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Harmful Algal Blooms


The term algae refers to a diverse group of aquatic plant life, which ranges from single-celled organisms to seaweeds and kelps. Algae are an essential part of fresh and saltwater environments by providing a food source for a wide variety of fish and macroinvertebrates. Additionally, half of the oxygen produced on Earth is from microscopic single-celled algae known as phytoplankton. Phytoplankton require nutrients (primarily phosphorus and nitrogen) and sunlight to produce oxygen in a process known as photosynthesis. Unfortunately, increasing levels of nutrients in water can promote the growth of cyanobacteria, photosynthetic bacteria which is also known as “blue-green algae”. Cyanobacteria can grow and spread at alarming rates in aquatic environments and these instances of unrestricted growth are known as Harmful Algal Blooms (HABs).

Harmful Algal Blooms

HABs form when excess nutrients are available in combination with warm temperatures and high amounts of solar radiation (sunlight). While nutrients, specifically phosphorus and nitrogen, are essential building blocks for aquatic life, excessive amounts can be extremely harmful and result in nutrient pollution. Nutrient pollution in combination with hot summer conditions and slow-moving water are the ideal circumstances for harmful algal blooms.

HABs can out compete native plants for resources, increase bacterial growth and produce dangerous toxins that can affect and even kill people and animals. On top of that, when the algae die and decompose, oxygen levels in the water are depleted resulting in hypoxic (low-oxygen) or anoxic (zero-oxygen) conditions. These conditions are deadly to fish and other aquatic species that need oxygen to survive. The Gulf of Mexico’s “Dead Zone” is an extremely large area of hypoxic/anoxic conditions that occurs at the mouth of the Mississippi River – as a result almost no marine life is able to live in the Dead Zone.

HABs have deleterious effects on environmental health that directly lead to economic impacts such as decreases in ecotourism, elevated water treatment costs, reduction in commercial fishing, and lower property values. For example, the National Oceanic and Atmospheric Administration (NOAA) estimated that the HAB on Lake Erie in 2014 resulted in $65 million impact to the economy and nationwide HABs cost the U.S. seafood and tourism industries $82 million a year.

Nutrient Pollution

Both algae and nutrients are essential to aquatic environments. Nutrients support the growth of algae, which is an important food source for fish and other marine animals. However, excessive amounts of nutrients, specifically nitrogen and phosphorus, lead to nutrient pollution and conditions that are ideal for HAB growth. Nutrient pollution can be sourced from several inputs, such as agriculture, industrialization, urban development, atmosphere and even groundwater. While both natural and anthropocentric actions have contributed to this issue, it is known that nutrient pollution from human activities have increased HABs.

Nutrient Pollution Infograph

A graphic displaying how nutrient pollution assimilates and how it drives algal blooms (Photo source: Paerl Lab).

EPA Fact Sheet of Nutrient Pollution

An infographic defining nutrient pollution, where it comes from and the impacts of it within the United States (Photo source: EPA).


Everywhere! Nutrient pollution and HABs affect air and water across the entire country. According to the EPA more than 100,000 miles of rivers and streams, around 2.5 million acres of lakes, reservoirs and ponds, and over 800 square miles of bays and estuaries just in the United States are affected by nutrient pollution and vulnerable to HABs.

A new survey done by the NRDC mapped HAB events across the U.S. between 2008 and 2018. Currently, 16 states, including Tennessee, do not collect data on HABs – hence the reason TN is grayed out on the map below. Without active monitoring programs it is impossible to know when and where HABs occur or if they are producing dangerous toxins.

NRDC Survey Map

An image of all the mapped HABs between the years of 2008 and 2018 created by the NRDC.


HABs are not only present in the United States, but all over. Below are a few pictures of some of the notable HABs worldwide.

HAB in China

A picture of a HAB on the coast of Lake Taihu in China (Photo source: Oregon State University)

North Sea HAB

A satellite image released by NASA in 2015 of the algae blooms in the North Sea (Photo source: NASA). 

Lake Erie HAB

The National Oceanic and Atmospheric Administration released satellite data of Lake Erie's harmful algal bloom in 2019 (Photo source: NOAA).

Florida HAB

A photo from the USGS of an aerial view of Lake Okeechobee in Florida showing an algae bloom. Harmful algal blooms remain a pressing problem here and throughout many other Gulf Coast states. (Photo source: AL article).

How to Recognize a Harmful Algal Bloom

Look for the colors green, red, blue or brown. HABs can look like spilled paint and/or spotted greenish dots on the surface of the water. They can also have a bad odor. Below are two pictures of harmful, toxin producing algal blooms.

A picture of a Harmful Algal BloomA picture of a Harmful Algal Bloom

Below are two pictures of non-toxic green algae for comparison. Healthy algal blooms will have a floating raft or stringy/hair-like appearance like pictured below.

A picture of healthy algae  A picture of healthy algae

Things YOU can do to Help

1. Test your soil before fertilizing it. You can use our soil testing kits to make sure you are not adding in excessive amounts of nutrients. This will help you avoid nutrient polluting and save you money!

2. Report any signs of HABs. You can do this by downloading the Water Reporter App and posting anytime you see potential harmful algal blooms or any other problematic situations.

3. Stay away from phosphorus heavy detergents, soaps, cleaners, etc. and be sure to use the appropriate amounts. Always try to avoid using too much of any detergents, soaps, or cleaners.

4. Always pick up after your pet!

5. Use less water and energy. Turn off the lights and appliances around the house, take shorter showers, etc.

For more suggestions and details on what you can do in your home to help please visit this site!

Additional Resources

For more additional information on HABs and what the EPA is doing to alleviate this rising problem please visit the EPA's web page on this topic.

If you are interested in learning more about associated illnesses and risks regarding this topic, visit the Centers for Disease Control and Prevention's web page.

The CDC also provides an online fact sheet of more details and information behind HABs and what impacts it can have on both humans and animals.