Aquatic life in Forest Lake – The good, the bad, and the ugly

It was a beautiful night for Arts in the Park; the air was cool, the storm clouds had passed, and a Beatles tribute band was rocking the pavilion. My Forest Lake friends – a damselfly larva and a crawling water beetle – were swimming to the beat of the music in a container full of water I’d scooped from the shoreline earlier that evening. When the damselfly eventually crawled out of the water and perched on the edge of the tray, I assumed it was resting. By the time I packed up to leave, however, I was surprised to discover that the little insect had metamorphosed in the span of an hour. Gone were the feathery gills, stretching behind its body like a three-pronged tail; instead, the damselfly had sprouted gossamer wings to fly and left a shell of its former self lying limp by the edge of the water.

A newly emerged damselfly dries its wings beside its discarded exoskeleton. Damselflies and dragonflies are some of the many insects that lay their eggs in water and spend their larval stages in the water as well.

Damselflies are one of many insect species that lay their eggs in water and spend the first half of their lives as aquatic animals. Other examples include dragonflies, mosquitoes, stoneflies, mayflies, and caddisflies. Biologists can learn a lot about the health of a lake or stream by studying the insects and other aquatic invertebrates that live there. Some aquatic insects, such as caddisfly larva, can only be found in clear and clean streams, while others, like mosquitos, will hatch in just about any puddle or pond. In general, the wider the variety of aquatic critters, the healthier the water and the more fish and wildlife it can support.

With a surface area of 2,220 acres, Forest Lake is the largest lake in Washington County and a treasured resource within the Comfort Lake – Forest Lake Watershed District. The lake has the healthiest fish community of all lakes in the watershed, with relatively low numbers of rough fish and healthy populations of predator and pan fish. In addition, Forest Lake also supports a diverse native aquatic plant community, which provides food and habitat for fish and other wildlife. The fish, aquatic insects, native aquatic plants, turtles, frogs and birds that call Forest Lake home are the kinds of aquatic life that we biologists like to see.

In recent years, a number of non-native aquatic invasive species have found their way into Forest Lake as well. The four species causing the most problems in the lake are curly leaf pondweed, Eurasian watermilfoil, flowering rush, and zebra mussels. These are the kinds of aquatic life we don’t like to see.

Curly leaf pondweed and Eurasian watermilfoil grow underwater in large patches, tangling on boat propellers and making it hard to swim. Curly leaf begins growing under the ice early in the spring and then dies in the middle of the summer, releasing a pulse of phosphorus into the water that can lead to algae growth. Flowering rush grows along the shoreline, crowding out beneficial native plants and making it hard for people to access the shoreline. In recent years, the Comfort Lake – Forest Lake Watershed District has worked with the Forest Lake Lake Association and Washington County to manage these three species through manual pulling and chemical treatments.

Invasive Eurasian watermilfoil (left) has 12 to 21 pairs of leaflets and grows into dense thickets in shallow water. Native northern watermilfoil (right) has only 5 to 10 pairs of leaflets and grows in sparse patches. The native milfoil provides good habitat for fish.

Most recently, zebra mussels were introduced to the lake in 2015. The mussels spread quickly and attach themselves to boat motors, rocks, docks, ladders, and water intake pipes, creating a nuisance for swimmers and boaters and disrupting the natural food web. One of the impacts of zebra mussels in Forest Lake has been clearer water in 2017 and 2018. The mussels are filter feeders that eat tiny food particles in the water, which can cause a ripple-effect within the lake’s ecosystem. On the upside, clearer water leads to better swimming conditions for people. On the downside, clearer water also makes it easier for aquatic plants to grow.

ZebraMussel - photo from Echo Lake Aquarium
Non-native, invasive zebra mussels attach to native mussels, as well as boats, lifts, docks, rocks, and pipes. (Photo from Echo Lake Aquarium)

This summer, many Forest Lake area residents have noticed fluorescent green globs floating in the water, especially in 2nd Lake. According to Steve McComas, an aquatic scientist who frequently works with the watershed district, the blobs are filamentous algae, a non-toxic green algal species. The clearer lake water has allowed more sunlight to penetrate and spur the algae’s growth. “Nutrient pulses are not the probable cause, this is a natural outcome of slightly improving water clarity due to zebra mussels in Forest Lake,” he explains. McComas expects the filamentous algae to show up in 1st and 3rd Lakes as well, but predicts it will fade away by mid-August. Luckily for us, though the algae looks gross, it is harmless.

The fluorescent green blogs underwater in Forest Lake’s 2nd Lake are masses of filamentous algae, a non-toxic green algal species.

Boaters can help to prevent the further spread of aquatic invasive species in Minnesota by cleaning, draining, and drying boats and other equipment after each use and disposing of unwanted bait, including minnows, leeches, and worms, in the trash. Learn more at

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