The folks at the Nature Conservancy have a terrific Blog post on the topic. We excerpt bits of it here.
Research: Can Restoring Oyster Reefs Combat Nitrogen Pollution?
By Jonathan Adams
Oysters filter nitrogen from water — and nitrogen pollution is a huge and growing problem along many coastlines, not just for the United States, but worldwide. So could restoring oyster reefs combat nitrogen pollution? And if the answer is yes, could that service generate enough funding for broad-scale oyster restoration?
A team of scientists is investigating those questions at the Nature Conservancy’s Virginia Coast Reserve — fittingly, just offshore from Oyster, Virginia — developing crucial data on just how much nitrogen oyster reefs can take out of the water. Their findings may aid in the recovery of oyster reefs across the country. That recovery may in turn spur the revival of estuaries — and the economies that depend on them — from Willapa Bay in Washington, to Chesapeake Bay, to the Gulf of Mexico.
“If we do it right, restoring oyster reefs can be a big part of how we reduce the amount of nitrogen and other nutrients in estuaries like Chesapeake Bay,” says Lisa Kellogg, a researcher at the Virginia Institute of Marine Science and the leader of the team working in the Virginia Coast Reserve.
THE PRESSING NEED TO REDUCE NITROGEN POLLUTION
Unfortunately, you do not need scientific expertise to see the need to reduce nitrogen pollution here in Oyster and elsewhere in and around Chesapeake Bay—or in many parts of the world.
When nitrogen reaches the Bay or any other shallow estuary –– as run-off from agriculture, as untreated sewage, or in the emissions of power plants and factories –– it provides nutrients for phytoplankton, which bloom effusively and then die and sink to the bottom. Bacteria break the plants down and in so doing suck much of the oxygen out of the surrounding water, killing the fish, crabs, shellfish, seagrass and just about everything else. As a result, when plankton blooms reach their summer peak, about two-thirds of the Bay is a dead zone.
The problem is far bigger even than that. Just in the US, the Mississippi River gathers runoff from millions of acres of farmland, and the nitrogen and other nutrients that pour into the Gulf of Mexico creates a dead zone every summer that can cover more than 8,000 square miles, and area the size of New Jersey. Worldwide, there are some 400 dead zones, and scientists expect that number to rise.
1,300 ACRES OF OYSTER REEF COULD EQUAL A STATE-OF-THE-ART WATER TREATMENT PLANT—AT FAR LESS COSTThe science team in Oyster has reason for at least cautious optimism: Kellogg found in earlier experiments that under the right conditions, an acre of restored oyster reef could remove nearly 500 pounds of nitrogen from the water per year, more than anyone had shown anywhere in the Bay and one of the highest ever reported in any marine environment.
At that rate, restoring about 1,300 acres of oyster reef would be about the same as building a state-of-the-art water treatment plant. That may sounds like a lot, but there are many thousands of acres in the Bay that could potentially support restored oyster reefs.
That result stirred considerable excitement. Under the Clean Water Act, communities must reduce the amount of nitrogen flowing into rivers and estuaries. Faced with the possibility of needing to build new water treatment facilities at the cost of hundreds of millions of dollars to get into compliance with the law, the idea of using oysters instead may well make economic and ecological sense.
“We have been chasing the denitrification argument for some time,” says Boze Hancock, a marine scientist with The Nature Conservancy. “Denitrification is one of the service that oyster reefs provide for us, and one of real value,” he says. But Hancock also adds a note of caution: the state of most oyster populations are so dire that it will take time for them to really contribute. “We have a lot of work to do to get these systems back on their feet before reefs are relevant to reducing nutrient pollution.”
HOW DO OYSTERS TAKE NITROGEN OUT OF THE WATER, ANYWAY?To reduce an exceedingly complex system to a simple schematic: oysters filter the water above them and consume the phytoplankton and other bits of organic matter. Some of the nitrogen in the plants ends up in the oyster itself or in its shell, and the rest gets excreted and lands on the sediment around the reef, along with everything else the oysters filter out of the water but do not actually eat.
Once there, aerobic bacteria living in the top layer of sediment and anaerobic bacteria deeper down transform the nitrogen in a series of steps that end with some of it being released as nitrogen gas, the most abundant element in the air we breathe.
The reefs provide other benefits as well, like protecting shorelines, providing food and habitat for fish and filtering water so more light reaches the bottom thus encouraging the growth of seagrass.