SEASONAL AND SPATIAL PATTERNS OF AN EPIBENTHIC DECAPOD CRUSTACEAN ASSEMBLAGE IN NORTH-WEST ATLANTIC CONTINENTAL-SHELF WATERS

To examine seasonal and spatial patterns in a mobile marine assemblage , monthly samples were taken in triplicate with a 2-m beam trawl (6-mm mesh) at three separate stations (landward of the ridge, on the ridge top, and seaward of the ridge). The assemblage was of epibenthic deca pod crustaceans, and was situated at a north-west Atlantic continental shelf, sandy ridge site. The assemblage was composed of nine species and was extremely variable over time and space. The sevenspine bay shr imp (Crangon septemspinosa), the Atlantic rock crab (Cancer irroratus) , the spider crab (Libinia emarginata) and the lady crab (Ovalipes oce llatus) were the numerical dominants, comprising >98% of all decapods collected. Three of these species (C. septemspinosa, C. irroratus, L. emarginata) exhibited marked spatial heterogeneity in abundance, with many fewer found on the ridge top than at either of the other two stat ions. Ovalipes ocellatus was not as spatially variable. Crangon showed two clear peaks, in spring and fall, as did Libinia, but neither appe ared to use the site as a nursery area. Ovalipes ocellatus and C. irro ratus each showed a single peak of very small individuals in the summe r and appeared to use this site for settlement. Komolgorov-Smirnov tes ts, analysis of variance and cluster analysis showed much less differe nce in assemblage structure between the landward and seaward stations than was demonstrated between either station and the ridge top. The pr esence of the sand ridge had a clear impact on the abundance and distr ibution of local decapod crustacean populations. (C) 1997 Academic Pre ss Limited.