A tenfold increase in the Bay’s oyster population would boost efforts to bring back underwater grass beds, according to a recent computer modeling analysis.

But the powerful filter feeders would not significantly reduce the size of the summertime oxygen-depleted “dead zone” that keeps deep portions of the Chesapeake off-limits to fish and shellfish, the analysis concluded.

The modeling exercise suggests that oysters would help to meet some of the Bay’s water quality goals, but are not a magic bullet for the Chesapeake cleanup that would offset the need for sharp nutrient reductions throughout the watershed.

“Oyster restoration alone will not bring the Bay back,” said Carl Cerco, the scientist with the Army Corps of Engineers who is responsible for developing and operating the Bay Program’s Water Quality Model. “I view oyster restoration as a supplement, not a substitute, for nutrient reductions.”

Large oysters can filter up to two gallons of water an hour during warm summer months, removing sediment and phytoplankton in the process. The Bay once contained huge oyster populations which, by some estimates, could have filtered a water volume equivalent to the Chesapeake in a matter of days.

That has spurred speculation by some that it was primarily the decline of oysters, rather than an increase in nutrients, which led to the demise of the Bay’s water quality in recent decades.

Today’s oyster population is estimated to be at less than 1 percent of historic levels because of overharvesting, disease and a decline in habitat quality. It would take that depleted population more than a year to filter a Bay full of water.

And indeed, the exercise with the Water Quality Model—a complex “virtual Chesapeake” which simulates the movement of water and pollutants through the Bay—confirms that today’s oyster population has almost no impact on water quality. Other benthic-dwelling clams and mussels have a greater influence.

That begins to change as oyster numbers are increased in the model. If the Bay Program were to meet its goal of increasing oyster populations by tenfold from 1994 levels—something considered unlikely soon because of disease and lack of habitat—it could have the effect of removing 24 million pounds of nitrogen a year from the Bay.

That’s nearly a quarter of the Bay Program’s nitrogen reduction goal, which seeks to cut the amount of the nutrient entering the Bay from an estimated 278 million pounds a year now to an annual average of 175 million pounds a year in 2010.

But those reductions would have mixed results. The model suggests a tenfold increase would have only modest impacts on dissolved oxygen in most of the Bay. “We definitely need the nutrient reductions from the watershed to get to our dissolved oxygen goals,” said Lewis Linker, modeling coordinator for the EPA’s Bay Program Office.

But in small, shallow areas along the flanks of the Bay and its tributaries, the return of oysters could significantly improve water clarity as they filter algae and sediment from the water, potentially allowing a resurgence of underwater grass beds, the model indicates.

Some areas, depending on the amount of oyster habitat and hydrology, get little benefit. Elsewhere, the difference is huge—the model shows that attainment of the tenfold oyster goal could more than double the amount of underwater grasses beyond what would be achieved by nutrient reductions alone in some places. “As the system gets shallower and smaller, the impact of oysters on things like submerged aquatic vegetation gets magnified,” Cerco said.

For water clarity, oysters might provide an important boost for attaining the Bay Program’s goals. The model suggests that relying on nutrient and sediment reductions alone would only clear the water enough to allow about 125,000 acres of grass beds in the Chesapeake. The Bay Program goal is 185,000 acres. (In 2003, the Bay had roughly 65,000 acres of underwater grasses.)

“We need help with the clarity because we know our goals have gone just about as far as we can go with nutrients and sediment,” Linker said.

An attempt to estimate the impacts of restoring historic levels of oysters in the Bay, estimated at roughly 100 times today’s population, shows that water clarity and dissolved oxygen levels would improve significantly in much of the Bay, although pockets of oxygen-depleted water—which would be off limits to fish—would still remain in deep portions of the Chesapeake.

Part of the reason that dissolved oxygen improvements are not greater is that while oysters are powerful filter feeders, large oyster populations in shallow water areas keep refiltering the same water. “The oysters don’t filter a different parcel of water all the time,” Cerco said. “They are filtering the same parcel of water over and over.”

The effectiveness of oysters to control dissolved oxygen is also limited by a series of ecological “mismatches” which results in the current distribution of oysters not filtering algae in the right places at the right time to have maximum impact, said Denise Breitburg, a scientist with the Smithsonian Environmental Research Center, who is part of a group working to develop a food web model for oyster populations.

Because of cold water temperatures, oysters are not actively feeding while large late-winter and early-spring nutrient flows into the Bay spur the development of an annual spring algae bloom, which typically sets up conditions to deplete water of oxygen in late spring and early summer.

“They can’t do a lot to prevent that spring bloom,” Breitburg said. “It is ramping up at slightly colder temperatures than when the oysters are at their maximum filtration.”

When that algae die, they sink into deep areas of the Bay where they are decomposed by bacteria in a process that depletes the water of oxygen.

In addition, oyster populations are not located in the right places to capture some of the huge phytoplankton blooms generated by excess nutrients from the Susquehanna, which is an important factor in triggering low dissolved oxygen conditions in deep areas of the Bay. Meanwhile much of Virginia’s oyster population is in the James—a river that has relatively little impact on dissolved oxygen in the Chesapeake.

Richard Fulford, who is also working on the oyster model at SERC, said their model generally agrees with the Bay Program model in suggesting that conditions should improve modestly if the amount of oysters increases because they will remove more algae.

But he said the Water Quality Model may overstate the benefits to water clarity because it assumes improvements in part by looking at reductions in chlorophyll a, a measure of total algae biomass. Oysters eat large algae cells, leaving smaller plankton in the water. Those small cells, he said, can account for 20–50 percent of the total chlorophyll biomass in the summer and that means around a third of the chlorophyll biomass will be unaffected by oyster filtration at a time when oysters are most active.

Small cells normally become more abundant in the summer, and an increase in oysters may possibly increase their relative abundance because oysters will be selectively removing larger phytoplankton. “By removing the big cells, the oysters are actually benefiting the small cells because they remain in the system and and then have access to all the excess nutrients,” Fulford said. Small cells, because they float in the water longer, can have a major impact on water clarity.

Roger Newell, the scientist with the University of Maryland Center for Environmental Science who published the first papers calculating the oysters’ water-filtering impacts on the Bay, said the model shows that oysters have the potential to significantly influence shallow water habitats.

“They do have some function in the Bay other than just providing food for humans,” he said. “If you take them out completely, then you change the system, and this model shows that some of our adverse water quality problems are due to oyster removal.”

But Newell said no one should count on oysters to help clean up the Chesapeake anytime soon. Since the Bay Program established its goal for a tenfold increase in its Chesapeake 2000 agreement, the oyster population has declined—not gone up—as it has continued to face pressure from disease, harvest and habitat decline.

Oyster restoration efforts have been stepped up in the past few years, but it’s too soon to know if they will be effective.

Some are eyeing the potential introduction of nonnative Crassostrea ariakensis oysters to replace the native species, but the outlook for those oysters is also uncertain. They have proven susceptible to deadly disease in North Carolina, and laboratory tests indicate they may be more vulnerable to predation than native oysters.

“Even if ariakensis were the magic oyster, it is still going to be many years before populations build up because you don’t have the crucial unsilted oyster shell habitat that oyster larvae need to settle on,” Newell said.

To improve water quality, he said investments in nutrient reductions were more reliable than counting on oyster recovery. “It’s a difficult thing for me to say, because I’ve been working for a number of years to uncover the function of oysters in the Bay’s ecosystem. But because of the uncertainty surrounding the ability of oysters to flourish in the Bay, I think we should avoid over-reliance on oysters for this type of activity.

“But this modeling exercise shows that an increase in oyster stocks is fundamentally important to the Bay ecosystem, so we should maintain a strong commitment to bringing back oysters into the Bay.”