Assessing Seasonal Nitrate Contamination by Nitrate Dual Isotopes in a Monsoon-Controlled Bay with Intensive Human Activities in South China

Nitrate (NO(3)(−)) 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 NO(3)(−) 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 NO(3)(−) biological processes were found, indicating that NO(3)(−) isotopic characteristics may reveal details of the mixing from various sources. The Bayesian mixing model showed that NO(3)(−) 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 NO(3)(-) 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 NO(3)(−) sources in the condition of the sufficient ammonium, and runoff exerted an important impact on the shift in NO(3)(−) sources between both the local source and the source from the upper river basin during the two seasons in this monsoon-controlled bay.