Development of ecosystem indicators for the Suwannee River estuary: Oyster reef habitat quality along a salinity gradient

The Suwannee River watershed is one of the least developed in the eastern United States, but with increasing urbanization it is facing potential long-term alterations in freshwater flow to its estuary in the Gulf of Mexico. The purpose of this study was to develop biological indicators of oyster reef state along a natural salinity gradient in the Suwannee River estuary in order to allow the rapid assessment of the effect of changing freshwater input to this system. Percent cover and density of three size classes of living oysters, as well as the abundance of several predominant reef-associated invertebrates, were measured along a broad salinity gradient in the estuary and were correlated with salinity estimates from a long-term database for the preceding 12-24 mo. All eastern oyster, Crassostrea virginica, parameters (percent cover and density of three size classes) were significantly and negatively related to salinity. Data from samples collected near the lower intertidal were more closely dependent upon salinity than were samples from the higher intertidal at the same sites. Salinity differences were most closely reflected in differences in total oyster cover. This relationship corresponded with a general decline in oyster habitat with increasing distance from the mouth of the Suwannee River. Species richness was significantly and positively correlated with all C virginica parameters (percent cover and density of three size classes), but the relationship explained only about half the variability. Density data of the hooked mussel, Ischadium recurvum and a mud crab, Eurypanopeus depressus, were positively and strongly correlated with C virginica parameters, likely reflecting the abundance of habitat provided by C virginiva shells. All of the biological indicators measured responded similarly along the salinity gradient, indicating they provide reliable indices of the effect of changing salinities in the Suwannee River estuary over the previous I or 2 yr. Some areas of positive relief defined as reefs 30 years ago are no longer oyster habitat, suggesting an ongoing decline, but nearshore C virginica were abundant.