Spatial and Temporal Changes in Subtidal Benthic Crustaceans Along a Coastal River-Estuarine Gradient in Mississippi

Benthic crustaceans were collected monthly between 24 August 1985 and 20 September 1986 from tidal freshwater (TFW), oligohaline (OH) and mesohaline (h4H) sites in Old Fort Bayou, a black-water tidal river of the Biloxi Bay estuary, Mississippi. Salinity varied seasonally and spatially and was primarily related to variations in rainfall during this study. Reduced rainfall after October 1985 resulted in the upstream movement of saline water and a concommitant shift in benthic crustaceans upstream with this change in the physical-chemical environment. This shift is supported by a significant positive concordance of ranks between salinity and relative abundance of Gammarus sp. A (Kendall's tau-c = 0.458 and Spearman's rs = 0.704, p<0.007) and G. mucromatus (Kendall's tau-c = 0.497 and Spearman's r, = 0.701, p <0.007) at TFW indicating that when salinity became high, so did the relative abundance of these two species in TFW. No specimens of either species were collected at this site during the same months in 1985. There was no relationship between salinity and relative abundance of G. bonnieroides (Kendall's tau-c = 0.201 and Spearman's rs = 0.381, p = 199) in the TFW site. At the OH site, there was a marginally significant concordance (Kendall's tau+ = 0.432 and Spearman's rp = 0.545, p = .053) between salinity and relative abundance of G. bonnieroides but not for the other two species. Riverine estuarines are by nature dynamic in their physicalchemical environment. Such variation influences macrobenthic distribution which has important trophic implications because these organisms form a large portion of the food base for higher trophic levels, such as fishes.