Roles of depth, current speed, and benthic cover in shaping gorgonian assemblages at the Palm Islands (Great Barrier Reef)

Gorgonians are a diverse and conspicuous component of coral reef ecosystems, providing habitat structure that supports unique assemblages of fishes and invertebrates. Evaluating their overall importance as ecological engineers requires an understanding of their spatial patterns of distribution, abundance and assemblage composition, and the biophysical factors that drive these patterns. No baseline data are available on the spatial patterns of distribution of gorgonians for the Great Barrier Reef. In this study, we quantified the abundance, genera richness, and composition of gorgonian assemblages using video surveys at three depths (5, 10, and 15 m) at 16 locations at the Palm Islands, an inshore island group in the central Great Barrier Reef. We compared gorgonian abundance and genera richness between depths and assessed the role of benthic habitat in structuring gorgonian communities. We also conducted a preliminary investigation of the potential role of water currents in driving gorgonian spatial patterns in the Palm Islands, using in situ current meters. Gorgonian abundance and genera richness consistently increased with depth, although the magnitude of the depth effect varied among locations. Abundance increased with increasing percent cover of rubble and conversely declined with increasing cover of hard corals. The composition of gorgonian assemblages also varied among depths, with whip ( Junceella , Viminella ) and fan ( Acanthogorgia , Anthogorgia , Annella ) growth forms being dominant at depths of 5 and 10 m, and branching ( Dichotella , Icilogorgia ) and candelabrum ( Ctenocella ) forms being dominant at 15 m. The shallow gorgonian assemblage was associated with high coral cover, whilst the deeper assemblage was associated with high per cent cover of rubble, turf and/or macroalgae. This study highlighted that the abundance, diversity, and composition of gorgonian assemblages on coral reefs in the Palm Islands are determined by a range of biophysical factors linked to depth. Further work is required to isolate the primary drivers of these depth-related effects and evaluate their relative importance.