Ecological changes in the Southern California Bight near a large sewage outfall: benthic conditions in 1980 and 1983

Abstract

The structure of the macrobenthic community, sediment toxicity, and sediment contamination changed greatly between 1980 and 1983 along a pollution gradient from the Los Angeles County Sanitation Districts' (LACSD) sewage outfalls on the Palos Verdes Shelf, California, USA to a reference site in northern Santa Monica Bay. Stimulation of the species richness, biomass, and density of the benthos along the 60 m contour S to l1 km from the outfalls was significantly reduced in 1983 when compared to 1980. The clam Parvilucina tenuisculpta and the polychaete Tharyx sp. A were the numerically dominant species in the area of benthic shmulation in both years. Stahons 1 to 3 km from the outfalls in 1980 were occupied by a de auperate fauna strongly dominated by the opportunistic polychaete C a p i t e h spp. In 1983 species 6 chness and biomass significantly increased close to the outfalls, CapiteUa spp. was much less abundant, and the assemblage was dominated by ostracods of the genus Euphilomedes that are charactenstic of background conditions or areas only slightly affected by wastewater discharges. At the reference site in Santa Monica Bay there were no significant changes in species richness, biomass, or density between 1980 and 1983, and the brittlestar Amphiodia urtica remained the dominant species. Sediment contaminabon by most measured chemicals and parameters of organic enrichment decreased on the Palos Verdes Shelf, but not at the reference station, between 1980 and 1983. Sediments collected within 3 km of the outfalls in 1980 were acutely toxic to the phoxocephalid amphipod Rhepoxynius abronius. There was no significant toxicity associated with any sediment from the Shelf in 1983. These ecological changes correspond with the reduction in the mass emission of BOD and chemical contamnants from the LACSD outfalls between 1980 and 1983. Severe stornls in the winter of 1982-83 may also have contributed to improvements in sediment quality and the partial recovery of the macrobenthos.