Assessing the impacts of deepwater sewage outfalls on spatially-and temporally-variable marine communities

Abstract

Until recently, Sydney's domestic and industrial sewage was discharged to the Tasman Sea through outfalls at the cliff-face at North Head, Bondi and Malabar, NSW, Australia. To overcome the resulting pollution of nearby beaches, three deepwater outfalls were constructed and effluent is now discharged from the seabed in 60–80 m of water some 2–4 km offshore. An environmental monitoring programme was set up to assess the impacts of the new deepwater outfalls. This study describes the underlying philosophy and sampling designs of this monitoring programme. In doing so, it provides an overview of the pre-commissioning phase studies of the ichthyoplanktonic, demersal fish and soft-bottom communities. The abundances of the organisms comprising the three communities fluctuated in space and time. This sampling highlighted marked differences in the depth-distributions of larval fish. Trawling and longlining further reinforced technique-dependent selectivity and overcame problems of environmental heterogeneity which are often manifest when sampling fish populations. Power analyses using data for six families of polychaetes demonstrate the concerns over Type II errors in environmental impact assessment, and this paper suggests ways of addressing this issue. Finally, an experimental design is discussed that incorporates estimates of spatial and temporal variation, thus allowing better (unconfounded) assessments of the impacts of sewage-disposal on marine biological communities.