Modelling oyster population response to variation in freshwater input

This paper describes the linkage of a three-dimensional hydrodynamic circul ation model with descriptive and experimental biological data concerning oy ster (Crassostrea virginica) population dynamics in the Apalachicola Estuar y (Florida, U.S.A.). Our intent was to determine the direct and indirect ro le of Apalachicola River flow in the maintenance of oyster production. Resu lts of a monthly field sampling programme conducted on the oyster reefs in the Apalachicola system during 1985-1986 were used to develop statistical m odels relating several life-history characteristics of oysters to physical- chemical aspects of water quality. The same life-history characteristics we re related statistically to output from a circulation model of Apalachicola Bay. Highest oyster densities and overall bar growth were found in the vic inity of the confluence of high salinity water moving westwards from St Geo rge Sound and river-dominated (low salinity) water moving south and eastwar ds from East Bay. With the exception of models for oyster mortality, the pr edictive capability of results from the parallel modelling efforts was low. A time-averaged model was developed for oyster mortality during the summer of 1985 by running a regression analysis with averaged predictors derived from the hydrodynamic model and observed (experimental) mortality rates thr oughout the estuary. A geographic information system was then used to depic t the results spatially and to compare the extent of expected mortality in 1985 and 1986. High salinity, relatively low-velocity current patterns, and the proximity of a given oyster bar to entry points of saline Gulf water i nto the bay were important factors that contribute to increased oyster mort ality. Mortality was a major determinant of oyster production in the Apalac hicola Estuary with predation as a significant aspect of such mortality. By influencing salinity levels and current patterns throughout the bay, the A palachicola River was important in controlling such mortality. Oyster produ ction rates in the Apalachicola system depend on a combination of variables that are directly and indirectly associated with freshwater input as modif ied by wind, tidal factors, and the physiography of the bay. River flow red uction, whether through naturally occurring droughts, through increased ups tream anthropogenous (consumptive) water use, or a combination of the two, could have serious adverse consequences for oyster populations. By coupling hydrodynamic modelling with descriptive and experimental biological data, we were able to determine the effects of potential freshwater diversions on oyster production in Apalachicola Bay. (C) 2000 Academic Press.