An algal bloom is a quick increase in the number of algae in freshwater or saltwater environments. These blooms can be harmless or harmful.

Algal blooms are often noticed when water changes color because of the 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 algae. 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 rapidly, which can harm the ecosystem.

The effects of algal blooms vary. Some blooms help by providing food for other living things. Other blooms can block sunlight, reduce oxygen in the water, or release harmful toxins. Algae are also 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 can cause fish to die, cities to stop water supplies, or fisheries to close. The process of too many nutrients causing algae growth and oxygen loss is called eutrophication.

Algal and bacterial blooms have played a role in major extinction events in Earth’s history, such as the end-Permian extinction linked to volcanic activity and the slow recovery of life after such events.

Description

The term "algal bloom" is defined differently by scientists in various fields. It can refer to a small, harmless growth of algae or a large event that causes harm. Since algae includes many types of organisms with different sizes, growth rates, and nutrient needs, there is no official rule for what is considered a bloom. Scientists describe and measure blooms in different ways, such as by the amount of new algae growing, the level of green pigments in the water, the harm caused by the bloom, or how much algae is present compared to other tiny water organisms. For example, some definitions include:

Algal blooms happen when a nutrient that algae need to grow is added to the water. This nutrient is often nitrogen or phosphorus, but it can also be iron, vitamins, or amino acids. These nutrients can enter water in several ways. In the open ocean and near coasts, deep water can rise to the surface due to wind or underwater landforms, bringing nutrients to the sunlit part of the ocean. In coastal areas and freshwater systems, nutrients can come from agricultural runoff, city waste, or sewage.

Algal blooms, especially large ones, can make water less clear and change its color. The green pigments in algae, like chlorophyll, and other pigments that protect algae from sunlight determine the bloom's color. Depending on the type of algae, its pigments, and how deep the bloom is in the water, the bloom can appear green, red, brown, golden, or purple. Bright green blooms in freshwater are often caused by cyanobacteria, which are sometimes called "blue-green algae," such as Microcystis. Some blooms may also be made up of large algae, like seaweed, which can wash up on shorelines.

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

Harmful algal blooms (HABs) are events where algae that produce toxins or cause harm grow rapidly. Many types of algae can lead to HABs. For example,

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 reduce algal blooms that include cyanobacteria, it is important to lower the amount of phosphorus entering water systems. In lakes that form layers during summer, mixing of these layers in autumn can release large amounts of usable phosphorus, which may cause algal blooms when enough sunlight is available. Extra nutrients can also enter water systems through runoff. Some studies suggest that too much carbon and nitrogen may also contribute. Residual sodium carbonate in water can help algae grow by providing carbon dioxide, which supports photosynthesis when nutrients are present.

When phosphates enter water systems, higher levels can lead to faster algae and plant growth. Algae grow quickly with lots of nutrients, but each alga lives for a short time. This results in large amounts of dead organic matter that begin to decay. Natural decomposers in the water break down the dead algae, using oxygen in the process. This can greatly reduce the oxygen available for other aquatic life. Without enough oxygen, many animals and plants may die, creating an area known as a "dead zone."

Algal blooms can also appear in freshwater aquariums if fish are overfed and extra nutrients are not absorbed by plants. This is harmful to fish, and the problem can be fixed by changing the tank water and reducing the amount of food given to fish.

Algal blooms in freshwater are not always caused by human activity. They can occur naturally in both eutrophic lakes (which have many nutrients like nitrogen and phosphates) and oligotrophic lakes (which have few nutrients). Oligotrophic lakes are defined by low nutrient levels, and the Trophic State Index (TSI) measures these levels. A TSI below 30 means the water is oligotrophic. Algal blooms have been seen in oligotrophic lakes, even though these lakes usually have little available nutrients.

In nutrient-poor environments, cyanobacteria can still cause algal blooms because they are good at taking in nutrients. These bacteria use nitrogen and phosphates in their life processes. Cyanobacteria help fix nitrogen by changing nitrogen gas from the air into a form that other organisms can use. This added nitrogen supports large algal blooms in oligotrophic lakes.

Cyanobacteria can also take in large amounts of phosphorus even when nutrients are scarce, which helps them survive in oligotrophic environments. Some cyanobacteria species, like D. lemmermannii, move between deep, nutrient-rich layers of lakes and shallower, nutrient-poor layers. This movement brings phosphorus to the shallower layers, increasing the chance of algal blooms.

Upwelling events occur when nutrients like phosphates and nitrogen move from deep, nutrient-rich layers of lakes to shallow, nutrient-poor layers. This can happen due to natural processes, such as seasonal mixing when lake surfaces freeze or melt. This mixing spreads nutrients throughout the lake, leading to algal blooms.

Marine algal blooms

Turbulent storms stir up the ocean in summer. This brings nutrients to the sunlit waters near the top. These nutrients cause a lot of feeding activity each spring, leading to large groups of phytoplankton. Tiny molecules inside these microscopic plants use sunlight to create energy through photosynthesis. A natural pigment called chlorophyll helps phytoplankton grow in Earth's oceans. Scientists can see these groups from space by looking at chlorophyll levels.

Satellites find where phytoplankton live and how many there are by checking chlorophyll amounts in coastal and open waters. More chlorophyll means bigger groups. Observations show these groups usually last until late spring or early summer. This happens when nutrients decrease and tiny animals called zooplankton begin to eat the phytoplankton. The image below uses data from NASA SeaWiFS to show where these groups are.

The NAAMES study from 2015 to 2019 looked at how phytoplankton change in ocean ecosystems. It also studied how these changes affect tiny particles in the air, clouds, and Earth's climate.

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

Wildfires can cause phytoplankton groups by sending tiny particles from fires into the ocean.

Harmful algal blooms

A harmful algal bloom (HAB) is a type of algal bloom that harms other living things. It can cause harm through natural toxins, physical damage, or other harmful effects. HABs are difficult to manage because they come in many different forms, and they create problems for areas near the ocean that are already at risk. These blooms are linked to large numbers of marine animals dying and have been connected to types of poisoning from eating shellfish. Because they harm people’s health and the economy, scientists closely watch for HABs.

HABs are harmful to humans. People can be exposed to toxins from harmful algae by eating seafood that contains the toxins, swimming in or being near water, or breathing in tiny air droplets that carry the toxins. Since people can get sick from eating seafood with these toxins, illnesses can affect the nervous system, stomach and intestines, lungs, liver, skin, and heart.

People who use beaches often get illnesses such as breathing problems, eye or nose irritation, and fever. These illnesses sometimes require medical treatment. Ciguatera fish poisoning (CFP) is a common illness linked to algal blooms. Drinking water can also become unsafe if it is polluted with cyanotoxins, which can cause water-related illnesses.

If an HAB event causes a very high number of algae in the water, the water may change color. It can look purple, pink, red, or green. However, not all algal blooms are thick enough to change the water’s color.

Dinoflagellates are tiny organisms that can both glow in the dark and produce toxins in algal blooms. They use a chemical reaction to create a blue light. There are seventeen main types of toxins made by dinoflagellates, including Saxitoxin and Yessotoxin. Both of these toxins can glow and are harmful. These two types of dinoflagellates often live in similar areas near the ocean. At night, large numbers of dinoflagellates 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 checks for toxins in shellfish to warn about harmful algal blooms and track their levels. These checks help decide if shellfish should be kept off the market to avoid contamination. Another part of mitigation 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 allow water to soak into the ground instead of flowing directly into water. Vegetation, like rain gardens, native plants, trees, and rooftop gardens, also helps by absorbing 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 mixed into water to group harmful algal blooms, making them sink faster. Biological control uses methods like releasing sterile males or using scents to reduce reproduction. Chemical control involves using toxic chemicals, but this can harm other animals. Genetic control changes how species survive or reproduce, though this may affect native species. Environmental control uses water movement and adding oxygen to the water to manage algal blooms.

Environmental impacts

Harmful algae blooms cause many problems for the environment and are becoming a bigger issue that is spreading to more areas. A small brown tide organism that was once only found in parts of 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 events. Harmful algae blooms can create areas in water with no oxygen, which can harm or kill living things in the water, cause fish to become poisoned, and lead to breathing problems or illness in people who visit beaches.

Harmful algae blooms greatly affect the Great Lakes and St. Lawrence River Basin. Invasive zebra and quagga mussels are linked to the environmental effects of these blooms. These mussels help phosphorus move through the water, which increases the growth of harmful algae in areas where they live. Harmful algae blooms continue to pollute water supplies in the Great Lakes Basin. Because the world is still recovering from the COVID-19 pandemic, solving this problem has not been a top priority. This issue has become a topic in political discussions in the United States, while countries like Canada have shown little concern.

Harmful algae blooms have a major impact on marine life. For example, in August 2024, the growth of toxic algae called Pseudo-nitzschia along the California coast made sea lions sick and caused them to act aggressively toward people on the beach. Scientists say this is a regular event that happens during certain seasons. The growth of Pseudo-nitzschia leads to the production of domoic acid, which builds up in fish like sardines, anchovies, and squids. This affects the food web and the main food source of sea lions. When sea lions eat the poisoned fish, the toxins can cause seizures, brain damage, and death. During this event, people reported sea lions biting and acting unpredictably. In this unhealthy state, the sea lions are frightened and behave aggressively to protect themselves. Pregnant sea lions are especially at risk from toxic algae poisoning and are more likely to die from its effects.

Fossil record

The first known algal bloom in the world is called Eoseira wilsonii. It lived during the Eocene period and is the only diatom found to have algal blooms in the Horsefly Shale and the Eocene Okanagan Highlands.