Estuarine Invertebrates and Fish: Sampling Design and Constraints for Long-Term Measurements of Population Dynamics

The first five to six years of a long-term data set are presented for invertebrates and fish representing an array of physiological tolerances, trophic levels, and life history strategies in a lower mesohaline subestuary of Chesapeake Bay. Population abundances were estimated for infaunal invertebrates with cores taken four to six times per year at five stations, for nearshore fish with seines taken once per year at 14 stations, and for epibenthic fish and crabs taken in trawls eight times per year at three stations. Physical/chemical parameters of estuarine water were monitored continuously. Analyses of variance and covariance were used to account for variation associated with seasonal cycles and to test for effects of year, station, and salinity on population abundances. A 16-year record of meteorological and water parameters showed that the study period spanned a multiyear period of regional drought in the 1980s, which resulted in markedly increased salinities and reflected a greater deviation from the long-term average than reduced salinities during major storms of the 1970s. ANOVA and ANCOVA models accounted for 12 to 82% of the variation in population abundance, depending on the species. All but two species showed significant differences in population abundances among years in six years' data for 19 infaunal invertebrate species, six years for 18 species of nearshore fish, and five years for five species of epibenthic fish and crabs. Most species exhibited severalfold differences in abundance among years. About half of the infaunal species, several of the nearshore fish, and four epibenthic species showed significant responses to elevated salinities. However, despite the overall importance of salinity on estuarine systems, only a small fraction (1 to 6%) of the variation in any one species was explained by salinity changes. Failure to account for more of the population variation by salinity resulted from: (1) the significant spatial (station) variation and the significant interaction of time and spatial variation in nearly all species; (2) the importance of other meteorological and water quality parameters in regulating populations; (3) salinity being important primarily during a critical period (for example, during spring recruitment) because other factors (for example, predation) may override the controlling influence of salinity during the other seasons; (4) the spatial scale of the study area being too small too exhibit major shifts in species abundance across the major salinity zones of the estuarine gradient; (5) the location of the study area at the low end of the mesohaline zones means that high salinities dominating the study period are unlikely to affect euryhaline species as much as low salinities; and (6) possible biases of short-term variation since most years of the study perios were characterized by unusually high salinity.