Harmful Algal Blooms in the Great Lakes

When Blooms Go Bad

Algae are naturally occurring plants that grow in water. When algae grow extremely rapidly in a confined area or grow to the point where you do not need a microscope to see it, it is referred to as an algal bloom. Blooms can be found within most bodies of water throughout the Great Lakes, but they thrive in shallow, warm, non-moving bodies of water like ponds and smaller lakes.

Most algal blooms are harmless, but certain types of algae may pose a risk to humans, animals and water quality. Algae and algal blooms are generally not considered harmful unless they are capable of producing toxins and you come in direct contact with them.

What is a Harmful Algal Bloom?

A Harmful Algal Bloom, or HAB, is a bloom of blue-green algae that potentially contains toxins. HABs can cause fish kills, foul up nearby coastlines and produce conditions that are dangerous to aquatic life, as well as humans.

Blooms can range in color from red to bright, neon green to more blue-green. A bloom can look like a scum, foam or mat on top of the water or like paint that has been spilled in the water. They are also sometimes accompanied by an earthy, pungent or musty smell. However, not all algal blooms give off an odor or affect the appearance water and toxins can remain present in the water even when a bloom has dissipated.

HABs in the Great Lakes region are made up of blue-green algae. While technically not an algae, blue-green algae are a cyanobacteria that contain chlorophyll similar to true algae. They produce rapidly, are typically found at or near the surface of the water and are known to produce toxins.

How Harmful are HABs?

Blue-green algae that form HABs have been known to produce a wide array of neurotoxins, liver toxins, cell toxins and skin irritants. Consumption of large amounts of these toxins by animals or humans can result in muscle cramps, twitching, paralysis, cardiac or respiratory difficulty, nausea, vomiting and liver failure. Skin irritants, found in nearly all blue-green algae blooms, can produce symptoms including skin irritation, rashes and gastrointestinal distress.

The most dominant blue-green algae in the Great Lakes is Microcystis which can produce Microcystin, a liver toxin and skin irritant.

What Causes HABs?

Harmful algal blooms do naturally occur, but have increased since the mid-1990s. Malfunctioning septic systems, products with phosphates like dishwasher detergent or phosphorus/nitrogen in lawn fertilizers, and urban and agricultural runoff are thought to contribute to more frequent HABs. Some scientists also link the increase of harmful algal blooms to the invasion of zebra and quagga mussels in the Great Lakes and the ability of the mussels to filter feed. Essentially, they eat the good algae and phytoplankton but release organisms like blue-green algae back into the water intact.

The harmful blooms often persist for several weeks to a few months, depending on air and water temperature, sunlight, water flow and naturally occurring bacteria levels.

World Health Organization (WHO) Recommended Guidelines for contact with Microcystin

  • Drinking water = 1µg/L
  • Low risk recreational = 2-4µg/L
  • Moderate risk recreational = 20µg/L
  • High risk recreational = avoid visible scums

How can we keep track of HABs?

NOAA’s Great Lakes Environmental Research Lab (GLERL) generates the HAB Tracker, a Lake Erie HAB forecast available in a variety of visual and text-based formats. The forecast looks like a weather map and shows where concentrations of algae are highest. Key information on the HAB Tracker page includes a 5-day surface forecast of HABs, a vertical distribution forecast at 12 specific monitoring stations, and the latest weekly microcystin monitoring data taken at 8 monitoring stations. The page also provides a 5-day forecast of wind and waves for reference, as well as the latest usable satellite imagery of Lake Erie. 



10-742 Harmful Algal Bloom illustration-web

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Factors that can contribute to HABs include:

  • Excess nutrients (e.g. phosphorus or nitrogen)
  • Sunlight
  • Low-water levels or low-flow conditions
  • Calm water (low-wind conditions)
  • Warmer temperatures
  • Zebra and quagga mussels selectively filter feed.

Play it Safe!

If there are visible scums of algae in the water, it might be best to avoid contact with water. Remember: When in doubt, stay out.

  • Whenever you go swimming in a lake, pond, or river always rinse yourself off.
  • Rinse off your pets that have been swimming as well.
  • Avoid drinking water from lakes and rivers, as they may harbor algal toxins or pathogens and bacteria. Boiling the water will not get rid of the toxins.
  • Pay attention to any warning signs posted at the beach.
  • If anyone becomes ill after swimming, seek medical attention immediately. Seek veterinary assistance if a pet appears ill.

Know the signs of HAB poisoning:

  • Humans: numbness of lips, tingling in fingers and toes, dizziness, headache, rash or skin irritation, abdominal pain, diarrhea and vomiting.
  • Pets: weakness, staggering, convulsions, difficulty in breathing and vomiting.

What can you do?

  • Reduce or eliminate your use of fertilizers
  • Eliminate the use of products like soaps and dishwasher detergents that contain phosphates.
  • Wash your car on the lawn so the runoff filters through instead of running straight to the gutters.
  • Consider purchasing or building a rain barrel to cut down on runoff.
  • Properly maintain your septic system.
  • Consider installing a pond aeration system for small ponds and lakes that have had algae blooms in the past.