Abstract: Hermatypic corals are the main constructor of coral reef ecosystems. The nitrogen (N) fixation by hermatypic corals associated microbes can relieve N limitation in oligotrophic tropical waters, and consequently sustain high primary productivity of the coral reef ecosystems. In this study, the acetylene reduction assay (ARA) method was applied to detect the N fixation rate of diazotrophs associated with the corals inhabited in Luhuitou Peninsula, Sanya, Hainan and Nanshazhou, Xisha Island. The concentrations of dissolved inorganic nutrients were measured to explore the relationship between N fixation rate and environmental factors. The N fixation rate of all investigated corals in Luhuitou Peninsula showed no significant seasonal change. There were significant differences in the N fixation rates among Pocillopora damicornis, Porites lutea, and Acropora gemmifera. The highest N fixation rate was detected in Pocillopora damicomis(P<0.05), being 5.925×10^-11 mol C₂H₄·g^-1·h^-1 and 6.471×10^-11 mol C₂H₄·g^-1·h^-1 in summer and winter, respectively. The N fixation rate of Porites lutea(5.401×10^-11 mol C₂H₄·g^-1·h^-1) from oligotrophic Xisha was significantly higher than that from Lunhuitou (1.971×10^-11 mol C₂H₄·g^-1·h^-1) (P<0.01). Our results revealed no seasonal variation of N fixing rate in coral reef ecosystems with high concentration of nutrients. In addition, the high concentration of soluble nutrients in seawater may reduce the N fixation activity in corals.

Key words: temperature zone, species diversity, vegetation type, the East China Sea, annual mean temperature, the Yellow Sea, annual mean precipitation