Regional infauna

In all, 1,620 animals representing 209 taxa were sorted from the grab samples at the three sites. The two major taxa encountered were polychaetes and crustaceans which made up over 84 percent of the total species found at each site. They were also the most abundant animals, accounting for more than 88 percent of the total number of individuals found, with echinoderms, molluscs, nemerteans, sponges and fish making up the remainder.

In a comparison of the three sites it was found that the sites at Bayu-Undan were characterised by low species richness (number of species present per sample), and abundance (number of animals per sample). In broad terms the infauna collected from the three regions were very similar (Figure 44). 

For example, the number of species of polychaetes and crustaceans at each of the sites was much the same, although the abundance of the particular taxa varied notably. The overall percentage of polychaetes at Bayu-Undan is nearly double the values for Elang Field and Mallee East-1. The mean number of species and mean abundances at Bayu-Undan sites were less than half those at the Mallee East-1 site and the Elang Field site (Figure 45). 

This may have been attributable to sediment size, which is known to directly affect community composition of benthic systems (Rhoads, 1974). Mallee East-1 and Elang Field seem to have greater similarity than Elang Field and Bayu-Undan, which are much closer together, providing some evidence that there may be significant spatial variability of benthic infaunal communities in this area of the Continental Shelf.

All sites had low species richness and abundance, in comparison with other studies conducted in the region (eg. Long and Poiner, 1994). This is not completely unexpected as these sites were at greater depths than the previous studies in the region. At depths of 20-80 metres, oxygen levels in the water begin to become stressful to benthic life (Alongi, 1990). It has been suggested by Alongi (1989c) that tropical, benthic habitats are subject to a wider range of environmental disturbances than temperate habitats, thus increasing variation in species diversity and abundance.

Most of the species found were usually present in very low numbers. A high proportion were found as either single records, or accounted for fewer than 5 percent of the samples. This seems to be a common trend in tropical benthic habitats (Saenger et al., 1980).