A lake with clear water is not necessarily cleaner than one that is green with algae, water quality researchers have found. Public domain

Clear water is not necessarily the best water — seemingly clean lakes can be among the least healthy, according to recent research on water quality.

Scientists have found a pattern in how a lake could appear depending on the amount of nitrogen and phosphorous that has been flushed into it, and they say they the green color we see in an algae-ridden body of water peaks and drops to a clearer-looking surface even as those elements are still gushing in. For a study in the journal Inland Waters, the team looked at data from dozens of lakes throughout Iowa, in places where there is a lot of agriculture. What they found was that even clear water could be full of runoff from fertilizers.

The relationship between the fertilizer going into a lake and the color of that lake flows in a curve. As the nitrogen and phosphorous that helps plants grow leaks into the water, it boosts the amount of algae and cyanobacteria, which turns the lake a green color because they contain chlorophyll. But according to a statement from Minnesota Sea Grant, when the amount of those elements oozing into the water reaches a certain level, it stops feeding those same microorganisms and kills them instead.

“One of the dangers here is mistaking increased water clarity for improved water quality,” lead study author Chris Filstrup said in the statement from Minnesota Sea Grant, an organization that is part of the U.S. National Oceanic and Atmospheric Administration’s Sea Grant Program. “In fact, the water quality in these instances is worse than in lakes that have more algae, yet lower levels of nutrients.”

This lake near the Iowa city Clear Lake is a typical example of the ones that water quality researchers were studying when they found that clear water is not necessarily more healthy than greenish water covered in algae. Minnesota Sea Grant

The scientists compared the curve to the one that appears with crops and grass — a little fertilizer does some good, while too much can kill them and affect the fertility of the land. They said the nitrogen and phosphorous are probably combining with other conditions in a way that breaks down cell membranes to take out the algae and cyanobacteria.

According to the group, judging water quality by its clarity, although common, might not be the best method in many areas to determine how healthy it is.

“In some of the Iowa lakes in our study we noted phosphorus levels 10 times what we’d expect to see,” Filstrup said. “We were astonished to see that the nitrogen levels were more than 30 times higher.”

The team had looked into other causes, such as the algae not receiving enough sunlight, but came up empty.

“The only explanation that makes sense, so far, is that high nitrogen is bad for algae,” study co-author John A. Downing, director of Minnesota Sea Grant, said in the statement.

Because fertilizer usage is rising, due to more demand for crops, the information from the study could help guide environmental protection efforts: “Applying too much nitrogen to fields, lawns and gardens wastes money, leads to unhealthy lakes and even damages the Gulf of Mexico,” Downing said. “Growing crops and managing animal waste according to best management practices is essential.”