For years, nutrient runoff from agriculture has been Minnesota’s #1 water quality problem, polluting potable groundwater with nitrate and causing widespread eutrophication of lakes and rivers. With 51% of MN land in agricultural use, the threat is widespread and shows little sign of easing up. It’s long been known that unnaturally high loading of nutrients causes harmful algal blooms that smell bad, kill fish, and prevent people and animals alike from enjoying water resources.
What’s just now being discovered is that zebra mussels, one of the most tenacious aquatic invasive species, may have a compounding effect on farm pollution. Lake Mendota in Wisconsin, which first discovered mussels 2 years ago, has seen severe blue-green algae blooms this year that killed thousands of fish in Lake Mendota and the Yahara River.
Stephen Carpenter, the director of the limnology center at UW Madison, said he was “shocked to see the bright green hue stretching as far as he could see” across the lake. Other scientists reported seeing fish gasping for breath on the surface or invertebrates crawling out of the water and dying.
In 2015, only a few zebra mussels were found in Lake Mendota. Now, they cover a significant portion of the lakebed, filter-feeding enough water to actually change their own ecosystem. The effect is twofold— By “cleaning” the lake of all food particles, the water grows clear enough to allow more light at the bottom. This promotes growth of nuisance vegetation that frequently washes up in foul-smelling piles on the beach.
Additionally, mussels DON’T eat the harmful bacteria that cause algal blooms, but they do eat nearly everything else. This effectively creates the perfect environment for the cyanobacteria, who suddenly have no food competition— Limnologists at UW Madison suspect that it is this effect in tandem with high nutrient loading that has caused “the worst… outbreak in decades”.
These cyanobacteria outbreaks aren’t just unsightly. They are toxic to humans and animals, causing rashes or illness and even killing several dogs over the years. The high density of bacteria removes oxygen from the water, suffocating fish and other aquatic animals, turning the lake into a “dead zone”. These dead zones, aside from killing thousands of fish in the last outbreak alone, have lead to hundreds of Madison beach closures in the last several years, with 34 so far this year.
Scientists anticipate the blooms to continue throughout summer— “Next time we have a week to 10 days of hot weather we may see another bloom”, said Carpenter, adding that it’s still very early in the summer.
Zebra mussels, once introduced to a lake, are nearly impossible to completely eradicate. This new information about the potential augmenting effects of mussels on farm pollution makes it more important than ever to keep our lakes and rivers free of invasive mussels.
The most important thing to remember is that it’s up to you, the user, to make sure your boats are clean, your bait containers are empty, and that you aren’t carrying any excess water anywhere in your watercraft. As a boater, you truly hold the power to stop the spread of zebra mussels and save our lakes, rivers, and streams. Cleaning, Draining, Drying, and Disposing of bait are the proven best management practices, and at CD3 Station, we’ve developed an innovative and easy to use product to help you do just that.