Biophysical Relationships And Assemblage Patterns

The Seabed Biodiversity Project sought to examine the relationships between the major driving factors outlined above and the biology observed by video and collected in epibenthic sled and scientific trawl samples at almost 1400 sites. These relationships are important in order to deliver integrated landscape maps of seabed assemblages, which are needed for management in the Marine Park. Such maps were produced by predicting species distributions and assemblage patterns in areas not sampled, on the basis of modelling their relationships with 28 more broadly available geo-physico-chemical variables. While this surrogate approach has limitations, it is the only feasible option given the vast size of the GBR, which means that despite considerable effort, sampling sites were relatively sparsely (average ~12 km apart) distributed over the shelf seabed.

The biophysical modelling indicated the major environmental factors that appeared to affect the distribution patterns of seabed habitats and assemblages in the GBR, including: sediment grain size (particularly the percentage of mud); force of water currents on the seabed (benthic stress); chlorophyll and/or turbidity; and, to a lesser extent, depth and some nutrients. While the correlations between the 28 physical factors in the GBR are complex and multidimensional, they have been simplified to just two dimensions (Fig. 6.1), after rescaling them in proportion to their importance for biological patterns. The most common environments

Figure 6.1 Biophysical map of the GBR continental shelf. Inset: colour key showing distribution of >170 000 seabed 0.01° grid cells on the first two principal dimensions of variation; contours show 99th and 50th percentiles of grid cell density; labelled arrows indicate direction of major physical factors. (Image: N. Ellis, CSIRO.)

Figure 6.1 Biophysical map of the GBR continental shelf. Inset: colour key showing distribution of >170 000 seabed 0.01° grid cells on the first two principal dimensions of variation; contours show 99th and 50th percentiles of grid cell density; labelled arrows indicate direction of major physical factors. (Image: N. Ellis, CSIRO.)

(50% of the seabed) are coloured light grey in the centre of the plot and represent mostly coarse sands, usually carbonate and often with sparse algae and benthos; the corresponding map shows the location of such areas. Less common and more extreme environments are shown in higher intensity colours and the arrows indicate the axes of the major physical factors, with the length indicating their relative influence on biological variation. By relating these axes to the colour key, the distribution of the dominant factors can be identified in the map. The colour key of Fig. 6.1 has been chosen to maximise the representation of the major physical environments of the GBR so that similar environments have a similar colour; nevertheless, the region is so varied that differences will occur among areas having the same colour. Further, only 65% of the environmental variation can be represented in the two dimensions of the key.

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