Seasonal changes of benthic megafauna in the R�a de Muros e Noia (Galicia, North-West Spain)

Abstract

A community of decapod crustaceans (Brachyura) was sampled seasonally (October 1978–July 1979) from three habitats (raft, middle and beach) in the Ria de Muros e Noia (North-West Spain), with the purpose of studying spatial and temporal changes in the community and comparing with communities in the neighbouring Ria de Arousa, which supports an intense mussel (Mytilus edulis)-raft culture. The Portunidae family dominated the decapod community. Polybius henslowi, a species with pelagic stages, which enters the rias periodically in large numbers, was the dominant species at all the stations throughout the sampling period, attaining densities of 1.6 individuals m-2 (18.7 g wet wt m-2) in summer. Macropipus depurator and M. puber were the next most important species at the raft station, M. depurator at middle stations, and M. vernalis and Carcinus maenas at beach stations. The highest population densities were recorded in summer, due to the great abundance of Polybius henslowi at this time. When the data were reconsidered omitting p. henslowi, highest densities were in autumn and winter in the inner ria. In general, the nature of the substratum, the presence of mussel rafts, depth and salinity were the main factors determining the structure of the community. Density and biomass in the Ria de Muros e Noia (P. henslowi omitted) were lower than in the Ria de Arousa (up to six times lower in some areas). The Ria de Muros lacks the extensive number of mussel rafts present in the Ria de Arousa which constitute an important food resource for decapods.