Oysters are valued for their reefs, which provide habitat and shelter for an array of fish, clams and other Bay creatures. They’re appreciated for their ability to clear murky water by filtering out algae and silt. And, of course, many simply like to eat them.

But some are asking whether oysters have another value: as a nitrogen-removing best management practice, like a cover crop or filter strip.

In that case, and if the amount of nitrogen removed can be quantified, oyster growers and others who put the bivalves in the water might get financial credit for helping to meet nutrient reduction goals set by the Chesapeake Bay Total Maximum Daily Load, or pollution diet.

Two recent reports, one from the Bay Program’s Scientific and Technical Advisory Committee and the other from the National Oceanic and Atmospheric Administration Chesapeake Bay Office, examined whether oyster aquaculture and oyster restoration projects could get credit for their nitrogen removal potential.

Their answer: probably — but it may not be much, at least not now.

Both reports concluded that only the amount of nitrogen stored in oyster shells and soft tissue could be confidently estimated when oyster growers remove them from the water.

That’s not a lot of nitrogen. Citing work by Virginia Commonwealth University scientists and others, Mark Luckenbach, a researcher with the Virginia Institute of Marine Science who participated in the development of both reports, said that removing 1 million market size oysters would remove about 290 pounds of nitrogen from the water.

Put another way, it would take a million oysters to offset the typical nitrogen runoff from 10–12 acres of corn.

“It can be part of the strategy in a place that has a relatively low nitrogen target to meet and has enough habitat available for aquaculture,” Luckenbach said.

The issue was raised because the fledgling oyster aquaculture industry in Virginia and Maryland has shown interest. It would help the cash flow of growers’ operations if they could get some marketable nitrogen removal credits that could be sold to someone who needs to offset nitrogen pollution elsewhere.

“It’s like any crop,” said Peter Bergstrom, a fisheries biologist with the NOAA Chesapeake Bay Office. “You put in all this money up front, and you wait. And with oysters, it is usually two years — it’s not just waiting until the fall harvest — to get your money back. They would love to have some mechanism, such as [nitrogen removal] credits, to get money back sooner.”

Some local governments have expressed interest in achieving some of their nutrient reduction goals by supporting oyster restoration efforts.

Oysters filter huge amounts of water as they feed. As they remove algae from the water, they are also removing nitrogen — but the fate of that nitrogen is largely unknown.

Some of it is absorbed into the oyster shell and flesh as it grows — and that amount can be estimated fairly easily and credited, both reports said.

But the rest is excreted, and determining the fate of that nitrogen is more difficult. Some of it is recycled back into the water column. An unknown amount is permanently buried in the sediment. And some is denitrified — converted to harmless nitrogen gas by colonies of bacteria living on the oyster reef or in the sediment...

A recent study of a restored oyster reef in the Choptank River showed an acre of reef could remove 543 pounds of nitrogen a year through denitrification (See “Ability of oysters to denitrify Bay surprises scientists,” March 2013). That finding fueled hopes that oyster restoration projects and aquaculture might get credit for their denitrification abilities.

Both reports said that is not possible with information currently available. “Incorporation of nitrogen removal via denitrification into TMDL implementation plans is currently unsupported both for oyster aquaculture and for oyster reef restoration,” the STAC report stated.

Few studies have been done to assess the denitrification rates of restored oyster reefs, and the limited work available suggests the nitrogen removal ability varies greatly from place to place based on water depth, light penetration and other factors.

Estimating denitrification for aquaculture oysters is even more difficult because they are grown in several different ways — some are grown directly on the bottom; others are grown in cages that may be placed on the bottom or float in the water. Aquaculture denitrification studies are even more limited than reef studies, and none show consistently enhanced denitrification, according to the reports.

But, Luckenbach said that conclusion likely reflects limited research on the issue. “I find it hard to believe that, with further experimentation, we are not going to see, under some conditions in some places, enhanced denitrification associated with aquaculture,” he said.

For now, credits could only be offered for nitrogen physically contained in aquaculture oysters when they are harvested — a number that can be quantified with confidence.

Restoration projects would need more research about denitrification rates, some of which is already being funded by NOAA, before they could get any credit at all. Because of the cost of oyster restoration, both Luckenbach and Bergstrom expressed doubt that nitrogen removal would be a driving factor for any restoration project. “It would be sort of an added bonus,” Bergstrom said.

In the big picture, Luckenbach cautioned that it is unlikely that oyster aquaculture is going to play a significant goal in achieving the Bay’s nitrogen reduction goals.

In a small tributary, like Maryland’s Rhode River, it would mean 140,000 oysters would have to be harvested annually to achieve 1 percent of the river’s nitrogen reduction goal, according to calculations by Luckenbach. Because it takes about two years to grow oysters to harvest, that means about twice that number would need to be in the river at any one time. “That is a reasonable scale,” Luckenbach said.

But for a larger system, such as Virginia’s Lynnhaven River, it would take an annual aquaculture production of 49 million oysters — or 98 million oysters being reared at any one time — to remove 1 percent of the nitrogen.

“There is not room for a farm that big,” Luckenbach said. “The populace wouldn’t allow it. Something on that scale gets in the way of sailboats and powerboats. That is kind of a reality check.”

The Scientific and Technical Advisory Committee report, “Evaluation of the Use of Shellfish as a Method of Nutrient Reduction in the Chesapeake Bay,” is available on the STAC website, .

The NOAA report, “Quantifying Nitrogen Removal by Oysters Workshop Report,” is available on the NOAA Chesapeake Bay Office website.