Toxic Blooms: Algae in Our Lakes

Have you noticed that Madison’s lakes and waterways have been especially green and soupy lately? Recent heavy rains and hot temperatures have contributed to a massive bloom of toxic blue-green algae around Lake Mendota, Lake Monona, the Yahara River and other parts of our watershed. How did this happen?

UW-Madison Center for Limnology

Wisconsin is known for its delicious dairy products, fresh produce, and exceptional farmers’ markets — but this close connection with farming means that our soils and waterways receive loads of fertilizer and manure from surrounding farms. While good for producing lots of crops, when heavy rains wash this phosphorus into waterways, it over-feeds plants in the water, changing our lakes and rivers.

Tiny cellular plants and photosynthetic bacteria, collectively known as algae, feed on these nutrients in the water and especially thrive in warm weather. When the conditions are right, they “bloom” into huge colonies, turning the water a bright green or turquoise color. Not only are these blooms toxic to humans, pets, and wildlife, but when the masses of algae die off, decomposition microbes use up the oxygen in the water, suffocating fish and other aquatic critters. Our lakes and rivers become “dead zones.”

From aquatic insects to crayfish to fish to this young duckling, the bloom spared very little that was in its path. Photo: Jake Vander Zanden, UW Limnology

This spring and early summer have been marked by higher-than-average rainfall and multiple intense storm events. All this stormwater carrying fertilizers into our lakes, where the waters have been especially warmed by hot temperatures, has led to this June’s toxic bloom. Invasive species such as the zebra mussel have also contributed.

We know we are expecting even warmer temperatures and more frequent, more intense storms to continue to impact Wisconsin in the decades ahead. As we look into the future of our watershed, what does this mean for our local lakes, summer recreation, fishing, wildlife, and the health of our kids, pets, and communities?

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