Macroalgal assemblage changes on coral reefs along a natural gradient from fish farms in southern Hainan Island

Degradation processes on coral reefs are associated with changes in macroalgal assemblages. In this study, a floristic study of the macrophyte species on coral reefs along a pollution gradient at 3 transects in southern Hainan Island, China, was conducted in April 2014. A total of 73 benthic macroalgal taxa were collected, of which 58% were reds, 23% browns and 19% greens. Along with the distance away from the outlet of fish farms, macroalgal species richness increased significantly from 40 species to 71 species. However, macroalgal biomass decreased from 1.58 kg m⁻² to 1.1 kg m⁻². Brown blooms of Chnoospora implexa, Padina australis, Sargassum polycystum and S. sanyaense changed to green blooms of Ulva lactuca and Caulerpa racemosa from moderately polluted sites to severely polluted sites close to the outlet. This study supports the results that severe pollution can lead not only to a significant decline of macroalgal species richness and changes in macroalgal assemblages, but also to the occurrence of green blooms. This study will hopefully provide more accurate cases to link nutrient pollution to changes in benthic organisms on coral reefs.