Cross-shelf benthic community structure on the Great Barrier Reef: relationships between macroalgal cover and herbivore biomass

The Great Barrier Reef (GBR) is one of the most extensively studied coral reef systems in the world, yet to date, there has been no comprehensive multi-spatial scale evaluation of its benthic community structure. Such descriptions provide a useful reference point for evaluating future community changes, Moreover, large-scale associations between macroalgae and herbivory on the GBR are yet to be investigated. Our aim, therefore, was to quantify all major substratum categories across multiple spatial scales and investigate cross-shelf relationships between macroalgal cover and herbivore biomass. Using point-intercept transects and timed underwater censuses, individual benthic components and roving herbivorous fish densities were quantified across 3 spatial scales: latitude, continental shelf position and within-reef habitat. Principal component analysis and analysis of similarities (ANOSIM) revealed a distinct separation of inner-shelf reef habitats from all mid- and outer-shelf habitats in both northern and central regions of the GBR. Macroalgal cover was generally high on inner-shelf reefs (36 to 66%) and decreased markedly on all mid- and outer-shelf habitats (0 to 15%). A significant negative correlation was found between macroalgal cover and total herbivorous fish biomass, but no relationship was found between macroalgal cover and macroalgal browser biomass. In contrast to macroalgae, the cover of crustose coralline algae and live scleractinian corals increased markedly across the shelf in both regions of the GBR. These patterns appear to be shaped by both physical and biological factors, including wave energy, sedimentation and grazing intensity.