Nitrate (NO3-) dual isotope analysis was performed in Zhanjiang Bay, which is a closed bay with intensive human activities in South China, to investigate seasonal changes in the main NO3- sources and their biogeochemical processes in the monsoon-controlled climate. The relatively low N/P ratios in Zhanjiang Bay suggests that nitrogen (N) is a limiting nutrient, which indicates that the increase of N is favorable for phytoplankton proliferation. However, a sufficient amount of ammonium was found in our study area owing to intensive human activities, which can support biological processes. Thus, less NO3- biological processes were found, indicating that NO3- isotopic characteristics may reveal details of the mixing from various sources. The Bayesian mixing model showed that NO3- in the upper bay originated from manure (43%), soil N (30%), N fertilizer (17%), and N precipitation (10%) during winter, which reflects the local human activities; while NO3- sources during summer were mainly N fertilizer (36%), soil N (32%), and manure (31%), indicating the source as the runoff from the upper river basin. Our results suggest that nitrate dual-isotope was very useful for tracing the main NO3- sources in the condition of the sufficient ammonium, and runoff exerted an important impact on the shift in NO3- sources between both the local source and the source from the upper river basin during the two seasons in this monsoon-controlled bay.
Keywords: Bayesian isotope mixing model; Zhanjiang Bay; nitrate dual isotopes; nitrate sources.