Relative importance of environmental gradients and historical trawling effort in determining the composition and distribution of benthic macro-biota in a large inverse estuary

In this paper we describe the composition and distribution of benthic macro-biota in Spencer Gulf, South Australia, and evaluate the contributions of prawn trawling effort and local environmental factors to observed spatial patterns. Quantitative samples of benthos were obtained from 120 sites using commercial prawn trawling gear, and 4.2 tonnes of benthos belonging to 395 species and 12 phyla collected. Fish, prawns and crabs dominated the catch and together comprised 96% of the total abundance and 82% of the total biomass. These motile taxa also represented 38% of the total species richness. Overall catch weights were strongly correlated with abundance, and both parameters were highest on the western side of the Gulf where nutrient-rich water from the shelf flows inwards. In contrast, species richness was highest in the east where nutrient-depleted water flows out of the Gulf. A combination of univariate (ANOVA) and multivariate (MDS, ANOSIM) statistical techniques were used to evaluate possible trawl-related impacts on individual species, species groupings and communities. ANOVA tests showed no significant (p < 0.05) trawl-related differences in overall species richness, abundance or biomass. Individual species did, however, show significant but contradictory patterns in relation to historical trawling effort. The abundances and biomasses of 6 of the 20 most common species were significantly (p < 0.05) higher on the most intensively trawled grounds, while 5 of the 20 most common species were significantly (p < 0.05) lower. ANOVA tests were also applied to species grouped by phyla to evaluate their susceptibility to past trawling effort. These tests confirmed that sponges, bryozoans and fish were significantly (p < 0.05) less prevalent on the heavily trawled areas. Despite such differences, MDS showed that benthic communities were similar at sites with different trawling histories. These same multivariate techniques also confirmed that putative trawl-related differences in community structure were small when compared to those associated with latitude. A strong environmental gradient was identified between the north and south of the Gulf, and effectively overwhelmed any trawl-related differences in benthos. This gradient was classified into four geographical regions supporting different biotic assemblages. Latitude and depth accounted for most (ρw=0.641ρw=0.641) of this spatial variation in community structure, but it is likely that other unmeasured factors play an important role in structuring the benthic biota of the Gulf.