Nitrate dynamics and source apportionment on the East China Sea shelf re- vealed by nitrate stable isotopes and a Bayesian mixing model

The excess input of nitrate is one of the primary factors triggering nearshore eutrophication. To estimate the source apportionment of nitrate on the East China Sea (ECS) shelf, the nitrogen and oxygen stable isotopes in nitrate (815N-NO3- and 818O-NO3-) collected in winter and late spring 2016 were analyzed alongside essential physical, chemical and biological parameters. The temporal and spatial distributions and characteristic values of nitrate-bearing water masses were presented. Accordingly, the end-member mixing model and Rayleigh model were applied to systemati-cally analyze biogeochemical processes. The biogeochemical processes of nitrate were weak in winter, except in the southern ECS, where assimilation and nitrification probably occurred. In contrast, the biogeochemical processes were intensive in spring. The stable isotopic fractionations of N and O were unified in the whole area, and the ratio between 818O-NO3- and 815N-NO3- was 1.81 +/- 0.04, which indicated significant assimilation accompanying nitrifica-tion in spring. Furthermore, a Bayesian stable isotope mixing model was used to reveal the source contributions of ni-trate on the ECS shelf for the first time, demonstrating that the Changjiang Diluted Water and Kuroshio Subsurface Water were always sustained and provided steady nitrate sources for the whole ECS. The nitrate inputs from the Yellow Sea to the northern ECS increased from approximately 30 % in spring to nearly 70 % in winter, while that from the Taiwan Strait Warm Water to the southern ECS decreased from approximately 40 % in spring to zero in winter. More-over, although the nitrate contributions from nitrification were significantly weak in the middle and northern ECS during winter, they were important over the entire ECS during spring. This study qualitatively and quantitatively im-proves the understanding of seasonal nitrate control from various sources, and these findings are important for nutrient management and policy making to mitigate nearshore eutrophication.