Nitrogen cycling in a tropical coral reef ecosystem under severe anthropogenic disturbance in summer: Insights from isotopic compositions

Coral reefs, one of the most productive ecosystems, have been dramatically declining in recent decades. While studies contend a prominent correlation between coral reef degradation and increased anthropogenic nitrogen (N) loads, a quantitative description of the N sources and cycling processes in these ecologically important ecosystems is lacking. Through a comprehensive depiction of the δ15N compositions of seawaters and sediments, we systematically accessed the N cycling processes in the Weizhou coral reef ecosystem. The correlations between the nitrate (NO3-) concentrations and isotopic compositions (δ15N/δ18O-NO3-) indicated the pelagic NO3- loads were largely regulated by mixing between precipitation and sewage. Biological NO3- turnover processes appeared to be weak. In the sediments, N2 fixation contributed about one-third of the sedimentary organic N, with the rest coming from the settlement of pelagic organic N. We also uncovered significant sedimentary mineralization-nitrification-denitrification processes in which the N loss was greater than the input. While pelagic N significantly contributed to the sedimentary N, the N export from the sediments to surface seawater was potentially short-circuited by the high N retention and recycling efficiencies of the organisms in the coral reef ecosystem. Overall, this study shows that the complex N cycling processes in the ecosystem are effectively reflected in the isotopic compositions of seawater and sediment, thus adding an important dimension to understanding the N cycling in coral reef ecosystems.