An algal bloom is a sudden increase in the number of algae in freshwater or ocean water. These blooms can be harmless or harmful.

Algal blooms often make water look different in color because of pigments in the algae. The word "algae" includes many types of water plants, such as large, visible plants like seaweed and tiny, single-celled organisms like cyanobacteria. Most algal blooms refer to the fast growth of tiny, single-celled algae, not large plants. An example of a large algal bloom is a kelp forest.

Algal blooms happen when nutrients like nitrogen or phosphorus enter water systems. These nutrients can come from sources such as fertilizer runoff or other pollution. When too many nutrients are present, algae grow quickly, which can harm the entire ecosystem.

The effects of algal blooms vary. Some blooms provide food for other living things, while others can block sunlight, lower oxygen levels in water, or release harmful chemicals. Despite these issues, algae are important because they produce about 70% of Earth’s oxygen, which supports life on land. Blooms that release toxins and harm animals or ecosystems are called "harmful algal blooms" (HABs). These blooms can cause fish to die, force cities to stop water supplies, or lead to closures of fishing areas. The process of too many nutrients causing algae growth and oxygen loss is called eutrophication.

In Earth’s history, algal and bacterial blooms have contributed to major losses of life linked to global warming, such as during the end-Permian extinction caused by volcanic activity in Siberia and during the recovery of life after such events.

Description

The term "algal bloom" is described differently in various scientific fields. It can refer to a small, harmless growth of algae or a large, harmful event. Since algae includes many types of organisms with different sizes, growth rates, and nutrient needs, there is no official definition for what qualifies as a bloom. Scientists use different methods to describe and measure blooms, such as the amount of new algae growing, the level of photosynthetic pigment, the harm caused by the bloom, or how much algae is present compared to other microbes in the water. For example, some definitions include:

• A chlorophyll concentration greater than 100 μg/L,
• A chlorophyll concentration greater than 5 μg/L,
• A count of a specific algae species exceeding 1000 cells/mL, and
• A change in the usual amount of a particular algae species.

Algal blooms occur when a nutrient needed by the algae enters the local water system. This nutrient is often nitrogen or phosphorus, but it can also be iron, vitamins, or amino acids. Nutrients can enter water through natural processes, such as upwelling in the open ocean or along coastlines, which brings nutrients from deeper water to the sunlit zone. In coastal areas and freshwater systems, nutrients can come from agricultural runoff, city waste, or sewage.

Algal blooms can make water less clear and change its color. The pigments inside algae cells, such as chlorophyll and other protective pigments, determine the bloom’s color. Depending on the type of algae, its pigments, and the depth of the water, blooms can appear green, red, brown, golden, or purple. Bright green blooms in freshwater are often caused by cyanobacteria, such as Microcystis. Some blooms are made of macroalgae, which are not phytoplankton and are visible as large, flat pieces of algae that may wash onto shore.

When nutrients are present in water, algae grow much faster than usual. In small blooms, this rapid growth can help the ecosystem by providing food for other organisms.

Harmful algal blooms (HABs) are events where toxic or harmful phytoplankton grow in large numbers. Many species can cause HABs. For example:

• Gymnodinium nagasakiense can create harmful red tides,
• Gonyaulax polygramma can reduce oxygen levels and kill fish,
• Microcystis aeruginosa can produce toxins, and
• Chaetoceros convolutus can damage fish gills.

Rivers like the Amazon carry nutrients from land into the ocean, leading to thick algal blooms along coastlines. Blooms often grow in dark, nutrient-rich water near the Amazon River’s mouth, as seen by NASA’s Aqua satellite. Changes in coastal phytoplankton blooms were observed between 2003 and 2020.

Freshwater algal blooms

Freshwater algal blooms happen when there is too much of certain nutrients, especially phosphates. These extra nutrients can come from fertilizers used on land for farming or recreation, as well as from household cleaning products that contain phosphorus.

To stop blooms that include cyanobacteria, it is important to reduce the amount of phosphorus entering water systems. In summer, lakes often form layers, and when these layers mix in autumn, large amounts of usable phosphorus can be released. This phosphorus can cause algal blooms when enough sunlight is available for photosynthesis. Extra nutrients can also enter water systems through runoff. Some research suggests that excess carbon and nitrogen might also contribute. Residual sodium carbonate can help algae grow by providing carbon dioxide, which supports photosynthesis when nutrients are present.

When phosphates enter water systems, they can cause more algae and plant growth. Algae grow quickly when nutrients are plentiful, but each alga lives for a short time. This leads to a large amount of dead organic matter that begins to decompose. Natural decomposers in the water use oxygen to break down the algae, which can greatly reduce the amount of dissolved oxygen available for other aquatic life. Without enough oxygen, many plants and animals may die, creating what is called a "dead zone."

Algal blooms can also occur in freshwater aquariums if fish are overfed and excess nutrients are not used by plants. This is harmful to fish, but it can be fixed by changing the tank water and reducing the amount of food given to the fish.

Algal blooms in freshwater systems are not always caused by human activity. They can happen naturally in both eutrophic lakes (which have many nutrients like nitrogen and phosphates) and oligotrophic lakes (which have few nutrients). The Trophic State Index (TSI) measures nutrients in water systems, and a TSI below 30 means the water is oligotrophic. Even in oligotrophic lakes, algal blooms have been observed, often caused by cyanobacteria.

In nutrient-poor environments, cyanobacteria can still grow because they are better at taking in nutrients than other organisms. These bacteria use nitrogen and phosphates in their life processes. They can also take nitrogen from the air (in the form of N₂) and change it into a form other organisms can use. This increases the amount of nitrogen in the water, which supports large algal blooms even in oligotrophic lakes.

Cyanobacteria can take in a lot of phosphorus even when nutrients are scarce, which helps them survive in oligotrophic environments. Some species, like D. lemmermannii, move between the nutrient-rich hypolimnion and the nutrient-poor metalimnion. This movement brings phosphorus to the metalimnion, giving other organisms more phosphorus and increasing the chance of algal blooms.

Upwelling events occur when nutrients like phosphates and nitrogen move from the nutrient-rich hypolimnion to the nutrient-poor metalimnion. This can happen during seasonal changes, such as when lakes freeze or melt, causing water layers to mix. This mixing spreads nutrients throughout the water system, leading to algal blooms.

Marine algal blooms

Turbulent storms mix the ocean in summer, bringing nutrients to the sunlit surface waters. This leads to a lot of feeding activity each spring, which causes large groups of phytoplankton to grow rapidly. Tiny parts inside these small plants use sunlight to make energy through photosynthesis. The green pigments, called chlorophyll, help phytoplankton survive in Earth's oceans and allow scientists to track blooms from space.

Satellites find where phytoplankton are located and how many there are by measuring chlorophyll levels in coastal and open waters. More chlorophyll means larger blooms. Observations show that blooms usually last until late spring or early summer, when nutrients decrease and tiny animals called zooplankton begin to eat the phytoplankton. The image below uses NASA SeaWiFS data to show bloom locations.

The NAAMES study, which happened between 2015 and 2019, looked at how phytoplankton changes in ocean ecosystems affect atmospheric particles, clouds, and climate.

In France, people are asked to report colored water through the PHENOMER project. This helps scientists learn more about marine blooms.

Wildfires can cause phytoplankton blooms by sending smoke particles into the ocean.

Harmful algal blooms

A harmful algal bloom (HAB) is a type of algal bloom that harms other living things. This can happen when the algae produce natural toxins, cause physical harm, or affect organisms in other ways. Because HABs vary widely, they are difficult to manage and create challenges, especially for coastal areas that are already at risk. These blooms are often linked to large-scale deaths of marine life and have been connected to several types of shellfish poisonings. Due to their negative effects on both the economy and human health, HABs are closely watched and studied.

HABs can harm humans. People may come into contact with toxic algae by eating seafood that contains the toxins, swimming in or being near contaminated water, or breathing in tiny air droplets that carry the toxins. Because humans can be exposed to these toxins through seafood, food-related illnesses can occur. These illnesses may affect the nervous system, digestive system, respiratory system, liver, skin, and heart.

People who visit beaches often report health issues such as upper respiratory problems, eye or nose irritation, and fever. They may need medical treatment to recover. Ciguatera fish poisoning (CFP) is a common illness caused by exposure to algal blooms. Water-related illnesses can also occur when drinking water becomes polluted with cyanotoxins.

When an HAB event produces a very high amount of algae, the water may change color, becoming murky or discolored. The water can range in color from purple to pink, but is most often red or green. Not all algal blooms are dense enough to change the water’s color.

Dinoflagellates are tiny, single-celled organisms that are part of algal blooms. Some of these organisms can produce light through a chemical reaction called luciferin-luciferase, which creates a blue glow. There are seventeen major types of dinoflagellate toxins, including Saxitoxin and Yessotoxin. Both of these toxins are harmful and can produce light. These two types of toxins are often found in similar coastal environments. At night, large numbers of dinoflagellates can create a blue-green glow. During the day, they appear red or brown, which is why these blooms are sometimes called "Red Tides." Dinoflagellates are known to cause seafood poisoning through their neurotoxins.

Management

There are three main ways to manage algal blooms: mitigation, prevention, and control. Mitigation includes regular monitoring programs to check for toxins in shellfish. These programs help detect harmful algal blooms early and track their levels. If shellfish are found to be contaminated, restrictions are put in place to stop them from being sold. Another mitigation step is moving fish pens away from areas with algal blooms.

Prevention focuses on reducing runoff that carries extra nutrients into waterways. This can be done by increasing permeable surfaces, such as streets and parking lots that let water soak through, which helps absorb pollution. Vegetation, like rain gardens, native plants, trees, and rooftop gardens, also helps by filtering and slowing runoff. Farmers can help by planting cover crops, creating forested buffers, using less fertilizer, and building fences to keep animals away from streams.

Control includes several methods: mechanical, biological, chemical, genetic, and environmental. Mechanical control uses clay to mix with harmful algal blooms, causing them to settle in the water. Biological control includes methods like using pheromones or releasing sterile males to reduce reproduction. Chemical control involves using toxic chemicals, but this may harm other organisms. Genetic control changes the traits of species to affect their survival or reproduction, though it can risk harming native species. Environmental control uses water circulation and aeration to manage algal blooms.

Environmental impacts

Harmful algae blooms cause many problems for the environment and are becoming more common in many areas. A small brown tide organism that was once found only in the northeastern United States and South Africa is now causing large blooms along the coast of China. These blooms are similar to those seen in other brown tide areas. Harmful algae blooms can create areas without oxygen, which can kill living things in the water, poison fish, and cause breathing problems and illness in people who visit beaches.

Harmful algae blooms greatly affect the Great Lakes and the St. Lawrence River Basin. Invasive zebra and quagga mussels are linked to the environmental damage caused by these blooms. These mussels help phosphorus move through the water, which increases the number of harmful algae blooms in areas where they live. Harmful algae blooms continue to pollute water supplies in the Great Lakes Basin. Because the world is recovering from the COVID-19 pandemic, solving this issue has become less of a priority. This environmental problem has become a topic in U.S. politics, while countries like Canada show little concern.

Harmful algae blooms have a major impact on marine life. For example, in August 2024, the toxic algae Pseudo-nitzschia grew along the coasts of California, making sea lions sick and causing them to act aggressively toward people on the beach. Scientists say this happens every year during certain times. The growth of Pseudo-nitzschia leads to the production of domoic acid, which builds up in fish like sardines, anchovies, and squids. This directly harms the food web and the main food source of sea lions. When sea lions eat these fish, the toxins can cause seizures, brain damage, and death. During this event, people reported bites and unpredictable, aggressive behavior from the sick sea lions. In this condition, the sea lions are frightened and act out of fear to protect themselves. Pregnant sea lions are most likely to be harmed by toxic algae and are more likely to die from its effects.

Fossil record

Eoseira wilsonii is the first known algal bloom in the world. It existed during the Eocene period and is the only diatom found in the Horsefly Shale and Eocene Okanagan Highlands that had algal blooms.