The influence of depth and substrate inclination on sessile assemblages in subantarctic rocky reefs (Magellan region)

Previous studies from other latitudes have demonstrated that depth and spatial heterogeneity, produced by surface inclination, can have strong effects on diversity and distribution of sessile assemblages on rocky reefs. Rocky reef communities in the subantarctic Magellan region have been rarely studied, and the factors influencing diversity, distribution and abundance of benthic communities remain poorly understood. Sessile benthic assemblages inhabiting rocky reefs habitats were studied by SCUBA diving at Punta Santa Ana, Magellan Strait from 0 to 30 m water depth. We describe the sessile assemblages assessing the effect of depth, inclination and other environmental factors on species richness and community structure. A total of 37 taxa of invertebrates and 31 taxa of macroalgae were identified based on 280 high-resolution photoquadrats. Species richness and percent coverage varied with depth and inclination. Macroalgae dominated in abundance in the shallows, while bryozoans and ascidians (mound and tree-like forms) increased their coverage with depth. Lithothamnion sp. dominated on vertical and inclined surfaces while sheet-like organism such as bryozoans increased their coverage on overhanging surfaces. Multivariate analyses showed that sessile assemblages at Punta Santa Ana are strongly influenced by the interaction between inclination and depth, which alter the effect of other physical factors such as light and sedimentation. In this regard, our results suggest that sedimentation may play a role structuring benthic assemblages in Punta Santa Ana, especially in deeper zones, where it may replace the structuring effect produced by light in the shallows.