Dead fish and green water?

Explanations for recent phenomena observed in Forest Lake

In recent weeks, several people have contacted the Comfort Lake-Forest Lake Watershed District (CLFLWD) to report fish kills and algal growth observed in Forest Lake. Read on to learn what’s behind these recent phenomena.


Fish Kill:

After conversation with the Minnesota DNR, watershed district staff have concluded that fish in Forest Lake may have been impacted by a bacterial disease called Columnaris, which often kills fish during the spring and early-summer. The bacteria is always present, but only becomes an issue when certain environmental and biological conditions exist. Columnaris is exacerbated by drought and a sudden influx of organic material being washed into a lake after an extended dry period, and is most problematic when water temperatures are between 65 and 70F. During the spring and early summer, fish’s immune systems are weakened due to hormonal stress associated with spawning, making them more susceptible to bacterial infection as well.

This spring, fish kills have also been observed nearby at Big Marine Lake. Fortunately, Columnaris usually only affects a small percentage of fish in a lake and does not affect the survival of the overall fish population.

Not all green is a sign of bad water quality. On the left, the water is filled with blue-green and filamentous algae. In the center, the water is clear but covered in duckweed. On the right, the water is clear but has aquatic plants growing below the surface of the water.

Green Water:

To the casual observer, green water is usually considered an indication of poor water quality. However, that is not always the case.

Sometimes a lake appears to be green because it is covered in duckweed, a beneficial native plant that floats on top of the water. From a distance, a duckweed covered pond or lake may look green and slimy, but if you look at the water up close, you can actually distinguish little tiny leaves floating on the water’s surface and tiny roots dangling down into the water. As the name implies, duckweed is a good food source for ducks and other waterfowl.


Filamentous green algae can make swimming and boating unpleasant but is not harmful to humans and wildlife. The filaments often tangle together to form stringy, hair-like strands or slimy, green mats that float on the water’s surface. Filamentous algae are a natural part of Minnesota lakes and provide cover for small aquatic insects and animals that fish eat. Chara, a form of filamentous algae found in lakes with good water quality, has long, stringy strands and looks like a plant without roots. The life cycle of filamentous algae is 30 to 60 days.

The biggest concern for human and aquatic health is blue-green algae, which usually looks like pea soup or spilled green paint in the water. Though small amounts of blue-green algae exist in all of our lakes and rivers, these algae can quickly multiply into large colonies during the summer, creating blooms that are sometimes toxic to people and animals. Algae blooms are also problematic if they grow large enough to cover an entire lake because the algae consume oxygen during the night to fuel their growth when sunlight is not available. As a result, dissolved oxygen levels in the water plummet and fish die.


The best way to limit algae growth in Forest Lake and other local lakes is to reduce the amount of phosphorus available in the water. Yard waste like lawn clippings, leaves, and seeds contains high levels of phosphorus that can be washed into our lakes via storm drains when it rains. You can help to keep our lakes blue by sweeping dirt and yard waste off of your sidewalk, driveway and curb-line throughout the year and adopting your local storm drain:

If you live on one of the lakes, you should also maintain a buffer of native plants along your shoreline, inspect your septic system regularly, and consider building raingardens to capture and filter stormwater runoff from your house and driveway.

To learn more about water quality in Forest Lake, visit or follow the watershed district on social media @clflwd.

You can help to protect lakes and rivers by adopting a storm drain near you and committing to clean up litter and yard waste that would otherwise wash into our waterways.