Interspecies and spatial diversity in the symbiotic zooxanthellae density in corals from northern South China Sea and its relationship to coral reef bleaching

Coral reef bleaching is usually characterized by expulsion of symbiotic zooxanthellae, loss of zooxanthellae pigmentation, or both. We collected 128 samples comprising 39 species of 21 genera of reef-building corals from Luhuitou and Xiaodonghai in Sanya of Hainan Island and Daya Bay of Guangdong Province, respectively, and analyzed the symbiotic zooxanthellae population density. The results show that: (1) the symbiotic zooxanthella density varies from 0.67x10(6) to 8.48x10(6) cell/cm(2), displaying significant interspecies variability, with branch corals usually having relatively less zooxanthellae (ranging from 0.67x10(6) to 2.47x10(6) cell/cm(2)) than massive species (from 1.0x10(6) to 8.48x10(6) cell/cm(2)); (2) corals inhabiting within 4 m water depth have higher levels of symbiotic zooxanthellae than those living at the bottom (similar to 7 m depth) of the reef area; (3) there is no discernable difference in the zooxanthellae density between corals from relatively high latitude Daya Bay (similar to 22 degrees N) and those from relatively low latitude Sanya (similar to 18 degrees N) at comparable sea surface temperatures (SST); (4) in partially- bleached corals, the density of zooxanthellae shows the following order: healthy-looking part> semi-bleached part > bleached part. Based on the above results, we suggest that (1) the zooxanthellae density difference between branching and massive coral species is the main cause that branching corals are more vulnerable to bleaching than massive corals. For example, symbiotic zooxanthellae levels are low in branching Acropora and Pocillopora corals and thus these corals are more susceptible to bleaching and mortality; (2) symbiotic zooxanthellae density can also be affected by environmental conditions, such as sediment loads, diving-related turbidity, and aquaculture-related nitrate and phosphate input, and their increase may reduce symbiotic zooxanthellae density in corals.