Patterns in subtidal marine assemblages associated with a deep-water sewage outfall

Abstract

The macrobenthic assemblages living on hard substrata in the vicinity of the Sydney deep-water ocean outfall at North Head were sampled from March 1991 to April 1994. The main objective of the study was to determine whether spatial and temporal patterns in macrobenthic diversity and abundance were affected by the outfall. The assemblages were sampled according to an experimental design that had three sites nested within each of three locations. These locations were North Head (outfall location), Long Reef and Bungan Head (control locations). Ten replicate quadrats at each site were photographed with a jump camera. These photographs were then sampled to provide estimates of the number and abundance of the major component phyla and also abundances of individual species. An asymmetrical analysis of variance detected a significant time × outfall versus control interaction for the total number of species, bryozoans and cnidarians and for the abundances of two bryozoan species and a silt matrix. Smaller-scale interactions among sites through time occurred for many taxa, representing both inherent 'noise' and some evidence for outfall effects at these smaller spatial scales. This study demonstrates that there is correlative evidence of an effect of the outfall, but in general the assemblages showed various spatial and temporal fluctuations that should not be attributed to sewage.