Geomorphological drivers of benthic community distribution on high energy temperate mesophotic and rariphotic reefs

Current knowledge of mesophotic benthic reef communities has predominantly focused on the distribution of sessile assemblages along the depth gradient. However, the influence of geomorphology on exposure to hydrodynamic processes may independently shape community assemblages, in addition to depth. We examined the distribution of sessile invertebrates at 50-150 m depths on exposed continental shelf reefs within a Marine Park in southwest Tasmania. Using marine still imagery and bathymetric mapping derivatives, we described the spatial response of biota to the prevailing hydrodynamic regime and geomorphology in this region, based on the growth forms of dominant morphospecies. The biota is exposed to large oceanic swells where high-wave energy mobilises sediments at depths of up to 140 m. As expected, the encrusting sponge morphospecies group was most prevalent but showed a niche habitat preference. Unlike wave-exposed reefs elsewhere, we observed a high cover of dominant morphospecies with three-dimensional growth forms alongside encrusting sponges over a wide range of depths. Notably, whip octocorals, cup-like, and tube sponge morphospecies groups had high cover on the most exposed slopes, while other groups such as encrusting and simple massive sponges were absent. However, this baseline assessment suggests there may be high species turnover due to the severe conditions during ocean storms, which may influence population dynamics. We recommend conducting future monitoring at ecologically relevant intervals to assess the natural variability in distribution due to local habitat effects.