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Harmful Algal Blooms: Basics

What are harmful algal blooms (HABs)?

Green WaterHarmful algal blooms occur when normally occurring cyanobacteria in the water multiply quickly to form visible colonies or blooms. These blooms sometimes produce potent cyanotoxins that pose serious health risks to humans and animals.

Cyanobacteria, also known as blue-green algae, aren’t actually algae, they are prokaryotes, single-celled aquatic organisms that are closely related to bacteria and can photosynthesize like algae. These microorganisms have been a natural part of aquatic ecosystems for thousands of years. Depending upon the species, cyanobacteria can appear as single cells, filaments of cells, or colonies. Their characteristic pigment (“cyan”) gives them their blue-green color, although they can also appear as blue, green, red, or brown in the water.

Although most algal blooms are not toxic, some types of cyanobacteria produce nerve or liver toxins. Toxicity is hard to predict in part because a single species of algae can have both toxic and non-toxic strains, and a bloom that tests nontoxic one day can be toxic the next.

What causes harmful algal blooms (HABs)?

The conditions that lead to harmful algal blooms (HABs) generally occur in the late summer and early fall. These include:

  • High nutrient levels, particularly phosphorus
  • Abundant sunlight
  • Warm water temperatures
  • Stagnant or slow-moving waters 

If these conditions are present for several days, cyanobacteria can multiply to form large blooms that can cover an entire lake or collect in smaller areas. Blooms generally die and disappear after one or two weeks. However, if conditions remain favorable, overlapping blooms can occur over the course of several months, giving the appearance of one continuous bloom. Algal toxins can linger for days after a bloom has dissipated, and depending on the type of cyanobacteria present, can even increase as toxins are released from dying cells.

Why do blooms sometimes seem to appear overnight?

Because algal blooms can be suspended at different depths in the water depending on the availability of light and nutrients, they are not always visible from the surface. Cyanobacteria can sink and float in response to changing conditions (weather, time of day, etc.), allowing them to move to areas where the light and nutrient levels are highest. At night, they may move to the surface, where they “suddenly” appear the next day as pond scum.

What kinds of toxins are produced by harmful algal blooms?

The poisons that are produced naturally inside the cells of certain species of cyanobacteria are called cyanotoxins. When the cells are broken open, the toxins may be released. In many species, the toxins remain within the cells while the bloom is growing and are only released into the water when cells die or rupture. Other species release the toxins during both their growth period and cell death.

Some of these toxins attack the liver (hepatotoxins), others target the nervous system (neurotoxins), while still others irritate the skin (dermatoxins). The cyanotoxins most often detected in blooms in the United States are:

Other cyanotoxins found in U.S. waters include Nodularin and Lyngbyatoxin a.

How can I tell if a bloom is toxic?

Unfortunately, you can't tell if a bloom is toxic just by looking at it. Because it is hard to tell whether an algal bloom is harmful or not, we recommend avoiding contact with any floating mats, scums, and discolored water. For a photo gallery of green and blue-green algal blooms, visit the Utah Department of Health website or New York Department of Environmental Conservation website.

What can be done to prevent harmful algal blooms?

Green WaterExcess nutrients, particularly phosphorus, in waterbodies can trigger algal blooms. Discharges from wastewater treatment plants, runoff from agricultural operations, and stormwater runoff can carry nitrogen and phosphorus into waterways and promote the growth of cyanobacteria.

While not all blooms produce toxins, nutrient loads appear to influence the types of cyanobacteria that dominate individual blooms. You can do your part to improve water quality by taking the following steps:

  • Reduce the amount of fertilizer you use on your lawn.
  • Use only phosphorus-free fertilizer when possible.
  • Fix leaking septic systems.
  • Use only phosphorus-free detergents in dishwashers.
  • Keep yard debris such as leaves or  grass clippings from washing into storm drains
  • Pick up pet waste

Reducing nutrient loads to waters is the best way to limit the occurrence of harmful algal blooms. Please visit DWQ’s Nutrient Reduction website for more information on Utah’s efforts to reduce nutrient pollution in our waters.

For more information, visit: Wisconsin Department of Natural Resources, the New York Department of Environmental Conservation, Oregon Health Authority, Washington Department of Ecology, and the U.S. Environmental Protection Agency.