Beavers have long been admired for their dam-building skills, which enable them to create ponds and slow the movement of water. Recently, a team of University of Rhode Island scientists has discovered that their environmental engineering tactics also help mitigate levels of nitrogen in the water.
According to Arthur Gold, URI professor of natural resources science, nitrogen levels have been increasing in northeastern streams for many years, due largely to the increased use of fertilizers. When nitrates travel from watersheds into estuaries, they trigger algal blooms that lead to dead zones, fish kills, and the degradation of marine habitats. Gold has spent his career studying the movement of nitrogen across the landscape to better understand how various habitats either remove or retain nitrogen.
“We found in our earlier studies of nitrogen movement that when a beaver pond was upstream it would confound our results,” Gold said. So he and doctoral student Julia Lazar decided to investigate beaver ponds to see what happens to nitrogen there.
They found that beaver ponds create ideal conditions for nitrogen removal, or denitrification. The organic matter that builds up behind a beaver dam plays a key role. That’s where bacteria transform nitrates into nitrogen gas, which dissipates into the air. Some nitrogen is also absorbed by aquatic plants and becomes stored in the sediments when the plants die. None of this would happen without the beaver dam because for substantial denitrification to occur, the water must remain somewhat still and in place for days or weeks at a time. In fast-moving streams, there is not time for the necessary biological and chemical processes that remove nitrogen.
“The great surprise to us was that beaver ponds are very effective at nitrogen removal,” Gold said. Depending on the pond and the amount of nitrogen present, as much as 45 percent of the nitrogen in a beaver pond is removed from the water. “And most of it goes out as a gas, which is advantageous since beaver ponds are temporary,” he added. “At some point when the dam breaks, the nitrogen stored in the sediments behind the dam moves back into the water, so it’s better when we see it go off as a gas. Then it’s gone for good.”
The researchers hope to continue to track the fate of beaver ponds to see how long the sediments maintain their nitrogen storage capacity. “It would be useful to know what happens to the stored nitrogen when the dam breaches,” Gold said. “It’s possible that it could just move on to the next beaver pond.”