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Why does this lake look like pea soup?

Barb Liukkonen and Mary Blickenderfer

If your lake or pond is covered with mats of green or looks like "pea soup" in July or August, it may be from an algae "bloom." Algae are single-celled organisms that support the entire food chain above them - right up to the fish you want to catch. Algae are necessary to a healthy lake and a healthy fishery, but too many algae can be difficult to live with.

Algae populations depend on many factors including available nutrients, temperature, and the number of fish or other animals that feed on algae. Phosphorus is the nutrient of most concern in Minnesota because the growth of aquatic plants in our lakes is usually controlled by how much phosphorus is available. Under the right conditions, a single pound of phosphorus can lead to 500 pounds of algae!

To reduce the phosphorus reaching your lake, change your practices on the land. Lawn and garden fertilizer, leaves and grass clippings, soil, and wastewater all contain nutrients and all help turn your lake green. Leave an un-mowed buffer strip between your lawn and the lake; never rake leaves or clippings into the water; maintain your septic system properly; and eliminate erosion and soil runoff. These actions will reduce the phosphorus coming from your shoreland property.

Occasionally blue-green algae blooms can produce toxins that are harmful to fish and other animals, including cattle and pets. In Minnesota several dogs have died from swimming in waters with blue-green algal blooms. If the water looks like pea-soup or smells swampy, it may be from blue-green algae. If you, your children, or your pets come in contact with blue-green algae, wash thoroughly as soon as possible, and keep dogs from licking the algae off their fur.

Treating lake water to reduce algae blooms is usually not practical, may be expensive, and can pose risks to fish and other aquatic life. For assistance and the permits required for treatment of algae, contact the MN Department of Natural Resources, Section of Fisheries at 888-MINN-DNR.

DNR permits & regulations

Reviewed by Karen Terry, 2008

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