Depth gradients drive changes in early successional community composition and associated coral larvae settlement interactions

Depth gradients are known to drive patterns of adult and juvenile coral distribution, but the influence on early successional communities and subsequent interactions with coral larvae settlement are poorly understood. We studied how early successional communities changed across a 40 m depth gradient to test whether patterns of coral larvae settlement could be explained by changes in preferred settlement substrates. Light decreased 83-fold from 3 to 40 m, and coral settlement orientation switched from tile undersides at 3 and 8 m to tile topsides at 25 and 40 m as expected. Regardless of the reduction in light, community composition on the topsides of settlement tiles did not change across depths, being dominated by turf algae (49%), crustose coralline algae (CCA; 29%), and biofilmed tile (12%). In contrast, settlement tile undersides were dominated by coralline and fleshy algae at 3 m, transitioning to ascidians and bryozoans at 8 m and 25 m, and biofilmed tile (80%) at 40 m. Following multiple regression analysis from an initial 21 bio-physical variables (R² = 0.34), the optimal reduced model for predicting coral settlement included just five variables and explained most of the variability (R² = 0.29). CCA contributed an R² of 0.14 to the model and was positively correlated with Acroporidae settlement, biofilmed tile contributed an R² of 0.09 to the model and was negatively correlated with Acroporidae settlement, while the other predictors explained nominal model variance. As assessed using electivity indices, Acroporidae switched settlement preference from CCA at shallower depths to biofilmed tile in deeper environments. In addition to the influence of light on larval settlement behaviour as described in previous research, our results show that CCA also provide an important facilitator of coral settlement behaviour across depth gradients. However, as in shallow reefs environments, further work is required to elucidate whether changes occurring at a finer taxonomic resolution within the CCA community can help to further explain switches in coral larval settlement behaviour in light-limited environments.