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 delta N-15 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 (delta N-15/delta O-18-NO3-) indicated the pelagic NO3- loads were largely regulated by mixing between precipitation and sewage. Biological NO(3)(-)turnover processes appeared to be weak. In the sediments, N-2 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.

Automated summary

The complex nitrogen cycling processes in the coral reef ecosystem are effectively reflected in the isotopic compositions of seawater and sediment, with nitrate sources being largely regulated by a mix of precipitation and sewage influence. The study also reveals significant nitrogen mineralization-nitrification-denitrification processes in sediments, with nitrogen loss exceeding input.