock, and wildlife. Reports can also include blooms without identified illnesses or foodborne cases linked to HAB toxins (e.g., contaminated seafood). HABs represent a quintessential One Health challenge because they simultaneously affect human health (through toxin exposure via water, air, or food), animal health (mass die-offs in fish, birds, marine mammals, and pets), and the shared environment (eutrophication, dead zones, and climate-amplified blooms). Prior to OHHABS, surveillance was fragmented across disciplines, limiting the ability to detect patterns or respond effectively. The system addresses this gap by integrating data across sectors, enabling a more complete picture of risks and supporting prevention as blooms increase globally due to nutrient pollution and climate change. By embedding One Health principles into routine surveillance, OHHABS demonstrates how integrated data collection can translate into practical prevention, protecting human and animal health while addressing the environmental drivers of HABs.
Food Eating fish or shellfish from lakes with a bloom nearby is not recommended. In 1987, a new illness emerged:
amnesic shellfish poisoning (ASP). People who had eaten
mussels 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. A 2013 study found that toxic
paralytic shellfish poisoning in the
Philippines during HABs has caused at least 120 deaths over a few decades. 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. In 2015, most shellfish fisheries in Washington, Oregon and California were shut down because of high concentrations of toxic domoic acid in shellfish. 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.
Drinking water during a harmful 2011 algal bloom 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. 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. In some locations visitors have been warned not to even touch the water. After a dog died in 2015 from swimming in a bloom in California's
Russian River, officials likewise posted warnings for parts of the river. Boiling the water at home before drinking does not remove the toxins. 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 and was forecast again for the summer of 2016. 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. 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. A smaller water shut-down in China affected 15,000 residents two years later at a different location. Australia in 2016 also had to cut off water to farmers. 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. 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.
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. Less than one percent of algal blooms produce hazardous toxins, such as
microcystins. The brevetoxins bind to
voltage-gated sodium channels, 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. 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. Over time, the PbTx-2 brevetoxin can be converted to PbTx-3 through metabolic changes. Reports of skin irritation after swimming in the ocean during a HAB are common. 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. People with severe or persistent respiratory conditions (such as chronic lung disease or asthma) may experience stronger adverse reactions. 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.
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, California, Vermont, and Utah. 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. The economic damage resulting from lost business has become a serious concern. According to a 2016 report, 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. 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. In 2013, another bloom in China, thought to be its largest ever, covered an area of 7,500 square miles,
Fisheries industry In 1976, a short-term, relatively small, dead zone off the coasts of
New York and
New Jersey cost commercial and recreational fisheries over $500 million. In 1998, a HAB in Hong Kong killed over $10 million in high-value fish. In 2009, the economic impact for the state of
Washington's coastal counties dependent on its fishing industry was estimated to be $22 million. In 2016, the U.S. seafood industry expected future lost revenue could amount to $900 million annually. $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. 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. 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...." The bloom covered a range from
Santa Barbara, California northward to
Alaska. 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, and in 2016 a farm in Chile lost 23 million salmon after
an algal bloom.
Environmental impact Dead zones The presence of harmful algal blooms can lead to
hypoxia or
anoxia in a body of water. The depletion of oxygen within a body of water can lead to the creation of a
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. 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. In the 1960s the number of dead zones worldwide was 49; the number rose to over 400 by 2008. In 2016, 23 million salmon which were being farmed in Chile died from a toxic algae bloom. 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. 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. 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. Similar fish die-offs from toxic algae or lack of oxygen have been seen in Russia, Colombia, Vietnam, China, Canada, Turkey, Indonesia, and France.
Land animal deaths Land animals, including livestock and pets have been affected. Dogs have died from the toxins after swimming in algal blooms. 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." In 50 countries and 27 states that year there were reports of human and animal illnesses linked to algal toxins. Marine mammals have been seriously harmed, as over 50 percent of unusual marine mammal deaths are caused by harmful algal blooms. In 1999, over 65
bottlenose dolphins died during a coastal HAB in Florida. In 2013 a HAB in southwest Florida killed a record number of
Manatee. 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. In 2003, off
Cape Cod in the North Atlantic, at least 12 humpback whales died from toxic algae from a HAB. 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. Birds have died after eating dead fish contaminated with toxic algae. Rotting and decaying fish are eaten by birds such as
pelicans,
seagulls,
cormorants, and possibly marine or land mammals, which then become poisoned. As dying or dead birds washed up on the shore, wildlife agencies went into "an emergency crisis mode." such as the dozens of
cetacean skeletons found at
Cerro Ballena.
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. 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. 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. 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 • 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
anoxia) from cellular respiration and bacterial degradation
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 channels and ingestion can cause
paralysis within 30 minutes. 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. Other animals that eat exposed shellfish are susceptible to the neurotoxin, which may lead to neurotoxic shellfish poisoning and sometimes even death. Most mollusks and clams filter feed, which results in higher concentrations of the toxin than just drinking the water.
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. The toxins released by the blooms can kill marine animals including
dolphins, sea turtles, birds, and
manatees. 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 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. In addition to causing manatee mortalities, red-tide exposure also causes severe 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. 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. HABs have been shown to have a negative effect also in the memory functions of sea lions. == Potential remedies ==