Light limitation selects for depth generalists in urbanised reef coral communities

Depth range is an important species trait for coral reef organisms, yet it remains to be quantified and analysed adequately among tropical coral species. Filling this knowledge gap is crucial as the depth limits of corals are related to important environmental factors such as light and temperature. Furthermore, the health and survivorship of corals may be threatened due to warming-induced sea-level rise, particularly for colonies living at the deeper limits of species depth ranges. Here we collected benthic and environmental data along the reef profile to characterise the depth ranges of coral species, and analysed species diversity and community structure in relation to possible depth-related biophysical parameters on the sediment-stressed reefs of Singapore. The results reveal clear environmental covariations with depth, expectedly with light availability showing the most marked decline as depth increases. Live coral cover, species richness and diversity are associated positively and significantly with light, which also structures coral communities along the reef profile more strongly than temperature or sediment levels. Relatedly, we detect species-specific depth distributions with two main strategies observed among coral species: shallow specialists and depth generalists. We suggest that corals in Singapore are unlikely to be impacted by light limitation specifically as sea level rises due to the wider depth range of the deeper species. Our data will inform conservation efforts especially in the selection of sites and depths for coral transplantation.