DNR Q & A: What causes many lakes, rivers and ponds to turn green by mid-summer?

Minnesota Department of Natural Resources field staff, resource managers and the DNR Information Center staff answer many questions every day about natural resources topics. Here is one of them:

Q.  What causes many lakes, rivers and ponds to turn green by mid-summer? Some even have an odor.

A. By mid-summer many waterbodies turn green due to the growth of small microscopic plants in the water called algae. Algae grow in all bodies of water when light and nutrients levels are sufficient.

In many lakes, algae abundance is determined by the amount of phosphorus dissolved in the water. The more phosphorus present, the more abundant algae become and the greener the water gets.

There are many different types of algae. During mid-summer one particular group of algae, called blue-green algae, are often particularly abundant. When this algal group becomes abundant, a strong musty or earthy odor many occur. Algae that have died and are decomposing cause the odor. Because algae abundance strongly depends on the amount of phosphorus available, the best long-term strategy is to improve land-use practices to prevent phosphorus and other nutrients from getting into our lakes and ponds.

— Dave Wright, DNR lakes and rivers unit supervisor

For more information, call the DNR information line at 296-6157 or go to the DNR Web site at www.dnr.state.mn.us.


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