Regional, watershed and local correlates of blue crab and bivalve abundances in subestuaries of Chesapeake Bay, USA

Blue crabs (Callinectes sapidus) and deposit- and suspension-feeding bivalves (Macoma balthicaand M. mitchelli) play important roles in the food web of Chesapeake Bay and may serve as indicators of ecological health, particularly in small subestuaries where conditions may be strongly linked to watershed and local factors. We sampled 19 subestuaries of Chesapeake Bay to test hypothesized relationships of blue crabs and bivalves with salinity (a regional indicator), watershed- and local-scale land use, and local habitat and water quality. We divided the subestuaries into five land-use categories: forested, developed, agricultural, mixed agricultural and mixed-developed. We measured water quality, sediment class, physical habitat and adjacent land use/land cover at each of six stations within each subestuary. Fyke nets were employed to estimate blue crab abundance and size-structure, while cores were used to estimate bivalve densities and biomass. Classification and regression tree (CART) analysis indicated that 51% of the variance in blue crab abundance was explained by salinity, watershed land use and shoreline marsh habitat. Crab abundance was greatest at salinities >16 ppt, but in lower salinities crabs were most abundant along marsh shorelines in forested and mixed land-use watersheds. Juvenile crabs <85 mm were more strongly associated with shoreline marshes, particularly in subestuaries with forested and mixed land-use watersheds. Macoma spp. were similarly associated with shoreline marshes but mainly in muddy bottoms at moderate-to-high salinities; however the best CART model only explained 25% of variance in bivalve abundance. These results were consistent with our predictions that shoreline wetlands and watershed land use may have important effects on these taxa along the estuarine salinity gradient, and are consistent with hypotheses based on previous descriptive and experimental research linking blue crabs and deposit-feeding bivalves to habitats rich in allochthonous detritus.