Mary Blickenderfer, University of Minnesota Extension Service, 888-241-0885

At the base of the food web, algae support nearly every aquatic creature. They are essential to a diverse and productive fishery and the overall health of our lakes. Many species of algae occur in lakes. The exact species and their population within a given lake reflect the available nutrients, water clarity, temperature, acidity, time of year, and abundance of algae grazers.

Many Minnesota lakes have algae “blooms” – the mats of vegetation or “pea soup” green water that occur on hot, calm days. On rare occasions blue-green algae blooms can produce toxins that are harmful to fish and other animals, including cattle and dogs.

Algae “blooms” occur under conditions that favor algae growth or when algae grazers are scarce. Turn up the water temperature and add some phosphorus and you have the perfect recipe for algae soup! The small amount of phosphorous that naturally occurs in our lakes is usually insufficient to support large algae blooms. However, phosphorus entering our lakes from the surrounding watershed (the large land area that drains to a lake) or resuspension of phosphorus that has settled on the lake bottom will fuel algae blooms – under optimal conditions, additions of only one pound of phosphorus can lead to 500 pounds of algae!

Fishing pressure on a lake can add to the problem. The saying, “tug on one part of the food web and you’ll affect all the other parts” holds true. Excessive removal of northern pike, walleye, bass, and other game fish from a lake affects populations of small fish and grazers and can ultimately lead to a greater abundance of algae.

The most cost-effective strategies that produce long-term results involve reduction of phosphorous inputs to a lake. Phosphorus commonly enters a lake attached to soil particles, dissolved in runoff, in seepage from failing septic systems and through resuspension of lake bottom sediments. On-land strategies to reduce phosphorus loading to your lake include maintaining septic systems, planting vegetative buffers along streams and lakes, and re-routing runoff into rain gardens and stormwater ponds. In-lake strategies to reduce phosphorus re-suspension include maintaining or restoring the native aquatic plant population, removing/ controlling carp (if they exist in your lake), reducing motorboat speed in shallow water and eliminating other activities that “stir up” sediments.

Lakes with high phosphorus levels will benefit from the strategies listed above, but may also require additional efforts to reduce existing phosphorus. These are best determined with the assistance of a limnologist or lake consultant (not a product sales representative). Your local Dept. of Natural Resources (DNR) office may provide direct assistance or help you find a consultant. Examples of treatments to consider are: phosphorus inactivation, sediment removal, artificial circulation, algae harvesting, foodweb manipulation, and algacides. Keep in mind, implementation of these treatments will require planning, substantial funding, and may require a Minnesota DNR permit. Depending on the method used, repeated treatments are often necessary, some may have negative impacts on a lake, and none of them alone will be affective in the long-term restoration of your lake unless phosphorus inputs from watershed and in-lake activities are also controlled.

Want to know more?

Information on the Web:

• http://www.extension.umn.edu/water/
• http://www.shorelandmanagement.org
• http://www.extension.umn.edu/distribution/naturalresources/DD7040.html

For permit information:

• http://www.dnr.state.mn.us/ecological_services/pubs_regulations.html

Posted in Lakes |