Seasonal nitrogen sources and its transformation processes revealed by dual-nitrate isotopes in Xiamen Bay, China

The eutrophication load in coastal waters is gradually increasing due to excessive nitrate (NO₃⁻) input caused by intense human activities. The tracing of NO₃⁻ sources as well as their biogeochemistry is crucial for the development of effective measures to alleviate the eutrophication load. In this study, seasonal seawater samples from Xiamen Bay were investigated to explore the sources and fates of NO₃⁻ using dual nitrate isotopes. The results indicated that the NO₃⁻ is less affected by biological processes and mainly influenced by the terrestrial sewage discharges (48 %) during rainy seasons. Contrarily, as runoff input decreases and external seawater intrusion increases, the assimilation of phytoplankton dominates the process of nitrate removal during dry seasons. This study suggests that the increase in the input of terrestrial sewage nutrients is responsible for the increase in eutrophication in Xiamen Bay, providing a basis for controlling water eutrophication and regional environmental governance.