Question a da week?

[Lee Borgersen]

            Question of the week

Q: What causes many lakes, rivers and ponds to appear brown in the fall?

A: The color of many Minnesota lakes is affected by the growth and abundance of small microscopic organisms 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.

In the fall, as the days get shorter and the lake water cools, algae don't stop growing. Fall conditions, however, often favor specific types of algae, such as diatoms, that tend give the lake or pond's water a brownish hue, due to yellow and brown pigments in the algae.

There are many different types of algae. During mid-summer one particular group, called blue-green algae, are often particularly abundant. When this algal group becomes abundant, a strong musty or earthy odor many also occur. For more information, visit the Minnesota Pollution Control website at www.pca.state.mn.us and type "blue-green algae" in the search box.

Because algae abundance strongly depends on the amount of phosphorus available, the best long-term strategy to prevent algal blooms is to improve land-use and waste-management practices. This helps prevent phosphorus and other nutrients from getting into our surface waters.

- Dave Wright, DNR lakes and rivers unit supervisor