Study finds clear lakes can be among the most polluted

Published 7:42 am Wednesday, October 11, 2017

DULUTH — A new study says appearances can be deceptive — clear lakes can be among the most polluted.

Scientists have long known fertilizer runoff from farm fields can fuel excessive algae growth, turning lakes the color of pea soup.

But a study of 139 Iowa lakes by the University of Minnesota Duluth and Minnesota Sea Grant finds that when lakes reach extremely high concentrations of nutrients, the water can be surprisingly clear. They found high levels of phosphorous and even higher levels of nitrogen in some.

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They concluded that extreme nutrient levels killed the algae in the lakes, similar to how too much fertilizer applied on land can kill plants.

Lead author Chris Filstrup tells Minnesota Public Radio it shows the need to measure nitrogen and phosphorous, not just water clarity.