Science

Study finds clear lakes can be among the most polluted

Posted October 10

— 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 the study of 139 lakes in heavily agricultural parts of Iowa, by researchers with the University of Minnesota Duluth and Minnesota Sea Grant, found 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 of the lakes, which would normally cause algae to grow. 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 of UMD's Large Lakes Observatory told Minnesota Public Radio their results show the need for water managers and regulatory agencies to measure nutrient levels, not just water clarity, when determining water quality.

"This study is indicating that under these really extreme nutrient concentrations, those may not be good measures, so we should be measuring things like nitrogen and phosphorous," Filstrup said.

Co-author John Downing, director of Minnesota Sea Grant, said they thought at first that maybe there was too much shade for algae to grow in those lakes, or that plankton might eat more algae when there's a lot of nitrogen in the water.

"But none of those hypotheses panned out," he said. "The only explanation that makes sense, so far, is that high nitrogen is bad for algae."

The study appeared Monday in Inland Waters, the official journal of the International Society of Limnology.

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