Abstracts
Session: The State of Oyster Disease
The Crucial Ecological Role of Oysters in Chesapeake Bay
Presented By:
Newell, Roger I.E., newell@hpl.umces.edu
Horn Point Laboratory, University of Maryland Center for Environmental Science

Originally oyster boats were outfitted with hand-powered oyster dredge winders. The winders were fastened to the deck aft on each side. Sometime before 1910, the hand winders were replaced with power dredge winders. Mounted to the deck amidships this technological improvement, made possible by the internal combustion engine, removed much of the back- breaking aspect of oyster dredging and allowed larger dredges to be used. Rothschild et al. (1994) showed that oyster bar acreage in Maryland waters declined by more than 50% from 1907 to 1982, and the quality of existing oyster bars has been diminished to the point where population dynamics, productivity, and yields per habitable acre are substantially reduced. Various methods of harvesting, primarily mechanical harvesting devices, used over the long term have destroyed the structural integrity of oyster reefs and depleted available substrate that is suitable for larval settlement. Rothschild et al. concluded that the effects of fishing manifested through modification of oyster reefs had a much greater influence on the long-term decline of the oyster than degraded water quality and the effects of diseases.

MSX disease first caused mass mortalities in Delaware Bay, on the Eastern Shore of the United States in 1957, and since then, has spread south to Florida and north to Maine

This feeding activity enables large populations of oysters to reduce phytoplankton assemblages, thereby decreasing turbidity and increasing the amount of light that reaches the sediment surface. In this process, oysters exert "top-down" grazer control on phytoplankton production and extend the depth to which ecologically important benthic plants, such as seagrasses and benthic microalgae, can grow. Unfortunately, the extensive populations of eastern oysters that once dominated Chesapeake Bay are but a mere remnant of their original abundances. If oysters harvests can serve as an index of population abundance, the decline from more than 10 million bushels a year in Maryland in the late 19th century to some 2 million a year in 1985 to less than 100,000 in these last several years reflects the catastrophic state of the Chesapeake oyster stocks. It is likely that the loss of this keystone suspension feeder has had profound adverse effects on the ecology of Chesapeake Bay.

Newell (1988) estimated that it took eastern oysters less than a week to filter the entire water volume of Chesapeake Bay when oysters were highly abundant in the 1880's before stocks were commercially exploited. Today, oyster stocks are at an all time low due to a combination of ongoing oyster disease epizootics and destructive harvest practices reducing oyster reef habitat quality. One way to gauge the ecosystem changes that may result from this loss of oysters is that it now takes the oyster stocks in the Bay about one year to filter the water volume of the Bay. Furthermore, the loss of oyster reef substrate that various invertebrate and invertebrate organisms in the Bay once utilized for shelter and feeding has altered animal community composition.

Some critics of Newell's (1988) proposition that oysters once exerted top-down control on phytoplankton stocks have argued that oysters simply recycle inorganic nutrients rapidly back to the water column and hence there would not have been any long-lasting reduction in phytoplankton biomass. To help distinguish between these scenarios, Newell et al. (2002) explored in laboratory incubations changes in nitrogen fluxes and denitrification under anoxic and oxic conditions in response to loading by different amounts of phytoplankton cells, representing an experimental analog of oyster biodeposits. When organics were regenerated under aerobic conditions, typical of those associated with shallow water oyster habitats, coupled nitrification-denitrification was promoted, resulting in denitrification of ~20% of the total added nitrogen. In contrast under anoxic conditions, typical of current summertime conditions in main-stem Chesapeake Bay where phytoplankton is microbially degraded beneath the pycnocline, nitrogen was released solely as ammonium from the added organics. This study indicates that denitrification of particulate nitrogen remaining in the biodeposits of oysters will enhance nitrogen removal from Chesapeake Bay. Phosphorus remaining in their biodeposits can become buried and sequestered within the aerobic sediments. In summary, it is now apparent that sufficient numbers of eastern oysters can exert both "top-down" control by grazing on phytoplankton stocks and influence "bottom-up" nutrient control on phytoplankton production by changing nitrogen and phosphorus regeneration processes within the sediment. Thus, restoration of the once abundant stocks of oysters to Chesapeake Bay may be a crucial complement to other management activities that seek to reduce phytoplankton production by curbing N and P inputs from point and non-point sources.

It is plausible that an ecosystem dominated by benthic primary production may develop in shallow waters when reduced turbidity associated with oyster feeding increases light penetration to a level that can sustain benthic microalgal production. These benthic microalgae compete with nitrifying bacteria for N regenerated from oyster biodeposits, thereby reducing or even precluding coupled nitrification-denitrification. Although these benthic microalgae are an important food source for many benthic animals, it means that nitrogen removal via denitrification will not be an important nitrogen removal pathway in the shallows.

Over the last four decades seagrass beds have either declined or disappeared throughout much of the Chesapeake Bay due to high water turbidity leading to reduced light availability for these benthic plants. We (Hood, Koch, and Newell) are developing a numerical model to explore the possible interactions between oyster and seagrass declines. Once complete, this will simulate the effects on seagrass growth of the interactions between wave-induced sediment resuspension, oyster filtration, and the direct influence of the physical structure of the oyster reef itself on wave action. Predictions from this model shows that under high wave height conditions the presence of oysters can reduce suspended sediment concentrations by nearly an order of magnitude, which significantly increases water clarity and the depth to which seagrasses can grow.

It is now widely believed that these ecological functions of eastern oyster populations are so vital that it is important to have extensive oyster populations in the estuaries along the Atlantic and Gulf coasts. Unfortunately, restoration activities in Chesapeake Bay over the last 5 y have largely been stymied by worsening Dermo and MSX epizootics. In Maryland, restoration primarily involves placing hatchery-reared spat in low salinity regions where a group of scientists, including me, expected that diseases would be less virulent over the long-term. This has proved to be an incorrect supposition as high salinities in recent years have allowed diseases to invade even these regions and kill oysters on many restored bars.

Recent incremental advances in the development of disease tolerant strains of oysters by Allen and coworkers means that in highly controlled aquaculture situations, where growth is rapid, oysters can now reach market size without appreciable disease mortality. Unfortunately, because of the much slower growth rates when growing on natural oyster bars, these strains are not yet sufficiently disease tolerant to survive for the extended period desirable to maximize ecological function for restoration projects. What is required is a highly disease tolerant strain of oyster that can be used for restocking oyster bars for ecological function and public harvest. It is likely that progeny from disease tolerant oysters on unharvested bars can help rebuild natural stocks, hence ultimately reducing reliance on hatchery production. I recommend that we rigorously evaluate recent research to determine if the development of a strain of oyster that is highly tolerant to MSX and Dermo is achievable in the next 5 to 10 y. If we believe it is attainable we should focus ODR funding efforts more strongly on the development of such a strain and put less emphasis on other ODR research activities.

Newell, R.I.E, J.C.Cornwell and M. S.Owens. 2002. Influence of simulated bivalve biodeposition and microphytobenthos on sediment nitrogen dynamics: a laboratory study. Limnol. Oceanogr. 47: 1367-1379. Free download at http://aslo.org/lo/toc/vol47/issue5/1367.pdf

Researchers working in the Chesapeake Bay in the 1980s and early 1990s concluded that oysters are such good filters they should be restored for their ecological benefit. What they found was that as oysters filter bay water, they remove sediment, algae and nutrients. Oysters feed on some of this waste and deposit remaining pollutants in small pellets that become part of the sediment.

How did msx get here?  Recently, scientists have discovered genetic evidence that implicates Japanese oysters as the cause of MSX, one of the diseases that has decimated the Chesapeake fishery in recent years.CLICK  Then scroll down to msx.
Chinese oysters may be introduced to repair oyster grounds that were ruined by an earlier introduction of Japanese oysters half a century ago. Forty years after MSX began devastating oyster beds in Delaware and Chesapeake bays, Gene Burreson of VIMS was able to prove that MSX was a natural parasite carried by Crassostrea gigas, a Japanese oyster carried into the Bay by oyster growers, scientists or international shipping. Gigas, it turned out, did not thrive in the Bay, but its parasite, MSX, does.

Note that the oysters that UMCES scientist Ken Paynter Paynter is planting are called Crassostrea ariakensis, or the Suminoe oyster. The known native ranges for the species include northern China, western Korea and southern Japan. Paynter's test oysters, however, never lived in Asian waters. They were conceived in a lab in southern Virginia — as were their parents and grandparents. These third-generation lab specimens were also crossbred for sterility by Stan Allen of VIMS. In both lab and field tests, they have grown faster and fatter in Chesapeake waters than the native Crassostrea virginica. They are not reproducing in the Bay because they are, for now, sterile animals bred for testing. Most importantly they are not dying off from the local diseases.

No wonder a lot of watermen and some scientists think the Asian oyster holds a key to the future economy and ecology of the Bay. Paynter, however, is cautiously optimistic, calling the prospects promising. "It is not in my mind an either/or sort of thing," he says. "But rather virginica, the native oyster can do its job in lower salinity waters where disease is not as prolific, and perhaps ariakensis will be more successful in higher salinities. And we can accomplish restorations in both areas."