La Crosse River Marsh and Duckweed - The Stewards of Water Blog

La Crosse River Marsh and Duckweed

 Duckweed. Image credit: Duckweed. Image credit:

     Duckweed, algae and invasive plants have slowly, or perhaps not so slowly, covered La Crosse River marsh this summer season, and though recent storms have washed away the majority of the plant matter, residents are wondering why it was so bad this year, and if it will be the same next year.

     The main culprits are duckweed and filamentous algae, and while duckweed is an important source of food for waterfowl and other animals, that's only true when the duckweed covers around 20% of the water's surface. Beyond that point, duckweed blocks sunlight from penetrating beneath the surface, and also leads to a lack of oxygen in the water when the biomass begins to decompose near the end of the season (which, in extreme cases, can lead to large fish kills). As well, once decomposed the matter settles on the bottom of the water column, slowly filling the water body and impeding water quality.

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