Atmospheric Nitrogen Deposition Increases the Possibility of Macroalgal Dominance on Remote Coral Reefs

Field surveys in the Xisha and Zhongsha archipelagos, in the midnorthern South China Sea, show macroalgal dominance on remote reefs that were previously dominated by corals. This study was conducted to investigate the underlying mechanisms of this phenomenon. Benthic composition analyses indicated that macroalgae that are rare on pristine reefs dominated 27% of 81 study sites, and live coral cover dropped to 16.3% in 2015. Nearly one third of corals came into direct contact with macroalgae. Further analyses on the algal distribution patterns, nutrient fluxes, and air mass backward trajectories suggest that atmospheric nitrogen deposition is the major external supply of nutrients. The greater cover of macroalgae on the coral reefs coincides with the considerable atmospheric nitrogen deposition from the southwest during the summer monsoon. Moreover, the remote coral reefs hosted sevenfold greater macroalgae cover than the coastal coral reefs because atmospheric nitrogen assimilates effectively in the N limited and high visibility water. Macroalgae promotion is projected to grow with the increasing atmospheric nitrogen supply in the coming decade, which constitutes a larger stress on remote coral reefs.