Oyster reefs may be more valuable economically and environmentally than previously thought, according to a new study from the UNC Institute for the Environment.
Published in PLOS One, researchers found that an oysters’ ability to filter nitrogen — through a burial process — makes them more than 40% more economically valuable than earlier estimates suggested.
Oysters are a vital part of coastal ecosystems. A single oyster can filter up to 50 gallons of water per day, according to the National Oceanic and Atmospheric Administration Fisheries Service, and the reefs they form can serve as barriers to storms and prevent erosion.
They also play a critical role in managing nitrogen levels.
While nitrogen is essential to all living things, too much of it is harmful to the environment, according to Anne Smiley, lead author of the study and postdoctoral researcher at the UNC Institute for the Environment.
“There's a real Goldilocks effect with nitrogen, where too little is bad and too much of it is bad,” Smiley said.
She said excess nitrogen can cause harmful, toxic algae to bloom, which creates public health issues and blocks sunlight to the sea floor. That makes it difficult for sea plants to photosynthesize and produce oxygen, which is necessary for the ecosystem to thrive.
Smiley said people are responsible for putting more nitrogen into the environment.
“We use a lot of fertilizers. We've got a lot of nitrogen in our detergents, like when we're washing our cars, when we walk our dogs and when they use the bathroom,” Smiley said. “There's a lot of nitrogen that we're putting in the environment that wouldn't otherwise be there, and we see negative ecological impacts.”
Oysters, however, naturally filter and remove nitrogen through a process called denitrification, in which they convert the harmful nitrogen into a harmless nitrogen gas. Smiley said scientists have studied this process for years.
“If we can leverage these natural processes and mechanisms to remove nitrogen, then we should,” Smiley said.
But now, Smiley’s research shows the burial process is another effective way the mollusks remove nitrogen from water.
Oysters filter the nitrogen through consumption of water, algae, and things in the water. They then dispose of it into sediments on the ocean floor, removing it from the environment.
Smiley said scientists suspected the burial process could be important in removing nitrogen, but they didn’t have any data to support it.
“We're trying to just understand all of these processes that regulate water quality,” Smiley said. “So that was a puzzle piece that we were sort of uncovering.”
The sediments she analyzed were collected in 2011 and 2014 from restored oyster reefs in Back Sound on North Carolina’s coast.
Smiley said faculty from the Department of Earth, Marine and Environmental Sciences and UNC’s Institute of Marine Sciences originally gathered the cores to study carbon burial, then moved on to other projects.
Years later, Smiley revisited the archived samples to measure nitrogen instead.
Smiley said since they knew how old the reefs were, they were able to figure out how much nitrogen was buried per year in the sediments. They analyzed three different reef structures: reefs that are exposed at low tide, reefs that are along the edges of salt marshes, and reefs that were submerged underwater.
“What we found was that burial was actually really strongly linked to structure,” Smiley said. “The height of the reef and the density of the oysters in the reef were strong predictors of how much nitrogen was buried in the sediments.”
She found that nitrogen burial is an important process, comparable to denitrification — and it’s all tied to reef design.
“I think it builds the case for why people should restore oysters, because it provides this tremendous value,” Smiley said.
The team determined that burial is a major process, comparable to denitrification. By applying North Carolina’s nutrient credit trading program, which sets a price on nitrogen removal, the team calculated that the economic value of these reefs is roughly 42% higher than previous estimates.
This increase comes from accounting for nitrogen burial, a natural solution, that provides a cost-effective alternative to expensive engineered solutions like wastewater treatment plants.
Smiley said this shows oysters’ value beyond them being a food source.
“What we're doing here is quantifying the non-market value that maybe is less obvious to people,” Smiley said. “I think it's really an effective way to communicate their intrinsic value and the intrinsic benefit they provide to society.”
Beyond water quality, oyster reefs can serve as natural infrastructure. Their hard, three-dimensional structures help reduce erosion, buffer coasts from storms and waves as well as provide habitat for other marine life. Smiley said there’s even research that shows oyster reefs can build vertically fast enough to keep pace with rising sea levels.
“Their ability to produce their own sediments and to capture external sediments and to hold on to them so that they can build vertically and laterally, makes them intrinsically resilient,” Smiley said. “I think they should be a priority habitat just because of their intrinsic resilience.”
Smiley said in recent history oyster populations have declined significantly. along with coastal water quality. But, she remains optimistic.
“Fortunately, though, we've got more and more restoration that's happening, more awareness of their value,” Smiley said.
Moving forward, Smiley said she wants to study older, natural reefs, so she can see exactly how long nitrogen stays buried.
“Now that we have identified nitrogen burial is an important component of water quality,” Smiley said. “How long are we expecting this nitrogen to be removed from the environment? Is it centuries? Is it millennia?”
