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Study shows clear lakes can be some of the most polluted

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For years scientists have known that nitrogen and phosphorous runoff from fertilizer applied to farm fields and lawns can fuel the growth of algae in lakes. Often it can lead to toxic blooms the color of pea soup.

But in a study of 139 lakes in Iowa over a 13-year period, researchers from the University of Minnesota Duluth and Minnesota Sea Grant found that when lakes reach extremely high nutrient concentrations, the water is shockingly clear.

That was surprising, said Chris Filstrup, the study's lead author and a research scientist at UMD's Large Lakes Observatory and Minnesota Sea Grant.

"Because every study to date has shown the more phosphorous, and the more nitrogen you have in systems, you tend to see more algae grow," he said.

In some of the Iowa lakes Filstrup and his colleagues studied, they found phosphorous levels 10 times what would be expected in a northern Minnesota lake. Nitrogen levels were more than 30 times higher. 

But rather than leading to even larger algae blooms,  researchers found that such extreme nutrient levels killed the algae in the lakes — similar to how too much fertilizer applied on land can kill plants and damage soil.  

Scientists at first thought that maybe there was too much shade for algae to grow, or that plankton eat more algae when there's a lot of nitrogen in the water.  

"But none of those hypotheses panned out," said John Downing, co-author of the study and director of Minnesota Sea Grant. "The only explanation that makes sense, so far, is that high nitrogen is bad for algae."

  The researchers concluded that really high levels of nitrates in the water caused algae cells to essentially burst apart, similar to how pouring hydrogen peroxide on a cut causes bacteria to burst. That's why it fizzes, explained Filstrup.  

A potential danger Filstrup forsees is that some people, or regulatory agencies, may mistake increased water clarity in these lakes with improved water quality.

He said water management agencies often measure how clear the water is in a body of water to help determine the quality of the water.

  "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.