Algae fixes may trigger more lake pollution

Phosphorus, a nutrient that is washed into Minnesota's lakes with leaves and lawn fertilizer, can cause algae blooms and poor water quality. But efforts to reduce it in lakes can have an unintended consequence.

According to a new University of Minnesota study published online Thursday in the journal Science, reducing phosphorus can also result in less of the microbial processes that eliminate another unwanted nutrient: nitrogen.

As a result, nitrogen can accumulate in large lakes and lead to nitrogen pollution downstream, the study says.

But rather than relaxing efforts to reduce phosphorus, the researchers said the new assertions mean efforts to reduce nitrogen in lakes should be stepped up even more.

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Nitrogen sources include wastewater treatment plants, farming, lawn fertilizer and pet waste. It's a concern because it can contaminate drinking water sources and has led to a large dead zone in the Gulf of Mexico where nutrient levels are too high to sustain most aquatic life.

The researchers looked at a dozen large freshwater lakes around the world. For example, they found that Lake Superior has low phosphorus but doesn't efficiently eliminate nitrogen.

Jacques Finlay, an associate professor in the College of Biological Sciences and the study's lead author, said researchers' discussions about Lake Superior motivated the work. The study noted that so far there hasn't been much research into what factors determine how efficient nitrogen removal is in freshwater lakes.

"We need to pay attention to the way that nutrients interact in ecosystems and maintain our focus on reducing phosphorus and nitrogen pollution," Finlay said in a news release. "If we do that, we'll be taking steps toward improving water quality locally as well as downstream."

Additional research is needed before drawing conclusions about how phosphorus and nitrogen interact in smaller lakes, Finlay said.