Algal Blooms

What are algae?

Algae are microscopic organisms that live in aquatic environments and use photosynthesis to produce energy from sunlight, just like plants. Algae can be found in all types of natural waters, including salt water, fresh water, and brackish water (a mix of salt and fresh water).

A few types of algae produce toxins. In these algae, toxin production can be stimulated by environmental factors such as light, temperature, salinity, pH, and nutrient levels. Algal toxins released into the surrounding water or air can seriously harm people, animals, fish, and other parts of the ecosystem.

What is a harmful algal bloom?

A harmful algal bloom (HAB) occurs when toxin-producing algae grow excessively in a body of water. The excessive algal growth, or algal bloom, becomes visible to the naked eye and can be green, blue-green, red, or brown, depending on the type of algae.

Some blooms are easy to spot, but others are hard to see because they grow near the bottom of water bodies. You cannot tell if a water body has a harmful bloom just by looking at it.

Why do HABs occur?

Many factors may contribute to HABs but understanding how these factors come together to create a harmful bloom of algae is a topic of ongoing research. Scientists know that certain environmental conditions, such as warmer water temperatures in the summer and excessive nutrients from fertilizers or sewage waste brought by runoff, trigger HABs, but they are still learning more.

As climate change gradually warms the earth’s climate, scientists expect HABs to become more frequent, prolonged, and severe, and in different geographic areas.

What are impacts from HABs?

HABs can damage the environment by depleting oxygen in the water, which can kill fish and other living creatures. HABs that bloom near the water surface can also block sunlight from reaching organisms deeper in the water.

The economic impacts of HABs to fisheries and recreational areas can be extensive. Businesses lose significant revenue if they are forced to close or reduce operations due to HABs.

Some HABs are associated with serious health effects, as explained below.

How are people exposed?

During an HAB, people can get exposed to toxins from fish they catch and eat, from swimming in or drinking the water, and from the air they breathe. In recent years, there have been numerous instances of HABs in lakes that provide drinking water, like Lake Erie. Importantly, cooking contaminated seafood or boiling contaminated water does not destroy the toxins. NIEHS research has helped us better understand how toxins accumulate in seafood and are later absorbed by our bodies.

People can get sick from eating seafood contaminated with HAB-related toxins. But it rarely happens from eating commercial seafood because state regulators monitor fisheries for HABs and close them during blooms.

Illness from HABs can be prevented by following local health advisories on the safety of recreationally caught seafood and drinking water sources.

What are the health effects of harmful algal blooms?

Depending on the type of algae, HABs can cause serious health effects and even death. For example, eating seafood contaminated by toxins from algae called Alexandrium can lead to paralytic shellfish poisoning, which can cause paralysis and even death. The algae Pseudo-nitzschia produces a toxin called domoic acid that can cause vomiting, diarrhea, confusion, seizures, permanent short term memory loss, or death, when consumed at high levels.

HABs that occur in freshwater, like the Great Lakes and other drinking water sources, are dominated by the cyanobacteria Microcystis. This organism produces a liver toxin that can cause gastrointestinal illness as well as liver damage.

As with many environmental exposures, children and the elderly may be especially sensitive to HAB toxins. Populations that rely heavily on seafood are also at risk of long-term health effects from potentially frequent, low-level exposures to HAB toxins.

The table below summarizes common HABs and their health effects.

Organism	Water Type	Color	Toxin	Health effects
Alexandrium sp.	Salt	Red or brown	Saxitoxins	Gastrointestinal (nausea, vomiting), and neurological (a floating sensation, headache, or muscle weakness)
Cyanobacteria	Fresh	Blue-green	Cylindrospermopsin	Nausea, vomiting, diarrhea, abdominal tenderness, pain, or acute liver failure
Gambierdiscus	Salt	Orange	Ciguatoxins	Nausea, vomiting, diarrhea, or stomach pain
Karenia brevis	Salt	Red	Brevetoxins	Gastrointestinal illness, muscle cramps, seizures, paralysis Respiratory problems, especially for asthmatics
Pseudo-nitzschia	Salt	Red or brown	Domoic acid	Vomiting, head weaving, nausea, seizures, diarrhea and abdominal cramps, bulging eyes, or headache
Microcystis	Fresh	Blue-green	Microcystin	Gastrointestinal illness, liver damage