A harmful algal bloom (HAB), also known as excessive algae growth, is a type of algal bloom that harms other living things. These blooms can produce natural toxins, reduce oxygen in water, cause physical damage, or harm organisms in other ways. Some definitions say HABs are only those that make toxins, while others include any bloom that lowers oxygen levels so much it harms marine or freshwater life. Blooms can last from a few days to many months. When the bloom ends, microbes that break down the algae use more oxygen, creating "dead zones" where fish and plants may die. If these zones are large and last a long time, neither fish nor plants can survive.

Sometimes it is unclear what causes specific HABs. In some areas, they occur naturally, while in others, human activities may be the cause. In some places, HABs are linked to nutrients like nitrogen (nitrates, ammonia, and urea) and phosphate. These nutrients come from agriculture, industrial pollution, fertilizers used in cities, and runoff from urban areas. Higher water temperatures and poor water movement also play a role.

HABs can harm animals, the environment, and human economies. They have grown larger and more frequent worldwide, and many experts believe this is due to climate change. The U.S. National Oceanic and Atmospheric Administration (NOAA) says more harmful blooms may occur in the Pacific Ocean. Ways to address HABs include using chemicals, building more reservoirs, using sensors to monitor blooms, reducing nutrient runoff, and improving research and management.

Nutrients from land, such as fertilizer, sewage, and animal waste, flow into seawater and cause blooms. Natural events, like river floods or the rising of nutrients from the ocean floor after big storms, also trigger blooms. Coastal development and aquaculture (raising fish or shellfish) can increase the chance of HABs near the coast. Effects of HABs may worsen in certain areas due to wind patterns that mix water and the biological impacts of the blooms.

Description and identification

Harmful algal blooms (HABs) from cyanobacteria, also called blue-green algae, can form as a foam, scum, or mat on or just below the surface of water. These blooms can appear in different colors based on their pigments. In freshwater lakes or rivers, cyanobacteria blooms often look bright green, sometimes with surface streaks that resemble floating paint. Cyanobacterial blooms are a problem found worldwide.

Most blooms happen in warm water with too many nutrients. The dangers from these blooms come from the toxins they produce or from using up oxygen in the water, which can cause fish to die. However, not all algal blooms create toxins. Some only change the water's color, make it smell bad, or give it a bad taste. It is not possible to know if a bloom is harmful just by looking at it. Testing through sampling and examining the algae under a microscope is needed. In many cases, looking at the algae under a microscope is not enough to tell if they are toxic or not. In such situations, tools can be used to measure toxin levels or check if the genes that produce toxins are present.

Terminology

Harmful algal blooms are sometimes defined as algal growths that produce toxins harmful to other living things. However, any algal bloom can lead to areas with very low oxygen levels, which can kill marine life. This means such blooms might be called "harmful" even without producing toxins. The term "harmful algal blooms" is used in different ways in media and scientific writing. A broader definition includes any organisms or events that harm human health, affect the economy, or damage aquatic ecosystems. This concept is more about societal concerns than a strict scientific definition.

In 2008, the U.S. Environmental Protection Agency described harmful algal blooms as including species that may or may not produce toxins. These blooms can cause low oxygen levels, harm marine life, or lead to large numbers of deaths in aquatic animals, even if toxins are not involved.

In coastal areas, harmful algal blooms are sometimes called "red tides." This name comes from certain types of dinoflagellates, like Karenia brevis, which can turn water red. However, this term is misleading because algal blooms can be many colors, and their growth is not connected to tides. Not all red tides are caused by dinoflagellates. For example, the mixotrophic ciliate Mesodinium rubrum creates non-toxic red-colored blooms by using chloroplasts from the algae it consumes.

The term "red tide" is being replaced with more precise language. "Harmful algal bloom" is now used for harmful species, while "algal bloom" refers to non-harmful ones.

Types

There are three main types of phytoplankton that can form harmful algal blooms: cyanobacteria, dinoflagellates, and diatoms. All three are made up of tiny, floating organisms that, like plants, can make their own food from sunlight through a process called photosynthesis. This ability makes most of them an important part of the food web for small fish and other living things.

Harmful algal blooms in freshwater lakes, rivers, and estuaries (areas where rivers meet the ocean) are often caused by cyanobacteria, which are sometimes called "blue-green algae." However, cyanobacteria are actually prokaryotic bacteria, not algae, which are eukaryotes. Some cyanobacteria, like the common type Microcystis, can produce harmful toxins called microcystins, which damage the livers of mammals. Other cyanobacteria can also create toxins that harm the nervous system, cells, and internal organs. Water treatment plants may not always remove these toxins, leading to warnings about drinking tap water, as happened in Toledo, Ohio, in August 2014.

In August 2021, 47 lakes in New York State were confirmed to have algal blooms. In September 2021, Spokane County in Washington issued a harmful algal bloom alert for Newman Lake after tests found dangerous levels of cyanobacteria toxins. At the same time, record-high levels of microcystins were found at Clear Lake in California, leading to a "Do Not Drink" advisory for 280 households. In Florida, water conditions are worsening due to increased nutrient levels, causing severe harmful algal blooms in both freshwater and ocean areas.

Harmful algal blooms can harm ecosystems by blocking sunlight needed for plants and algae to grow, or by reducing the oxygen available for fish and other aquatic animals. When oxygen levels drop for long periods, areas may become hypoxic (low oxygen) or anoxic (no oxygen), which are called "dead zones." These zones can form in both freshwater and saltwater environments due to natural or human causes.

Many harmful algal species have a two-stage life cycle. They alternate between a resting stage near the ocean floor and an active, growing stage near the water's surface. In the resting stage, the cells wait for the right conditions to move upward. When they transition to the active stage, they grow and multiply, leading to algal blooms. This change between stages can stop a bloom quickly. These species also move rapidly between different water layers, requiring a lot of energy to pass through temperature, salinity, and density changes in the water.

Other harmful algae include dinoflagellates and diatoms, which are mostly found in marine environments like oceans and bays. These blooms are natural but can be made worse by human activities such as pollution or climate change. They often grow when water temperatures, salinity, and nutrient levels reach certain points. Most harmful algae in the ocean are dinoflagellates. They can be seen in water at concentrations of 1,000 cells per milliliter, and in dense blooms, their numbers can reach over 200,000 per milliliter.

Diatoms produce domoic acid, a nerve toxin that can cause seizures in animals and humans. This toxin builds up in shellfish, sardines, and anchovies. If these animals are eaten by sea lions, otters, whales, birds, or people, it can harm the nervous system and cause serious illness or death. In 2015, governments in Washington, Oregon, and California closed shellfish fisheries due to high levels of domoic acid in shellfish.

In the ocean, tiny, plant-like organisms called phytoplankton or microalgae live in the sunlit surface layer of water. These organisms form the base of the marine food web, supporting nearly all other sea life. Of the more than 5,000 species of marine phytoplankton worldwide, about 2% are harmful or toxic. Harmful algal blooms can affect marine ecosystems in many ways, depending on the type of algae, the environment, and how they harm other life.

Some common types of diatoms and dinoflagellates that cause harmful algal blooms in the ocean include:

Causes

It is sometimes difficult to know what causes certain harmful algal blooms (HABs). In some places, HABs happen naturally, such as when ocean currents bring nutrients to the surface through a process called coastal upwelling. In other areas, HABs are linked to human activities, like pollution from farms or cities. Many types of algae can form HABs, and each type needs different conditions to grow best. In some parts of the world, HABs have become more common and severe because of extra nutrients from human activities. In other areas, HABs happen regularly during certain seasons due to natural ocean processes.

Marine phytoplankton, both toxic and non-toxic, usually grow slowly when there are not enough nitrates and phosphates. These nutrients are often found in areas where ocean currents bring them to the surface (like coastal upwelling) or in runoff from farms. The type of nitrates and phosphates available also matters because phytoplankton grow at different rates depending on which nutrients are more plentiful.

Other nutrients, like iron, silica, or carbon, can also affect algal blooms. Human activities, such as pollution from factories or the use of iron to boost phytoplankton growth, and rising ocean temperatures may contribute to HABs.

Common causes of algal blooms include:
– Too many nutrients, like phosphorus and nitrates, from fertilizers or sewage in water (called nutrient pollution).
– Heat from power plants and factories (thermal pollution).
– Lower water levels in lakes or rivers, which can raise water temperatures.
– Invasive filter feeders, like zebra mussels, that eat non-toxic algae and allow harmful algae to grow.

Nutrients enter water systems through runoff from farms, lawns, and golf courses, as well as from sewage treatment plants that do not control nutrients well. Nutrients can also come from pollution in the air. Coastal areas, such as wetlands, estuaries, coral reefs, and swamps, are especially vulnerable to too many nutrients. For example, many cities along the Mediterranean Sea discharge untreated sewage into the ocean. In some developing countries, up to 70% of wastewater from large cities may not be treated before entering water systems.

Even treated wastewater can add nutrients to water, which can lead to eutrophication. Eutrophication causes water systems to become dominated by cyanobacteria, which can create seasonal HABs. As more wastewater treatment plants are built, more treated wastewater is returned to nature, increasing the amount of nutrients in water.

These extra nutrients combine with other sources to build up nutrient levels in water, which can lead to long-term changes in water quality. This harms ecosystems, including dams, lakes, rivers, and reservoirs, which are now called "ecological infrastructure." These changes also increase the pressure on wastewater treatment and water purification systems, which can worsen seasonal HABs.

Iron fertilization is when iron-containing compounds, like iron sulfate, are added to parts of the ocean that lack iron. This is done to help phytoplankton grow, which can increase ocean productivity and reduce carbon dioxide in the atmosphere. Iron is a key nutrient for photosynthesis in plants like phytoplankton, but it is rarely found in ocean water. Adding iron to areas with little iron can cause large algal blooms.

Iron is a trace element in the ocean and is essential for plant life, including phytoplankton. When iron is added to areas where it is missing, it can help phytoplankton grow.

Climate change is making ocean and lake waters warmer, which can help algae grow in more places, including areas farther north. Warm, shallow water combined with high nutrient levels in lakes or rivers increases the risk of HABs. Summer lake temperatures have risen by about 0.34°C every decade since 1985, and this warming may increase HABs by 20% in the next 100 years.

Although the exact causes of HABs are not fully understood, they have become more common and widespread since the 1980s. This is partly because of human activities, like adding too many nutrients to water and climate change, especially rising water temperatures. Factors that affect HABs include ocean warming, marine heatwaves, oxygen loss, eutrophication, and water pollution.

HABs are dense groups of algae that make water look discolored, often reddish-brown. They are a natural event, but the exact reasons for a specific HAB are not always clear. Three natural factors—salinity, temperature, and wind—are thought to play a major role in algal blooms. HABs can cause economic harm, so scientists monitor them closely. For example, the Florida Fish and Wildlife Conservation Commission and the Texas Parks and Wildlife Department provide current information about HABs in their regions.

In some places, HABs occur naturally, such as when coastal upwelling brings nutrients to the surface. In other areas, HABs are caused by human activities, like adding too many nutrients to water.

Marine phytoplankton growth is often limited by the availability of nitrates and phosphates, which are found in agricultural runoff and coastal upwelling zones. Other factors, like iron-rich dust from deserts such as the Sahara, may also contribute to HABs. Some algal blooms along the Pacific Coast are linked to natural climate events like El Niño. HABs can also be influenced by heavy rainfall. Historical records show that HABs in the Gulf of Mexico were observed as early as the 1500s by explorer Cabeza de Vaca.

Number and sizes

The number of reported harmful algal blooms (cyanobacterial) has been increasing worldwide. It is not clear if the increase in how often and how badly these blooms happen is real or if it is because people are watching more closely and using better tools to identify species.

In 2008, the U.S. government created a report called "Harmful Algal Bloom Management and Response: Assessment and Plan." The report showed how serious the problem is.

Scientists have found harmful algal blooms in Europe, Africa, and Australia. These include blooms in some of Africa’s Great Lakes, such as Lake Victoria, which is the second-largest freshwater lake in the world. India has reported more blooms each year. In 1977, Hong Kong recorded its first coastal harmful algal bloom. By 1987, they saw about 35 blooms each year. In Canada, harmful algal blooms have been reported in popular lakes like Beaver Lake and Quamichan Lake. These blooms caused the deaths of some animals and led to warnings about swimming.

Global warming and pollution are causing algal blooms to appear in places that were once thought to be unlikely or impossible for them to exist. These places include under ice sheets in the Arctic, Antarctica, the Himalayan Mountains, the Rocky Mountains, and the Sierra Nevada Mountains.

In the United States, every coastal state has experienced harmful algal blooms in the last 10 years. New types of blooms have been found in areas that were not previously known to have problems. Inland, major rivers have seen more and larger blooms. In 2015, the Ohio River had a bloom that stretched 650 miles (1,050 km) into other states. Tests showed the bloom contained toxins, which caused problems for drinking water and recreation. In 2016, part of Utah’s Jordan River was closed because of a toxic algal bloom. Scientists predicted that from 2024 to 2090, the average number of harmful algal bloom days in U.S. lakes would increase from 7 days to 18–39 days.

Off the west coast of South Africa, harmful algal blooms caused by a type of organism called Alexandrium catenella happen every spring. These blooms harm fish populations because the toxins in the phytoplankton make shellfish in the area unsafe for people to eat.

Harmful effects

When algal blooms grow, they use up oxygen in the water and block sunlight from reaching fish and plants. These blooms can last for a few days or many months. With less sunlight, plants under the bloom may die, and fish may not get enough food to survive. The large number of algae in a bloom reduces oxygen levels at night because the algae use oxygen for breathing. When the algae die, microbes that break down the dead algae use even more oxygen, which can cause more fish to die or leave the area.

If oxygen continues to drop due to blooms, it can create hypoxic dead zones, where fish and plants cannot survive. In the Chesapeake Bay, these dead zones are common and may be a major source of methane.

Scientists have found that harmful algal blooms (HABs) were common during past mass extinction events, such as the End-Permian Extinction.

Tests show that some toxins near blooms can be in the air and inhaled, which can harm health. This happens when toxins are turned into tiny particles by wind and water movement. Not all algal blooms release toxins into the air, as it depends on the type of algae and environmental conditions. Some microalgae or bacteria release toxins when their cells break apart due to physical forces like waves. Other species release toxins when their cells break apart because of natural processes, viruses, or chemicals in the water. These toxins are often carried into the air by waves, which create bubbles that pop and release water droplets containing broken cell material. A 2017 study found that these aerosolized toxins can travel up to 1,000 kilometers by wind.

It is important to know local policies and response plans for HABs, even if you do not live near the coast, because toxins can travel long distances.

The One Health Harmful Algal Bloom System (OHHABS), started by the U.S. Centers for Disease Control and Prevention (CDC) in 2016, is the only national system in the United States that collects data on HABs and the illnesses they cause in people, animals, and the environment. Health departments across the country, working with environmental and animal health partners, report HAB events in freshwater, marine, or brackish water, along with illnesses in humans, pets, livestock, and wildlife. Reports may also include HABs without known illnesses or food-related cases, such as contaminated seafood.

HABs are a key example of a One Health challenge because they affect human health (through exposure to toxins in water, air, or food), animal health (such as fish, bird, or pet deaths), and the environment (like dead zones and climate-related blooms). Before OHHABS, data collection was scattered, making it hard to track patterns or respond effectively. OHHABS improves this by combining data from different areas, helping to understand risks and prevent HABs as they become more common due to pollution and climate change.

Data from OHHABS helps officials take action, such as issuing warnings, closing beaches or water areas, and teaching people how to avoid exposure (like not swimming in or drinking water from blooms or protecting pets). Reports about animals help veterinarians provide care to save pets and livestock. National data, such as 372 HAB events, 95 human illnesses, and over 102,000 animal illnesses reported in 2022 alone, help identify high-risk areas and support efforts to reduce pollution. Outbreaks are also shared with the National Outbreak Reporting System for coordinated responses.

By using One Health principles, OHHABS shows how combining data can help prevent health risks and address environmental causes of HABs.

Eating fish or shellfish from lakes near algal blooms is not safe. Toxins from the algae can build up in shellfish, and eating them can cause illnesses like amnesic shellfish poisoning, diarrhetic shellfish poisoning, neurotoxic shellfish poisoning, or paralytic shellfish poisoning. A 2002 study found that algal toxins may cause up to 60,000 cases of poisoning worldwide each year.

In 1987, a new illness called amnesic shellfish poisoning (ASP) was discovered after people in Prince Edward Island ate mussels contaminated with domoic acid, a toxin produced by a type of algae. A 2013 study found that toxic paralytic shellfish poisoning in the Philippines caused at least 120 deaths over several decades.

In 2014, health officials in California warned people not to eat certain parts of anchovies, sardines, or crabs caught in Monterey Bay due to a HAB. In 2015, shellfish fisheries in Washington, Oregon, and California were closed because of high levels of domoic acid in shellfish. People have been warned that breathing in air from waves or wind during HABs can cause asthma or other breathing problems.

In 2018, officials in Utah warned that crops might be contaminated if irrigated with water from HABs, though they admitted it is hard to measure contamination accurately. They issued warnings as a precaution.

People are generally told not to swim in or drink water from algal blooms and to keep pets away from the water because many pets have died from exposure. In one case, people got sick before warnings were issued. There is no treatment for animals, including livestock, if they drink water from HABs with toxins.

In some areas, visitors are warned not to touch the water. Boaters are told that toxins in the water can be inhaled from spray caused by wind or waves. Beaches, lakes, and rivers have been closed due to HABs. In 2015, a dog in California’s Russian River died after swimming in a bloom, leading to warnings for parts of the river. Boiling water at home does not remove toxins.

In August 2014, Toledo, Ohio, told 500,000 residents not to drink tap water because an algal bloom in Lake Erie made the water unsafe. The emergency required using bottled water for most activities, which disrupted public services and businesses. The bloom returned in 2015 and was expected again in 2016.

In 2004, a bloom in Kisumu Bay, Kenya, which supplies drinking water to 500,000 people, caused similar water contamination. In China, water was cut off to 2 million people in 2007 due to a bloom in its third-largest lake. A smaller water shutdown affected 15,000 people in 2009 at a different location. Similar events have occurred in Australia.

Potential remedies

Algal blooms often occur when large amounts of nutrient-rich runoff enter water bodies. Programs that treat wastewater, reduce fertilizer use in farming, and control runoff can help reduce severe algal blooms in areas like river mouths, estuaries, and oceans near rivers.

Nitrates and phosphorus in fertilizers can cause algal blooms when they wash into lakes and rivers after heavy rain. Farmers can reduce runoff by applying fertilizers only where and when they are most needed for crops. One successful method is drip irrigation, which delivers water and nutrients directly to plant roots through tubes and emitters. This method prevents fertilizer from washing away and helps reduce algal blooms in drinking water reservoirs while saving up to 50% of water used in agriculture.

Some experts suggest creating buffer zones with plants and wetlands to filter out phosphorus before it reaches water. Other methods include using conservation tillage, changing crop rotations, and restoring wetlands. With proper management, some dead zones can shrink within a year.

In some cases, chemical control has been successful. For example, after Norway’s lobster fishery collapsed in 1986 due to low oxygen levels, Denmark reduced phosphorus output by 80%, which helped restore oxygen levels. Similarly, dead zones in the Black Sea and along the Danube River improved after farmers reduced phosphorus use by 60%.

Nutrients can be removed from wetlands by harvesting wetland plants, which reduces their flow into nearby water. Researchers are testing floating mats of cattails to remove nutrients from deep water where wetland plants cannot grow.

In the U.S., surface runoff is the main source of nutrients in rivers and lakes, but it is not well regulated under the federal Clean Water Act. Local efforts, such as those in the Great Lakes region and the Chesapeake Bay, aim to reduce nutrient pollution. Ohio’s plan in 2016 focused on reducing phosphorus runoff to help control algal blooms in Lake Erie.

Algaecides, chemicals that kill algae, have been used in small water bodies but can harm fish and wildlife in larger areas. For example, copper-based algaecides may not work well over time because cyanobacteria can develop resistance. In 2019, a new chemical treatment successfully eliminated toxic algae in Chippewa Lake, Ohio, within one day. This treatment was also used in China, South Africa, and Israel.

In 2020, a new algicide was tested in South Africa’s Roodeplaat Dam to combat a severe bloom of Microcystis. The treatment released hydrogen peroxide only on the water’s surface, reducing harm to aquatic life.

Bioactive compounds from plants, such as seaweeds, have shown promise in controlling harmful algal blooms. These compounds can also fight bacteria, fungi, and help with antioxidants.

Modified clays, like aluminum chloride-treated clay, have been tested to remove cyanobacteria by trapping them in sediment. This method limits the growth of harmful algae.

In the Netherlands, a hydrodynamic separator was used to remove algae and reduce phosphate levels in water. The treated water became clearer, and future projects will study its effects on marine life. Removed algae and phosphate may be used as fuel in bio digesters.

Some experts suggest building reservoirs to stop algae from moving downstream, but this can lead to algae growth within the reservoirs. Declines in shellfish populations, like oysters, may also contribute to algal blooms, so restoring these populations is being studied.

Other solutions include better monitoring, predicting blooms, and testing new methods. Some countries near the Baltic Sea have considered using large-scale geoengineering, like aerating water layers, to reduce dead zones.

Mathematical models help predict future algal blooms. Scientists agree that forecasting harmful algal blooms is urgent to protect public health. Sensors are being developed to warn about potential blooms and help water treatment facilities prepare for higher toxin levels. Currently, sensors are only used in the Gulf of Mexico.

Notable occurrences

• 1530: The first reported case of a red tide off the Florida Gulf Coast is not supported by evidence. According to the Marine Lab at the University of Miami, the first possible red tide in Florida occurred in 1844. Earlier reports of "dead fish" along the coast were likely due to fish being thrown overboard by boats, not red tide.
• 1793: The first recorded red tide event occurred in British Columbia, Canada.
• 1840: No human deaths have been linked to Florida red tide. However, people may experience coughing, sneezing, or eye irritation when the red tide organism, Karenia brevis, is present along the coast and winds carry its toxins into the air. Swimming is usually safe, but skin irritation or burning may occur in areas with high red tide concentrations.
• 1844: The first possible red tide event off the Florida Gulf Coast was identified by the Marine Lab at the University of Miami. It is believed to have been caused by ships near the coast, with no known residents reporting the event at the time.
• 1901: Lingulodinium polyedrum creates bright, glowing light in warm coastal waters. This phenomenon has been regularly observed in Southern California since at least 1901.
• 1916: A large fish kill occurred along the southwest Florida coast. Harmful air was thought to result from an underwater explosion releasing chlorine gas.
• 1947: A large red tide bloom lasting nearly a year severely damaged the commercial fishing industry and sponge beds in southwest Florida. The poisoned seawater forced beach evacuations.
• 1972: A red tide in New England was caused by the toxic dinoflagellate Alexandrium (Gonyaulax) tamarense. This organism produces saxitoxin and gonyautoxins, which accumulate in shellfish. Eating contaminated shellfish can lead to paralytic shellfish poisoning (PSP), which may be fatal.
• 1972 and 1973: Red tides in the area west of Port Moresby, Papua New Guinea, killed two villagers. In March 1973, a red tide invaded Port Moresby Harbour and destroyed a Japanese pearl farm.
• 1972: A red tide in New England was caused by the toxic dinoflagellate Alexandrium (Gonyaulax) tamarense.
• 1976: The first case of paralytic shellfish poisoning (PSP) in Sabah, Malaysian Borneo, involved 202 people affected and 7 deaths.
• 1987: A red algae bloom in Prince Edward Island caused over a million dollars in economic losses.
• 1991: The largest algal bloom on record occurred in the Darling River, Australia, from October to December 1991. It covered over 1,000 kilometers of the Barwon and Darling Rivers and was mainly caused by Anabaena circinalis.
• 2005: A red tide was found farther south than in previous years by the ship R/V Oceanus. This event closed shellfish beds in Maine and Massachusetts and warned authorities as far south as Montauk, Long Island, New York, to check their beds. Experts warned that the red tide could spread to Long Island in the future, threatening the fishing industry and tourism.
• 2008: Large blooms of the algae Cochlodinium polykrikoid were found in the Chesapeake Bay and nearby rivers, such as the James River, causing millions of dollars in damage and beach closures.
• 2009: In Brittany, France, recurring macroalgal blooms were caused by fertilizer runoff from pig farming. This led to lethal gas emissions, resulting in one human unconsciousness and three animal deaths.
• 2010: Dissolved iron from the Eyjafjallajökull volcano ash triggered a plankton bloom in the North Atlantic.
• 2011: Red tides occurred in northern California and the Gulf of Mexico.
• 2013: An algal bloom in Qingdao, China, was caused by sea lettuce.
• 2013: A red tide occurred on the west coast of Sabah, Malaysian Borneo, in January. Two people died after eating shellfish contaminated with red tide toxins.
• 2013: A red tide bloom appeared at Sarasota Beach, Florida, mainly Siesta Key. This caused a fish kill, harmed tourism, and led to respiratory issues for beachgoers.
• 2014: Myrionecta rubra (formerly Mesodinium rubrum), a ciliate protist that consumes cryptomonad algae, caused a bloom on the southeastern coast of Brazil.
• 2014: Blue-green algae caused a bloom in the western basin of Lake Erie, poisoning the water supply for 500,000 people in Toledo, Ohio.
• 2014: A massive "Florida red tide" stretched 90 miles long and 60 miles wide.
• 2015: Twelve people were hospitalized in the Philippine province of Bohol due to red tide poisoning.
• 2015: Several beaches in the Netherlands between Katwijk and Scheveningen were affected by a red tide. Authorities advised swimmers not to enter the water.
• 2015: A red tide bloom occurred in the Gulf of Mexico, affecting Padre Island National Seashore in Texas.
• 2017 and 2018: A red tide caused by Karenia brevis led to warnings against swimming, a state of emergency, and the deaths of dolphins and manatees. The event worsened due to the Caloosahatchee River and peaked in summer 2018. A rare red tide also occurred along Florida’s east coast in Palm Beach County in September 2018.
• 2019: Blue-green algae, or Cyanobacteria, caused problems again on Lake Erie. Satellite images showed a bloom covering up to 1,300 square kilometers, centered near Toledo, Ohio. The largest Lake Erie bloom occurred in 2015, with a severity index of 10.5. Water quality testing was ongoing in August 2019.
• 2019: A bloom of Noctiluca algae caused a bioluminescent glow off the coast of Chennai, India. Similar blooms have occurred annually in the northern Arabian Sea since the early 2000s.
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