Causes and Ecosystem Impacts

Still water covered in green algae

Learn what causes blooms of harmful algae and cyanobacteria (blue-green algae), how to recognize them, and why they are a growing concern.

Blooms basics

Algae

Algae are a group of plants that are usually found in water. Like all plants, algae have a pigment called chlorophyll that they use to turn sunlight into food. Algae can be found in all types of waters, including salt water, fresh water, and brackish water (a mix of salt and fresh water).

Algae that live in the water can be grouped into two categories, seaweed and phytoplankton. Seaweed are large plants made up of many cells and phytoplankton are small, single-celled plants.

Seaweed, or macroalgae

Seaweed, or macroalgae

Lake's still water covered with phytoplankton or microalgae

Phytoplankton, or microalgae

Are algae good or bad?

On their own, algae and cyanobacteria are not good or bad. They are organisms that are important to the earth because they produce the oxygen needed to sustain life. However, if too much algae or cyanobacteria grow at once or if they make toxins, they can harm people, animals, and the environment.

Blooms of harmful algae and cyanobacteria

Both seaweed and phytoplankton sometimes grow quickly, or bloom. Some blooms can harm people, animals, or the environment. Most harmful blooms that make people and animals sick are caused by phytoplankton.

Blooms can harm people, animals, and the environment when they

  • Produce toxins (poisons)
  • Become too dense
  • Use up the oxygen in the water
  • Release harmful gases

These harmful blooms can be caused by many types of phytoplankton. However, three main types of phytoplankton cause most blooms that make people and animals sick:

  • Cyanobacteria (sometimes called blue-green algae)
  • Dinoflagellates (sometimes called microalgae or red tide)
  • Diatoms (sometimes called microalgae or red tide)

Not all blooms are harmful.
Blooms known as “nuisance blooms” can discolor water, smell bad, and cause the water or fish to taste bad. Nuisance blooms are not usually dangerous to people, pets, and livestock because they do not produce toxins. However, they can discourage people from visiting beaches, drinking tap water, or eating fish from water with an ongoing bloom.

You cannot tell if a bloom is harmful just by looking at it.

Cyanobacteria (blue-green algae) cause most freshwater blooms of public health concern

Cyanobacteria are a type of phytoplankton found in water and moist soil. Although they are not true algae, cyanobacteria are also known as blue-green algae. Blooms of cyanobacteria are more commonly seen in fresh water but sometimes can be found in salt water or brackish water.

Even though cyanobacteria are a type of bacteria, they do not cause harm by infecting people or other animals. Instead, cyanobacteria can cause harm by making toxins or growing too dense. Learn more about illnesses caused by cyanobacteria.

Cyanobacteria cells

Cyanobacteria can be found as single cells or as groups that form threads, balls, or sheets.

Lake's still waters edge covered with green Cyanobacteria

Cyanobacteria can make the water different colors, including green, blue, red, or brown, and look like foam, scum, mats, or paint floating on the water’s surface.

Dinoflagellates and diatoms cause most saltwater blooms of public health concern

Marine (Saltwater) Algal Blooms

Dinoflagellates and diatoms can make the water different colors, including red, brown, or golden.

Blooms caused by dinoflagellates and diatoms are called algal blooms. Dinoflagellates and diatoms are two different types of phytoplankton and are most often found in salt water or brackish water, including in estuaries.

Harmful algal blooms of dinoflagellates or diatoms are often called red tides because they can make the water appear red. Dinoflagellates are the most common cause of algal blooms in salt water.

Dinoflagellates and diatoms can cause harm to people and animals by making toxins or growing too dense. Learn more about illnesses caused by dinoflagellates and diatoms.

Blooms of harmful algae and cyanobacteria occur across the world

Algal and cyanobacterial blooms can grow in fresh water, salt water, and brackish water (a mixture of fresh and salt water) around the world, including in water people use for drinking or recreation. Harmful blooms tend to form in warm water with high levels of nutrients such as nitrogen and phosphorus.

Fresh water
In the United States, cyanobacterial blooms have been found in freshwater bodies in all 50 states, Puerto Rico, and the U.S. Virgin Islands. These blooms more often occur in still water, such as lakes or ponds, but can also happen in rivers or streams. Cyanobacteria cause most freshwater blooms of public health concern.

Edge of land with fresh water covered in cyanobacterial blooms

Marine (salt) water
Blooms in marine environments such as oceans and bays have affected all coastal U.S. states. They are most commonly caused by dinoflagellates or diatoms but can also be caused by cyanobacteria.

cyanobacteria blooming on a salt water beaches edge

Brackish water (mix of fresh and salt water)
Blooms can occur in brackish waters in the United States, including estuaries and coastal waters. Blooms in brackish water are most commonly caused by dinoflagellates or diatoms but can also be caused by cyanobacteria.

Brackish water (mix of fresh and salt water)

Signs of a bloom

Blooms can change how water looks and smells

When in doubt, keep out! Look for signs of a bloom of harmful algae or cyanobacteria before going in water and stay out if water looks or smells bad.

What’s that smell?
As algae or cyanobacteria in a bloom die and start decaying, they can release gases that smell like rotten eggs or rotting plants.

Cyanobacterial (also called blue-green algae) blooms in fresh water

Cyanobacterial blooms can grow on rocks beneath the water. They can also grow on the surface of the water and look like

  • Foam
  • Scum
  • Mats
  • Spilled paint

A cyanobacterial bloom can change the color of the water to

  • Blue
  • Green
  • Brown
  • Yellow
  • Orange
  • Red

Some blooms are easy to spot, but others are hard to see. For example, cyanobacterial blooms are sometimes present below the water’s surface.

Looks can be deceiving.
Some blooms grow on or near the bottom of water bodies (called the benthic zone) like lakes, rivers, and oceans. They can be hard to see because they don’t change how the surface of the water looks.

Some of these blooms make toxins and release them into the water. These blooms can also wash up on the shore. People and animals can get sick from these blooms.

You cannot tell if a water body has a harmful bloom just by looking at it. Find tips for protecting yourself and your loved ones.

Signs of a cyanobacterial bloom include changes in the water’s color and how the surface of the water looks.

Green water from signs of cyanobacterial bloom

Edge of rocks with green water from signs of cyanobacterial bloom

Bright orange water from signs of cyanobacterial bloom

Large rock surrounded with green water from signs of cyanobacterial bloom

Residential dock with brownish water from cyanobacterial bloom

Water and large rocks surrounded by thick green cyanobacterial bloom

Algal blooms in salt water

Algal blooms can change the color of the water to

  • Red
  • Brown
  • Orange
  • Yellow

You can sometimes see other signs of an algal bloom in the water or on the shoreline:

  • Foam
  • Scum
  • Mats of algae
  • Dead fish or other creatures

Signs of saltwater algal blooms include changes in the water’s color and how the surface of the water looks.

dark brown ocean's water with signs of algal blooms

dark brown beach water with signs of algal blooms

Still ocean water and rocks with dark orange water with signs of algal blooms

Orange water from signs of algal blooms

dead fish at the edge of a beach

Land and beach edge with orange colored water from signs of algal blooms

Ecosystem impacts

Algae, cyanobacteria, and their toxins can harm people, animals, and local environments

Ecosystems are made up of living organisms and their physical environment. People, animals, and local environments are all part of an ecosystem. Harmful algae and cyanobacteria are a One Health issue and can affect ecosystems in many ways.

Toxins

Toxins can be in the cells of the phytoplankton or released into the water. Learn more about how people and animals can be exposed to harmful algal blooms and what you can do to protect yourself from toxins.

Common toxins made by

Cyanobacteria

Dinoflagellates

Diatoms

Cyanobacteria

Microcystin
Cylindrospermopsin
Anatoxin
Saxitoxin
Nodularin
Lyngbyatoxin

Dinoflagellates

Brevetoxin
Azaspiracid
Ciguatoxins
Okadic acid
Saxitoxin
Dinophysistoxin

Diatoms

Domoic acid

Dense blooms

If a bloom becomes so dense that sunlight cannot go through, it can block other plants and animals in the water from getting the sunlight they need to survive. Dense blooms can also clog the gills of fish, shellfish, and other animals, preventing them from breathing.

When a bloom dies off, the decay process may use up all the oxygen in the water, causing other organisms in the water to suffocate (not be able to breathe). As a bloom decays, it may also release gases that can harm people, such as methane and hydrogen sulfide (which smells like rotten eggs).

Contributing factors

Many factors can help phytoplankton grow quickly, or bloom:

  • Levels of nutrients such as phosphorus and nitrogen increase in water. Nutrient pollution sourcesexternal icon include:
    • Fertilizer (for example, from home lawns and agricultural land)
    • Sewage from people and animals
    • Run-off from cities and industrial buildings
  • Deep ocean water rises towards the surface and increases nutrient levels. This rising of the water is called upwelling. It can happen along the western U.S. coastline when there are changes in temperature between the ocean and the atmosphere above the eastern Pacific Ocean.
  • Water temperature increases. Blooms are more likely to happen in summer or fall but can occur any time of year.
  • Water flow is low and moves slowly, such as during a drought.
  • Water conditions like pH or turbidity (how much “stuff” is floating in the water) change. When turbidity is low, light can shine through the water easily. This helps phytoplankton grow.
  • There are changes in the local ecology. Ecology is how organisms interact with the environment and with each other.
Man holding a large tube hose that is inside a septic tack
Help prevent harmful algal blooms

Reduce the amount of nutrients such as phosphorous and nitrogen that get in the water:

  • Use fertilizers properly
  • Maintain your septic system

Climate change can affect harmful algae and cyanobacteria

Climate change can increase the growth of harmful algae and cyanobacteria in fresh, salt, and brackish water. It can make blooms occur more often and be more severe. For example, warming temperatures in Lake Erie have contributed to extensive blooms of the cyanobacteria Microcystis aeruginosa that last into the early winter months. In the past several years, such blooms have been found more often and in more places across the United States.

The National Centers for Coastal Ocean Science (NCCOS)external icon are working to monitor and address the impacts of climate change on coastal bloomsexternal icon. NCCOS has documented and monitoredexternal icon blooms regionally in the Gulf of Mexico, the Great Lakes, the Northeast, the Pacific Coast, the Southeast, and the Caribbean/Pacific Islands.

Page last reviewed: April 19, 2021