Photosynthetic usable energy explains vertical patterns of biodiversity in zooxanthellate corals

The biodiversity in coral reef ecosystems is distributed heterogeneously across spatial and temporal scales, being commonly influenced by biogeographic factors, habitat area and disturbance frequency. In contrast to terrestrial environments, a potential association between gradients of usable energy and biodiversity has received little empirical support for explaining such patterns. Here, we analyzed the productivity and biodiversity variation over depth gradients in symbiotic coral communities, whose members rely on the energy translocated by photosynthetic algal symbionts (zooxanthellae). A mechanistic model was used to simulate the depth-dependent variation in photosynthetic usable energy to corals and explore its relationship with gradients of species diversity, comparing reefs with contrasting water clarity and biodiversity patterns along global hotspots of marine biodiversity. The productivity-biodiversity model explained between 64% and 95% of the depth-related variation in coral species richness, indicating that much of the variation in coral biodiversity with depth is driven by changes in the fractional contribution of photosynthetically fixed energy by the zooxanthellae. These results suggest a fundamental role of solar energy availability and photosynthetic productivity on global-scale patterns of coral biodiversity and community structure across depths. Accordingly, the maintenance of water optical quality is fundamental to protect coral biodiversity and prevent reef degradation, in addition to climate change mitigation policies.