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Some clear lakes are actually more polluted than green ones, a new study finds

And there's a very interesting reason why.

When you see a lake that's green, you can tell it's not that clean. 

But now some Minnesota biologists are saying even if a lake's water looks clear you shouldn't assume that means it's healthy. It turns out those clear-looking lakes – especially in farm country – may be even more polluted that the green ones. 

Researchers with the University of Minnesota Duluth and Minnesota Sea Grant have seen that in farming areas, the algae in lakes sometimes makes the water as green as the surrounding fields. 

In some heavily farmed parts of Iowa, though, there are a lot of really clear lakes. 

Wondering what that was all about, they studied 13 years' worth of data from 139 lakes in Iowa. Their findings were published Monday in the journal Inland Waters

Why lakes become green

The nitrogen and phosphorus in fertilizers help make plants grow.

Rain and snowmelt wash fertilizer off of farm fields, feed lots or lawns, and into nearby lakes. The nitrogen and phosphorus stimulate growth – but now it's algae and blue-green bacteria, rather than crops.

What researchers found in the clear lakes 

The scientists who studied those relatively clear lakes in the middle of Iowa farmland were surprised by the huge amounts of nitrogen and phosphorus the numbers showed. 

The lead author of the study, Chris Filstrup, said in a statement phosphorus levels were 10 times what you'd see in a typical northern Minnesota lake, adding: "We were astonished to see that the nitrogen levels were more than 30 times higher." 

The scientists concluded that when there's that much fertilizer in a lake, it no longer helps plants grow – it kills them instead. 

Co-author John Downing of Minnesota Sea Grant says researchers went into the study with various theories about what might explain the clear water. 

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

The authors pointed out that even on land, if you overdo it with fertilizer, it hurts or kills the plant instead of helping it. They think the same is going on in the lakes with algae. 

Why does it matter?

The findings could make a difference in the way we measure how healthy a lake is. 

The researchers say some agencies use clarity as a sign of water quality. But this study shows that in agricultural areas clear lakes can actually be more polluted, not less. 

The Minnesota Pollution Control Agency says about 40 percent of the lakes in the state are polluted. 

Things are especially bad in the heavily agricultural southwestern part of the state, where no lakes are clean enough for swimming and fishing.

Gov. Mark Dayton has made improving water quality a priority of his second term. 

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