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Megabenthic Assemblages of Coastal Shelves, Slopes, and Basins off Southern California





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Megabenthic invertebrate assemblages from soft sediments on the mainland shelf and San Pedro and Santa Monica Basin slopes and floor were identified based on 1203 otter trawl collections made between 1971 and 1985 at depths ranging from 10 to 915 m. Major changes in species composition and abundances occur at mid-slope (~300 m) and at the basin sills (~715 m). The mainland shelf assemblage (10-1 37 m) is dominated by the sea urchin Lytechinus pictus and the prawn Sicyonia ingentis. This assemblage is heterogeneous in space and time, with sub-assemblages that reflect severe contamination and large storms and/or El Nino. The basin slopes are dominated by the echinoids, Allocentrotus fragilis, Brissopsis pacifica, and Brisaster latifrons. The latter two species may exist in kilometer scale herds on the slopes, but they are absent below the basin sills (737 m). Galatheid crabs are the most abundant megafauna in the San Pedro and Santa Monica Basins. Species composition of all assemblages appeared to be seasonally stable, but 6". ingentis and the tuna crab Pleuroncodes planipes increased by several orders of magnitude during El Nino. Numbers of megafaunal species, individuals, and biomass were highest on the basin slopes reflecting elevated levels of organic material in the sediment. Numbers of individuals and biomass increased on the lower slopes (500-800 m), following El Nino. Although organic material is highest in the basins, the numbers of species and individuals were much lower than on the lower slopes. Near anaerobic conditions in the basins probably exclude many species.