Light availability regulated by particulate organic matter affects coral assemblages on a turbid fringing reef

Recently, increasing evidence suggests that reef-building corals exposed to elevated suspended solids (SS) are largely structured by changes in underwater light availability (ULA). However, there are few direct and quantitative observations in situ support for this hypothesis; in particular, the contribution of SS to the diffuse attenuation coefficient of the photosynthetically active radiation (K_d-PAR) variations is not yet fully understood. Here, we investigated the variations in ULA, the structure of coral assemblages, and the concentration and composition of SS on the Luhuitou fringing reef, Sanya, China. Light attenuation was rapid (K_d-PAR: 0.60 ± 0.39 m^-1) resulting in a shallow euphotic depth (Z_eu-PAR) (<11 m). Benthic PAR showed significant positive correlations with branching and corymbose corals (e.g. Acropora spp.), while massive and encrusting species (e.g. Porites spp.) dominated the coral communities and showed no significant correlations with PAR. These results indicate that the depth range available for coral growth is shallow and the tolerance to low-light stress differs among coral species. Notably, K_d-PAR showed no significant correlations with the grain size fractions of SS, whereas significant positive correlations were found with its organic fraction content, demonstrating that the light attenuation of SS is mainly regulated by particulate organic matter (POM). Intriguingly, our isotopic evidence revealed that POM concentration contributed the most to changes in K_d-PAR, with its source being slightly less important. Combined, our results highlight ULA regulated by POM is an important factor in contributing to changes in coral assemblages on inshore turbid reefs, and reducing the input of terrestrial materials, especially POM, is an effective measure to alleviate the low-light stress on sensitive coral species.