Aquatic plants abundant this summer in the upper Yahara River

by Jan Axelson and Sheila Leary

If you’ve paddled or viewed the shoreline of the upper Yahara at Cherokee Marsh this summer, you have noticed large numbers of aquatic plants growing on, above, and under the water’s surface.

Perhaps most noticeable has been the filamentous algae, which can form floating mats covering large areas. Filamentous algae feels slimy when wet, but if you grab a handful and squeeze the water out, you’ll be left with thin, cottony strands (filaments). 

Filamentous algae provides food and shelter for insects, fish, and other wildlife. But large mats of the algae are bothersome for paddlers, boaters, and swimmers, and as the algae turns brown and decays, it depletes oxygen in the water. 

Duckweed is a tiny floating aquatic plant sometimes mistaken for algae. Duckweed collects in stagnant water, such as along shorelines. You may see mallards dining on duckweed.

American lotus appears later in summer, first developing large, round, floating leaves. Later, the plant sends up leaves on stalks that rise above the water. In August, large, pale yellow, fragrant flowers bloom on stalks above the water.

Most of the aquatic plants at Cherokee Marsh are native, though a few non-natives such as curly pond weed and Eurasian milfoil are present in small numbers, and non-native cattails line the shorelines.

Cyanobacteria (sometimes called blue-green algae, though it's bacteria, not algae) may appear as a blue or green foam, or with the appearance of "spilled paint" on the water’s surface. Cyanobacteria produces toxins that can result in illness and other health effects in humans, pets, and wildlife. It has been found in Dane County’s waters, so it’s good to keep an eye out for it if you’re in or near the water. 

Exposure to cyanobacteria can occur by skin contact, swallowing water, breathing droplets, and eating contaminated fish. To test if a plant is cyanobacteria, use a stick to carefully lift plant material from the water. If it drips like paint, it may be cyanobacteria. If it looks like strands of hair, it likely isn’t cyanobacteria (unless it looks like black hair! That may be a particular cyanobacterium.) 

The recent increase in aquatic plants is likely due in part to continued phosphorous runoff from farms and development upstream. It's been a rainy summer, and rain events increase runoff entering the lake, bringing waterfowl excrement, pet waste, soil, trash, fertilizer, and other pollutants into our waterways. The area known as Cherokee Lake, along Burning Wood Way in the Cherokee Park neighborhood, was a wetland that was dredged in the early 1960s to create a “lake” for the neighborhood being built. The increase in plant life may be a sign that the lake is returning to wetland.

More information:

Aquatic plants (Clean Lakes Alliance)

Cyanobacterial Harmful Algal Blooms(Wisconsin Department of Health Services)