Quantitative assessment of nitrate sources in Jiaozhou Bay and its terrestrial inflows using nitrate stable isotope techniques

Excessive inputs of nitrate from land-based sources can lead to coastal eutrophication, and accurate identification of nitrate sources is an important prerequisite for the effective control of eutrophication. In this study, we selected Jiaozhou Bay, a typical eutrophic bay, as the case study area. Using nitrate concentration, nitrogen and oxygen stable isotope data, along with analytical techniques such as the Bayesian-based SIAR model and the LOICZ approach, we systematically assessed the sources and budgets of land-sea nitrogen pollution under China's new policies for land-based total nitrogen prevention and control. The results revealed that sewage and faeces were the main sources of nitrate in the Dagu River and the Moshui River, accounting for 42.3 % and 62.0 % of contributions, respectively, with significant assimilation observed in the former. Land use type was a key factor influencing differences in source contribution ratios. Land-based nitrate inputs contributed 26.1 % to Jiaozhou Bay, while nitrification accounted for up to 69.0 % of nitrate production, with both terrestrial and marine environments contributing equally to this process. Nitrate removal in the bay was dominated by sedimentation and export to the Yellow Sea. These results highlight the critical role of nitrification in nitrate pollution within eutrophic bays and provide critical data to support the development of total nitrogen control strategies along the river-sea continuum.