Algal bloom

An algal bloom, also known as a marine bloom or a water bloom, is an overgrowth of algae on a body of water. Algae are simple plants that feed on sunlight and nutrients in the water; a single alga can range in size from microscopic to the large seaweed and kelp found in the ocean. Under certain conditions, smaller forms of algae increase rapidly in number to form an algal bloom. Algal blooms may occur naturally, but sometimes they result from outside influences, such as industrial or agricultural pollution. Some algal blooms are relatively harmless, but others affect the surrounding environment in several ways. In certain cases, algal blooms may become deadly to the humans, animals, and plants around them.

Background

The microscopic, single-celled organisms that make up an algal bloom are known as phytoplankton or microalgae. Like plants on land, they rely on sunlight and carbon dioxide to grow and are found in the photic zone at or near the surface of a body of water. They can be found in freshwater, saltwater, or brackish water (where seas and rivers combine). Algal blooms may form in water sources as small as a large puddle or as big as an ocean.

Algae use nutrients in the water, such as phosphorus and nitrogen, as part of the photosynthesis process. Through photosynthesis, algae and other plants convert sunlight, carbon dioxide, and the nutrients to produce food and oxygen. In most cases, algae are harmless; they provide an important food source for many larger organisms and add oxygen back into the water and the surrounding environment.

A number of factors contribute to the growth of an algal bloom. Algal blooms are most likely to occur in warmer weather, so in the Northern Hemisphere early spring through early fall is the prime time for blooms to occur. A bloom is more likely to happen in sunny areas and in areas where water is clear with low turbidity, meaning water is relatively clear and free of suspended particles. Algae growth is also affected by the chemistry of the water, including pH, salt, and chlorine levels and the presence of other chemicals.

The level of sunlight and the chemical content of the water may result from natural conditions, such as the weather. Calm weather allows for longer periods of sunlight and still water conditions, while rainstorms can cause a sudden increase in the amount of water and its movement. Human action on the environment can increase the chances of an algal bloom by removing too much forest land or by polluting water with sewage, industrial waste, or agricultural chemicals, such as fertilizers. Whether directly deposited or carried into the water basin by rain runoff, pollutants can change the chemical composition of the water and allow algal blooms.

Impact

When conditions are favorable, algae grow at an increased rate. The resulting algal bloom can overwhelm the environment in which it forms, creating a cloud of algae in the water. Not all algal blooms are visible without a microscope. Those that are form a film that looks like scum, foam, a puddle of paint, or a carpet of wet gel floating on the water. Depending on the type of algae, a bloom may make the water turn red, brown, blue, green, yellow, or orange.

In most cases, the bloom will have little or no effect on the environment beyond making the water smell and taste bad and affecting the taste of fish from the water. The bad smell and unpleasant appearance may deter people from coming near the water and may affect tourism, but these types of algal blooms pose no threat of illness or death. They are sometimes called nuisance blooms. Other times, an algal bloom can be very harmful to other living things.

Algal blooms limit the amount of sunlight that reaches below the layer of algae formed on the water’s surface. If the bloom is thick and covers a large area of a small body of water, such as a small pond, it can limit the amount of sunlight that gets beneath it and affect the makeup of the water source enough to harm other plants and animals that live in the water. The longer the bloom lasts, the greater the chances become that it will negatively affect the aquatic life around it.

In some cases, the bloom is large enough to kill fish by clogging their gills and kill plants by smothering the light they need. These blooms negatively affect the ecosystem in which they are found. However, only a few dozen of the more than five thousand known forms of algae are toxic. When these types of algae form a bloom, animals and people who drink the water or are otherwise exposed to the bloom can suffer illness or death. The US Centers for Disease Control and Prevention (CDC) reported that in 2022, fifteen states reported 372 harmful algal blooms (HABs) that resulted in 95 human illnesses and at least 102,071 animal illnesses. People can be negatively affected by HABs if they come in direct physical contact with the water, drink water or eat seafood from the affected area, or inhale small airborne droplets near HAB sites. People who are exposed may experience symptoms ranging from skin and eye irritation to neurological problems. Animals that drink water affected by HABs often die in just a few days. It is not possible to tell simply by looking whether an algal bloom is harmful; however, less than one percent of all algal blooms are potentially toxic. Still, HABs pose a growing threat to marine and coastal ecosystems, adversely affecting environmental sustainability, public health, and blue economy industries, including fisheries, aquaculture, and tourism.

Some common types of HABs include blue-green algal blooms and red tide. Each is caused by a form of algae that turns water the color by which the blooms are known. In some cases, certain areas see an annual recurrence of a certain type of HAB. For instance, a red tide occurs nearly every year along the Gulf Coast of Florida and Texas, and parts of California are also affected annually. However, at Texas coast, they typically occur every two to five years, less frequent than the coast of Florida and California. Other times, algal blooms occur in areas that have never before been affected. Scientists are unsure why this happens, but they think changes in temperature and ocean currents may be contributors. Studies also underscores the importance of geographic information system (GIS)-based monitoring systems as scalable, real-time tools for bloom detection, resource management, and multi-agency decision support.

Bibliography

“Are All Algal Blooms Harmful?” National Oceanic and Atmospheric Administration, oceanservice.noaa.gov/facts/habharm.html. Accessed 3 June 2026.

“Gulf of America/Florida: Harmful Algal Blooms.” National Oceanic and Atmospheric Administration, 25 Feb. 2025, oceanservice.noaa.gov/news/redtide-florida/. Accessed 3 June 2026.

“Harmful Algal Blooms.” Central and Northern California Ocean Observing System, www.cencoos.org/learn/blooms. Accessed 3 June 2026.

Seyyed Roohollah Masoomi, et al. “Harmful Algal Blooms as Emerging Marine Pollutants: A Review of Monitoring, Risk Assessment, and Management with a Mexican Case Study.” Pollutants, vol. 6, no. 1, Jan. 2026, p. 4, doi:10.3390/pollutants6010004. Accessed 25 Jan. 2026.

“Summary Report–One Health Harmful Algal Bloom System (OHHABS), United States, 2022.” Centers for Disease Control and Prevention, 23 Sept. 2024, www.cdc.gov/ohhabs/data/summary-report-united-states-2022.html. Accessed 3 June 2026.

“Summary Report–One Health Harmful Algal Bloom System (OHHABS), United States, 2020.” US Centers for Disease Control and Prevention, 26 Feb. 2024, www.cdc.gov/habs/data/2020-ohhabs-data-summary.html. Accessed 3 June 2026.

“Understanding Algal Blooms.” St. John’s River Water Management District, www.sjrwmd.com/algae/. Accessed 3 June 2026.