ASSESSING THE IMPACTS OF DEEP-WATER SEWAGE OUTFALLS ON SPATIALLY-VARIABLE AND TEMPORALLY-VARIABLE MARINE COMMUNITIES

Until recently, Sydney's domestic and industrial sewage was discharged to the Tasman Sea through outfalls at the cliff-face at North Head, B ondi and Malabar, NSW, Australia. To overcome the resulting pollution of nearby beaches, three deepwater outfalls were constructed and efflu ent 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 t he impacts of the new deepwater outfalls. This study describes the und erlying philosophy and sampling designs of this monitoring programme. In doing so, it provides an overview of the pre-commissioning phase st udies of the ichthyoplanktonic, demersal fish and soft-bottom communit ies. The abundances of the organisms comprising the three communities fluctuated in space and time. This sampling highlighted marked differe nces in the depth-distributions of larval fish. Trawling and long lini ng further reinforced technique-dependent selectivity and overcame pro blems of environmental heterogeneity, which are often manifest when sa mpling fish populations. Power analyses using data for six families of polychaetes demonstrate the concerns over Type II errors in environme ntal impact assessment, and this paper suggests ways of addressing thi s issue. Finally, an experimental design is discussed that incorporate s estimates of spatial and temporal variation, thus allowing better (u nconfounded) assessments of the impacts of sewage-disposal on marine b iological communities.