A recent study by Sea Grant-funded researchers from Boston University supports the notion that oyster reef restoration and aquaculture efforts may be an important method for removing nutrients that can over-fertilize coastal waters.
The study, published in the July issue of Marine Ecology Progress Series, measures the rate of inorganic nitrogen and phosphorous processed in the eastern oyster Crassostrea virginica, the only native oyster species along the Atlantic seaboard, and is the first to show how bacteria that live in oyster digestive tracts and on their shells regulate these rates.
Oysters are filter feeders, like all bivalves, but with a high capacity to filter water up to 50 gallons per day. They remove phytoplankton, sediment, and other organic debris. As they filter and grow they also process nutrients such as nitrogen and phosphorus. Most of the nutrients in our coastal waters come from human activities, such as fertilizing lawns and farm fields or from our sewage. But due to disease, overharvesting, and pollution from heavy metals and other toxins, wild oyster populations have collapsed in the last century.
“The oyster reef is arguably the number one hardest hit [coastal habitat], where on the high end we’ve lost about 85% globally,” said Dr. Robinson Fulweiler, one of the authors in the study, during a Coastal State Discussion talk this spring.
That also means loss of habitat for other marine animals and plants, as well as filtering services for cleaner water. This is one of the reasons why multiple efforts are underway in estuaries like Narragansett Bay and the surrounding salt ponds to restore oyster reefs and/or enhance aquaculture development.
But for these efforts to be successful, a better understanding of how different oyster and other bivalve species process nutrients like nitrogen is needed to make better decisions regarding reef restorations and to determine the most efficient aquaculture practices, suggested by the study by Fulweiler and her team, which includes Ph.D. candidate Nick Ray, and an undergraduate researcher Maria Henning.
Their research shows that the oyster digestive tract and the shell host a small but diverse micro-community of bacteria that process nitrogen separately and differently.
Bacteria found in the digestive tract, where there is little oxygen and light, were found to support denitrification––a process that removes nitrogen in coastal ecosystems and returns it back to the atmosphere as nitrogen gas––at a 22% efficiency rate––similar to bacterial communities found in the sediment below a restored reef.
In contrast to the dark and no oxygen gut environment, bacterial communities on the shell, are exposed to oxygen and light. These shell bacteria were found to oxidize ammonium produced by algae and other organisms attached to the shell and create a form of nitrogen known as nitrate that can be converted back to nitrogen gas.
Researchers found that the total release of nitrous oxide, a greenhouse gas with almost 300 times the global warming potential of carbon dioxide, from these two processes was very small at less than 1%––a nominal tradeoff compared to nitrogen removed.
The study also found that the amount of ammonium released via the digestive tract is three times greater than the nitrogen removed and the ammonium released from the shell is twice as much as the digestive system. The authors note that the nitrogen from the ammonium produced is not new nitrogen but rather nitrogen that was already stored in phytoplankton and is now being recycled. This may be an important consideration for nutrient management planning if the plan involves reducing ammonium concentrations.
Overall, while the rate of nitrogen removal at an individual level is small, oysters tend to be found in dense populations whether on a reef or a farm, and collectively show a significant impact in removing nitrogen from surrounding waters via their digestive tract while also stimulating nitrogen removal in the sediments below from the production of ammonium and nitrate.
More research into understanding how oysters and other bivalves process nitrogen will be important as restoration and aquaculture activities continue with the intent to control nutrient loads in coastal waters.
– Meredith Haas