Habitat complexity enhances primary productivity on coral reefs.

Energy fluxes throughout ecosystems can be predicted by scaling relationships linking metabolic rates with the structural attributes of organisms and habitats. Coral reef ecosystems are structurally complex and highly productive, yet quantitative links between habitat structure and productivity have not been identified. Here we use benthic metabolic chambers to quantify rugosity-productivity relationships across shallow reef plots in Australia and Hawai'i. In each region, habitat rugosity explained 56-58% of variation in daytime community metabolic rates, despite naturally varying light, temperature and benthic composition (for example, coral versus algae cover). Allometric scaling with habitat rugosity was found for gross photosynthesis and respiration (scaling exponents 1.23 ± 0.19 and 1.45 ± 0.23, respectively), and sites with higher rugosity produced a greater surplus of photosynthetic carbon after meeting community respiration demands (higher net community production). Nevertheless, the proportion of gross photosynthesis allocated to net community production was diminished on high-rugosity reefs (lower carbon use efficiency), possibly because of increased respiration from cryptic, heterotrophic organisms. Our study shows that habitat complexity is a strong predictor of energy fluxes in a variable reef environment, with consistent scaling properties in two distinct regions. Moreover, reefs with complex habitat structure fix more net organic carbon for biomass accumulation and export to other organisms.