Effects of Listriolobus pelodes (Echiura) on coastal shelf benthic communities and sediments modified by a major California wastewater discharge
The Palos Verdes coastal zone off Los Angeles receives over 1·4×10 9 l of primary treated wastewaters daily. Benthic effects from this discharge include elevated organic matter and trace contaminants in sediments, generation of H 2S among sediment porewaters and altered biological community structure. In 1973 large numbers of the echiuran Listriolobus pelodes settled in shelf sediments several kilometers from the outfalls. By 1975 the centre of Listriolobus distribution had shifted towards the outfall region, where peak numbers (1500 m -2) and biomass (2 kg m -2) developed. Populations declined in 1977 and the echiuran had virtually disappeared from the shelf by 1978. The echiurans' burrowing, respiratory, and feeding activities aerated and reworked sediments, and thus reduced wastewater impacts on the ocean bottom, enabling a diverse fauna to develop in the outfall region. With the disappearance of the echiuran, sediment quality and benthic communities reverted towards former conditions. However, because of improvements made in effluent quality during the echiuran period, wastewater impacts were of lower magnitude than before the Listriolobus invasion.