Impact of a combined sewer overflow on the abundance,distribution and community structure of subtidal benthos

A biological survey of the subtidal fauna near an intertidal combined sewer overflow (CSO) discharging into Puget Sound, Washington, USA, was conducted during April and May, 1977. The purpose of this study was to determine the effect of the CSO on the surrounding marine invertebrate communities.The shallowest subtidal sites studied (9 m below MLLW) appeared to be more affected by the CSO than deeper sites (13 and 24 m below MLLW). The subtidal area affected, as reflected in the levels of organic material, heavy metals and pesticides in the sediments, as well as the composition of the benthic macrofauna, appeared to be highly localised. The numerically dominant species at sites closest to the CSO was the polychaete Capitella capitata while the abundance of the bivalve Axinopsida serricata increased with distance from the CSO.Burrowing deposit feeders were the numerically dominant polychaete feeding type at sites nearest the CSO. Surface deposit feeders were more abundant at sites farther from the CSO, perhaps due to reduced sedimentation rates.Site-classification analyses indicated that sites adjacent to the CSO were more similar to one another than to sites which were farther away and apparently less affected.The positions of the sites along the first three principal component ordination axes were significantly correlated with depth and volatile organics (traceable to the CSO) in the sediments.