Microorganisms Keep Large Waterways Clean, Study Shows

Nitrogen in water can cause algal blooms that consume oxygen in lakes, rivers and oceans, causing the death of fish and other aquatic animals.

To understand how nitrogen accumulates in large rivers and oceans miles away from where it entered the environment, 31 scientists examined 72 small streams across the United States and Puerto Rico.

Walter Dodds, a professor of biology at Kansas State University, K-State, looked no farther than his own campus for a testing site. Campus Creek at K-State was one of nine small waterways in the Manhattan area that Dodds used to study how nitrogen is removed from streams.

With a grant from the National Science Foundation, he and his 30 colleagues from across the United States and Puerto Rico studied nitrogen removal in streams in their own geographical areas. Their research appears in the March 13 issue of the journal "Nature."

Agricultural runoff laden with nitrogen enters a Wisconsin stream. (Photo courtesy Wisconsin DNR)

Dodds said the study included data from waterways in environments ranging from tropical forests to deserts to prairies.

"The significant thing about this project is that we all did the same experiments everywhere," he said.

In previous studies, researchers looked at pristine waterways, but Dodds said that this time researchers also wanted to evaluate urban and agricultural waterways where humans leave a mark.

The researchers investigated the dynamics of nitrogen in streams, using special nitrogen tracers analyzed by mass spectrometry to see how far nitrogen travels through waterways.

"When trying to account for nitrogen, we see that not all of it makes it down to the ocean," Dodds said. "We wanted to know how this is happening in streams."

What Dodds and the other scientists learned is that organisms like bacteria, algae and fungi are responsible for removing nitrogen from small streams.

This reduces the amount that makes its way into larger rivers, lakes and in oceans, where nitrogen can trigger excessive growth of algae and aquatic plants.

The researchers found that the filtration process works best if small streams are allowed to remove the nitrogen before merging with larger waterways. But small streams can do a better job of filtering if less nitrogen gets into the streams in the first place.

"An important thing to take away from this research is to understand how we're saturating our streams with nitrogen," Dodds said. "As you increase nitrogen, small streams become overwhelmed and cannot do their job."

Knowing the role that microorganisms play in removing nitrogen from small streams presents opportunities for research that can make waterways cleaner.

"What we can ask," Dodds said, "is which microorganisms are removing nitrogen and what management approach is most likely to stimulate that occurring."

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