ELSEV IER Marine Environmental Research, Vol. 41, No. 1, pp. 45-71, 1996 Copyright 01995 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0141-1136/96 $9.50+0.00 0141-1136(94)00001-1 Assessing the Impacts of Deepwater Sewage Outfalls on Spatially- and Temporally-Variable Marine Communities N. M. Otway,* C. A. Gray, J. R. Craig, T. A. McVea & J. E. Ling Fisheries Research Institute, PO Box 21, Cronulla, NSW 2230, Australia (Received 30 June 1994; revised version received 12 December 1994; accepted 30 December 1994) zyxwvutsrqponmlkjihgfedcbaZ 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 efluent is now dis- charged 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 ichthyoplank- tonic, demersal fish and soft-bottom communities. The abundances of the organisms comprising the three communities zyxwvutsrqponmlkjihgfedcbaZYXWVUT fluctuated in space and time. This sampling highlighted marked dtjherences in the depth-distributions of larval fish. Traw ling and longlining further reinforced technique- dependent selectivity and overcame problems of environmental heterogeneity w hich 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. INTRODUCTION Ocean-disposal of sewage dates back to the earliest of civilisations (Shepherd & Cannon, 1989) and now poses problems for developed *To whom correspondence should be addressed. 45