Alan Williams, CSIRO - CERF Surrogates Program

The first systematic collection of epibenthic megafauna from Australia’s western continental margin was made in 2005.  The distributions of five major invertebrate taxa broadly aligned with those of fish but with some intriguing differences. 

Epibenthos was sampled at ~nineteen 1º intervals from Barrow Island to Bald Island (Figure 1), guided by the sub-biomes identified in Last et al.’s (2005) bioregional analysis of fishes.  Samples were taken from the outer continental shelf (~100 m depth) and the upper continental slope (~400 m depth) at all sites; additional depths (200 m, 700 m and 1000 m) were sampled at 7 of the 19 sites. Museum experts identified all octocoral, decapod (see article by Anna McCallum in this issue), mollusc, echinoderm, ascidian and pycnogonid specimens to species, resulting in 118 samples of standardised abundance (numbers per m2) for 1,602 species in 875 genera from 290 families. Seventeen physical covariates were derived from multibeam acoustics at the dredge scale (depth and mean and variance of backscatter), from depth specific CARS data interpolated to the sample midpoint (mean, range and standard deviation of temperature and oxygen) and from the CERF Geoscience Australia sediment data set at 1 km grid scale (percentage carbonate, gravel, sand and mud).

Megafaunal community distribution was most influenced by bottom temperature, oxygen concentration, and latitude, which vary on large spatial scales (>100s km), and seabed type at smaller megahabitat scales (10s to 100s of km).  Many covariates were driven by the same physical processes and were correlated (e.g. to depth or latitude); thus it is not possible to ascribe causal relationships with faunal distributions. Regional scale transitions in seabed temperature and oxygen concentration are determined by the properties of several major currents that interact with the margin seabed in different ways depending on location.  Nested within these scales is high spatial heterogeneity of seabed type that, even when classified simply as ‘hard’ or ‘soft’ terrain, differentiates consolidated attachment sites for sessile fauna from sediment classes suited to the needs of mobile and burrowing fauna.  Collectively, these patterns of heterogeneity can be captured using a hierarchical conceptual framework which consists of biogeographic provinces, biomes, biogeomorphic features, terrains, and several finer scales (see article by Vincent Lyne in this issue). 

Important findings from this study are that the provincial structure of invertebrate megabenthos broadly aligns with a provincial structure based on fishes, but that differences in distribution occur between major taxa at the provincial and megahabitat scales. Achieving representative coverage of rarer taxa or taxa with limited distributions might not be feasible at the same time as achieving adequate representation of the major faunal groups. The hierarchical scales of heterogeneity of the megabenthos in this area, the diversity between taxa, and the high proportion of apparently rare species makes it clear that adequately managing the area outside the National Representative System of Marine Protected Areas (NRSMPA) will be as important as managing the NRSMPA itself.

This research is part of the Census of Marine Life Continental Margins Program.

Figure 1 - Sample locations from the 2005 survey of Australia’s western continental margin coded by assemblage groups (contour lines: 100 m, 400 m, 700 m and 1000 m).