For the first time, scientists have been able to measure the amount of nitrogen that restored oyster reefs can pull out of the water to help clean the Chesapeake Bay.
According to a recent study by Jeff Cornwell, Mike Owens, and Ken Paynter of the University of Maryland Center for Environmental Science and Lisa Kellogg of the Virginia Institute of Marine Science, oyster reef restoration can significantly increase the amount of nitrogen removed from the Bay’s waters by oysters that turn it into a harmless gas and even use it build their shells.
In the Chesapeake Bay, populations of the native oyster have been reduced to less than 1% of historic levels through a combination of overharvesting, poor management practices and disease. It has long been known that oysters play an essential role in filtering the water, but their direct impact on reducing harmful levels of nitrogen was unclear.
“Back when we had vastly more oysters in the 1970s, the Bay had ways to remove nitrogen that no longer exist,” said Cornwell. “We were able to determine that a square meter of restored oyster reef does 30 times as much as an unrestored area in terms of taking nitrogen out of the Bay.”
Nitrogen and phosphorus are essential for plants and animals to grow and thrive. However, an overabundance of these nutrients enter the Bay through runoff of lawn fertilizer and animal waste, causing low levels of oxygen in the water, algae blooms, and cloudy water that make it difficult for life to thrive. The study found that oysters on restored reefs remove some of this excess nitrogen from the water by using it to build their shells, and these reefs also provide a habitat for other organisms that provide the same benefit.
Cornwell and his colleagues scooped up a portion of the restored oyster reef near in Shoal Creek near Cambridge and sealed it into a water-filled box to measure the changes in gases in the water. They found that the oysters converted 25 percent of the nitrogen into an inert form that wouldn’t hurt the Bay.
Since 1990, efforts have been made to restore oyster reefs throughout the Chesapeake Bay. Oyster larvae are produced in the Horn Point Oyster Hatchery In Cambridge, Maryland, set on recycled adult oyster shells and transplanted to restoration sites. The expense of oyster reef restoration has raised the question of whether the benefits of oyster reef restoration are work the investment.
“We can look back to the mid 1970s when there were still plenty of oysters and understand that they were doing a lot of good for the Chesapeake Bay,” said Cornwell. “If we can get back to that, the Chesapeake Bay will be a better place.”
The study was funded by the Oyster Recovery Partnership and GenOn Energy. “Denitrification and assimilation on a restored oyster reef” was published in the April 22, 2013, issue of Marine Ecology Progress Series. The paper is available here.
