Distribuição espacial dos crustáceos decápodes e anfípodes de substratos não-consolidados sublitorais da Baía de Sepetiba, Rio de Janeiro, (Brasil)

Abstract

Sepetiba Bay, located at Rio de Janeiro, Brazil, is a tropical semi-enclosed estuary under a strong industrial and urban pressure. Fishing, intense navigation, ballast water discharge, sewage and industrial residues (such as heavy metals) are potential threats which have been contributing to degrade the local ecosystems. Crustaceans represent an important group of the infralitoral soft-bottom macrobenthos, and usually detach in species richness, abundance, density and biomass, as described by many authors. This paper aims to analyze the spatial variation of crustaceans among Sepetiba Bay belonging to the macroinfauna. During the years of 1998, 2000 and 2001 a total number of 68 stations were sampled. From each station triplicate sediment samples were obtained with a 0.1 m2 van-Veen grab, washed aboard with a 1 mm mesh sieve and preserved. The macrozoobenthos organisms presents were sorted and fixed in 70% ethanol, being the Crustacea separated from the other groups and enumerated. Also at each station sediment samples were analyzed for grain size distribution, organic matter and carbonates contents. The sediment composition along the Bay followed a gradient, with coarser sediments and lower organic matter contents towards the outer sector, in a general view. The existence of environmental gradients corroborates the existence of three sectors previously established: inner, intermediary and outer. Remarkable differences between the sectors were observed for the most analyzed sedimentological parameters. The macrobenthic crustaceans were apparently dominated by amphipods and decapods. Its density, species richness, diversity and equitability increased significantly in the same direction. An elevated proportion of azoic stations and rare species were encountered, mainly in the Bay s inner and outer sectors, respectively. The correlations techniques revealed a probable influence from sediment on the crustaceans spatial patterns (mean grain size, selection degree and organic matter). The estuary was dominated by the amphipod Ampelisca brevisimulata and the decapod Pinnixa sayana, however, the dominance patterns shifted according to the sectors of the Bay. Ordination and clustering techniques showed, for the amphipods, at least two stations grouping which were related with the intermediary and outer sector. Regarding the decapods, it was observed a high homogeneity between the sampling sites. These discrepancies between decapod and amphipods were probably associated to its respective feeding behaviors. The species dendogram revealed the presence of three major associations, probably connected with differences in sediment characteristics. Future investigations in a long-term issue, concerning the reproduction and feeding habits of the species found, might be helpful to a better understanding of the spatial patterns described.