Algae

Julie Klocker, Sauk River Watershed District, 320-352-2231

In late summer, people can become disenchanted with lake living when they see smelly green scum, called “blue-green algae,” taking over their lake. There are many types of blue-green algae. Some are a food staple in parts of the world while others are poisonous to livestock, pets, and humans. They have adapted to live in nearly any type of environment from the hot sulfur springs in Yellowstone to the glaciers in Alaska. They can occur as free-floating individuals or attach to rocks and docks in long strings or thick mats. The free-floating algae commonly produce the green scum we see on lakes, but a similar scum can result from large colonies of mat- or string-forming blue-green algae becoming dislodged and floating to the water’s surface.

What’s blooming?

If these blue-green algae occur in all our lakes, why do they become a problem in only some lakes? And why during the warmest part of the summer? To answer the first question, you need to know that phosphorus and water temperature are usually the main factors influencing algae growth in Minnesota’s lakes. Water bodies having the greatest blue-green algae growth include lakes that contain high amounts of phosphorus (1 pound of phosphorus can produce up to 500 pounds of algae) and/or shallow lakes and bays in which water warms earlier and reaches greater temperatures than in deeper lakes and open water.

To answer the second question, note that algae blooms occur on calm days during the heat of the summer when algae numbers are greatest. In the absence of turbulence that normally keeps free-floating algae at depth, large numbers of algae float to the water surface and create a green scum. Once at the water’s surface the intense solar radiation kills the algae. This die-off is followed by a rapid increase in bacteria that consume dead algae - a process that depletes oxygen in the water and results in a nasty, rotten-egg smell. In addition, low-oxygen water makes it difficult for fish to “breathe.”

What can we do?

We can’t control the water temperature or the wind, but everyone has a part to play in reducing phosphorus entering the lakes. You can help by eliminating phosphorus fertilizer use (it’s now Minnesota law), preventing soil erosion, and keeping leaves, grass clippings, and pet wastes from washing into lakes. These are small steps that can help control the smelly, green scum that we see so often on lakes in late summer.

Posted in Care of Shoreland Property, Lakes |