Harmful Algal Blooms: Contributing Factors and Impacts

Key points

  • Harmful algal blooms can hurt local ecosystems, including people, animals, and plants.
  • Harmful algal blooms are more likely to grow in warm, slow-moving water with many nutrients.
  • Help prevent harmful algal blooms from growing by using fertilizers properly and maintaining your septic system.
Two ducks swimming in red-orange, algae-covered water


Algae are an important part of ecosystems in fresh waters, salt waters, and brackish (a mix of fresh and salt) waters. However, under the right conditions some types of algae can be harmful when they grow too quickly or make toxins.

Terms to know

Ecosystems are made up of living things and their physical environment. People, animals, plants, and local environments are all part of an ecosystem.

Contributing factors

Many factors can help harmful algal blooms grow:

High nutrient levels

Too many nutrients, such as nitrogen and phosphorus, in water help harmful algal blooms grow.

Nutrient pollution in water can come from:

  • Fertilizer (used on farms and home lawns, for example)
  • Sewage or poop from people and animals
  • Run-off from cities and industrial buildings (during rainstorms, for example)

Deep ocean water rising towards the surface also increases nutrient levels. This rising of water is called upwelling. It can happen along the western U.S. coast when there are changes in temperature between the ocean and the air above the eastern Pacific Ocean.

Warm water

Harmful algal blooms are more likely to grow in warm water. Blooms happen more often in summer or fall but can occur any time of year.

Slow-moving water

Harmful algal blooms are more likely when water levels are low and water is moving slowly, such as during a drought.

Changing conditions

Changes in water conditions like pH or turbidity (how much "stuff" is floating in the water) can impact the growth of harmful algal blooms. For example, when turbidity is low, light can shine through the water easily. This helps algal blooms grow.

Changes in the local ecology can also impact the growth of harmful algal blooms. Ecology is how living things interact with the environment and with each other.

Climate change

Climate change can increase the growth of harmful algal blooms 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 large harmful algal blooms of cyanobacteria (blue-green alage) that last into the early winter months. In recent years, such blooms have been found more often and in more places across the United States.

Researchers are still learning how climate change is affecting harmful algal blooms and how different parts of the country are impacted.

Learn more about climate change and harmful algal blooms on the U.S. Environmental Protection Agency's (EPA) website and the National Oceanic and Atmospheric Administration's (NOAA) website.

Heron standing in a pond covered in streaky algae. The pond is surrounded by grass and homes.
Harmful algal blooms tend to grow in warm water with high levels of nutrients.

Ecosystem impacts

Algal blooms can harm people, animals, or the environment when they:

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


Producing toxins is the main way harmful algal blooms make people and animals sick. Toxins can be in the cells of the algae or released into the water.

People and animals get sick when they come into contact with toxins through water or food.

Dense blooms

Sometimes algal blooms become so dense that sunlight cannot go through them. This blocks 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.

Decaying blooms

When a bloom dies off, the decay process may use up all the oxygen in the water. Without oxygen, other living things in the water suffocate (are not able to breathe). As a bloom decays, it may also release gases that can harm people. These gases include methane and hydrogen sulfide, which smells like rotten eggs.

Not all algal blooms are harmful‎

Algal 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 make toxins. However, they can discourage people from visiting beaches, drinking tap water, or eating fish from water with an ongoing algal bloom.

You cannot tell if an algal bloom is harmful by looking at it. Testing the water for toxins is the only way to know for sure if an algal bloom could make you sick. If water has signs of a harmful algal bloom, it is safer to stay out.

What you can do

You can help prevent harmful algal blooms from growing by limiting nutrient pollution in water. Even if you do not live right next to a body of water, the nutrients from your home and yard can make their way into nearby lakes, rivers, and oceans. For example, extra fertilizer on your lawn can wash off during rainstorms and run downhill into a nearby stream.

Reduce the amount of nutrients that get into water by: