Responses of infaunal composition, biomass and production to discharges from a marine outfall over the past decade

The largest municipal outfall on the west coast of Canada discharges into the southeastern Strait of Georgia, where high sedimentation from the Fraser River has maintained stable sediment geochemical and contaminant conditions from 2001 to 2011. Outfall exposure has not affected trophic structure or diversity (H′), but has significantly affected faunal composition and species richness, resulting in loss of crustaceans and echinoderms near-field. Organic biomass and production have mostly remained within expected background ranges for the Strait, due to recent increases in a low oxygen tolerant polychaete in the high deposition zone.A significant regional shift in faunal composition occurred after 2003, followed by gradual declines in richness, abundance, calcareous organisms and production. This cannot be attributed to changes in outfall exposure, but is exaggerated by it. We hypothesize that changing river flow, extreme events and shifts in offshore upwelling water temperature, oxygen and pH may be increasing geochemical stress in benthos.

► Outfall effects on faunal composition are clear and consistent over past decade. ► Outfall exposure results in taxa declines and loss of crustaceans and echinoderms. ► Organic biomass and production remain within background for the Strait of Georgia. ► Regional declines in taxa, calcareous organisms and production unrelated to outfall. ► Changing background river flow, temperature, oxygen may be increasing benthos stress.