A three-year study of benthos of muddy environments in Port Phillip Bay, Victoria

Abstract

Samples taken four times a year over three years (1973–1975) at three widely separate stations in muddy environments in Port Phillip Bay were used to compare species diversity and composition of the benthic community between areas, seasons and years. Analysis of variance of five community statistics: the number of individuals, the number of species, Shannon's diversity and two evenness measures; showed that differences between the three stations and between years contributed the largest fractions of the variance in number of individuals and species but little to diversity or evenness. Seasonal differences contributed negligibly to variance in any statistic except diversity. Smallscale spatial patchiness (from replicate samples within stations) contributed a significant fraction of variance to all five statistics. The fauna at all stations was dominated by the deposit-feeding bivalve Theora fragilis and the decapod shrimp Callianassa limosa; only minor species were restricted to single stations. The density of few species varied seasonally but several were more abundant in some years than others. Hierarchical classification using the Canberra metric dissimilarity coefficient grouped samples into station groups. Use of the Bray-Curtis coefficient grouped some samples by station and some by season and it was concluded that the main differences between stations were in overall species composition, not in the densities of dominant species. Temporal changes in community structure were non-seasonal but, rather, were irregular changes occurring at single stations. Differences in fauna between stations were related in part to small differences in sediment-type but not to physico-chemical features of the overlying water. The irregular fluctuations in density of the common species and in the identity of the minor species suggest that multiple-stable points in community structure may exist for the silty-clay community. The two major species seemed not to have altered each other's environment over the sampling period. Considerable variation in species composition may occur in soft-bottom benthos in the absence of marked environmental perturbations.