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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
Horn Point Laboratory, University of Maryland Center for Environmental Science
Suspension feeding bivalve molluscs serve to couple pelagic and benthic processes by filtering suspended particles from the water column and transferring undigested remains in their feces and pseudofeces to the sediment surface. This activity can be extremely important in regulating water column processes where bivalves are abundant in coastal waters and in seasons when water temperatures are warm enough to promote active feeding. Of all bivalve species worldwide, eastern oysters (Crassostrea virginica), are among the most powerful in this regard because of their unusually high weight specific filtration rates (~7 to 10 L h-1 g-1 dry tissue weight at typical summer water temperatures of 25oC.) Adult eastern oysters are well adapted to living in estuaries, such as Chesapeake Bay, where inorganic particles comprise a large fraction of the seston because they can sort filtered particles prior to ingestion and reject less nutritious particles as pseudofeces.
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. 1988. Ecological Changes in Chesapeake Bay: Are they the result of overharvesting the Eastern oyster (Crassostrea virginica)? Pages 536-546 In: M.P. Lynch and E.C. Krome, (eds.) Understanding the Estuary: Advances in Chesapeake Bay Research. Chesapeake Research Consortium Publication 129 (CBP/TRS 24/88), Gloucester Point, VA. free download from http://www.vims.edu/GreyLit/crc129.pdf
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
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