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Learning How Oysters Do, and Don’t, Reduce Nitrogen

A Virginia Sea Grant graduate research fellow is researching whether aquaculture can help remove excess nitrogen from the Bay through denitrification.

©Abby Lunstrum

Abby Lunstrum collects a sediment core from Ruby Salts Oyster Farm to measure sediment nitrogen processes, like denitrification. ©Abby Lunstrum

By Julia Robins, Staff Writer

Nitrogen is an essential nutrient for many living things, but so much of it is now entering waterways due to human activities that it may cause some of the health problems experienced by organisms in the Chesapeake Bay. Reducing and removing nitrogen from the Bay has become a priority, and some hope that oyster aquaculture can help.

Abby Lunstrum, Virginia Sea Grant graduate research fellow at University of Virginia, is researching whether aquaculture can help remove excess nitrogen from Bay waters through denitrification.

“People are really interested in this concept of denitrification, because if oyster aquaculture can stimulate denitrification in the sediment, then that’s an added value of oyster aquaculture,” she says.

In denitrification, microbes convert nitrogen in the water into relatively harmless nitrogen gas, which is not consumed by most living organisms. Says Lunstrum: “It’s essentially removing pollution from the ecosystem.”

“Some studies have suggested that the denitrification rate should be high in aquaculture systems,” she says. But at Ruby Salts Oyster Farm, where Lunstrum has been measuring removal of nitrogen since 2014, denitrification is low.

In fact, 90 percent of the nitrogen removed from Ruby Salts occurs through the harvest. Oysters get their food by filtering water. When they are harvested, nutrients in their tissues—including nitrogen—are removed with them.

“Oysters get a lot of attention because they filter water,” Lunstrum says. This can lead to the assumption that oysters are constantly removing nutrients from the water, but not all of the nutrients that oysters filter stay in their tissues. Many get deposited into the water or sediment as waste.

While scientists have studied and quantified the removal of nutrients through harvested oysters, little is known about what happens to nutrients in waste. According to Lunstrum, this is important to consider when quantifying the amount of nutrients oysters remove. She found that the main form of nitrogen coming out of the sediment due to waste was in ammonium.

“In polluted systems like the Chesapeake Bay, ammonium is a type of pollution,” says Lunstrum, and in her research, nitrogen removed from the water through denitrification is very small compared to the amount of ammonium that stays.

“This study indicates that harvest is the most important source of nitrogen removal and denitrification is relatively unimportant,” says Lunstrum. Although the denitrification rate is lower than people interested in nitrogen removal through aquaculture had hoped, she affirms that harvest removal can be incorporated into nitrogen trading programs.

“The whole motivation behind this work is the question of how much nitrogen does oyster aquaculture remove from the environment,” says Lunstrum, “because it’s important for water policy to consider if shellfish aquaculture should be included in nitrogen trading programs.”