Dissolved N pollution and its biogeochemical constraints along a river-sea continuum of a typical dense oyster mariculture coastal water, northwest South China Sea

Dissolved nutrients, including nitrate (${\text{NO}}_3^{\text{–}}$-N) and its dual isotopes (δ¹⁵N-${\text{NO}}_3^{\text{–}}$ and δ¹⁸O-${\text{NO}}_3^{\text{–}}$) were systematically studied along a river-sea continuum area, wherein dense oyster mariculture is implemented, to constrain the pollution sources and biogeochemical cycling mechanisms of nitrogen (N). Total dissolved N, mainly composed of inorganic N, showed strong anthropogenic influence. Based on MixSIAR model results, N pollution was predominantly sourced from sewage/wastewater (55.9~64.3%). Nutrient stoichiometry revealed DIP and DSi stress, and surface water in the riverine region was severely eutrophic. The occurrences of eutrophication and changes in nutrient stoichiometry were significantly related varying N pollution sources in different in summer and winter. N dynamics were controlled by anthropogenic influence and physical mixing. However, due to insignificance biological processes involving denitrification, phytoplankton assimilation, N₂ fixation, and nitrification, including insignificant isotopic fractionations associated with these processes, and poor fitting of both the Rayleigh Model and Open system Model on the measured data, it is speculated that several folds of reductions in N load and eutrophication along the river-sea continuum, could be attributed to the combinations of significant N drawdown by dense oyster mariculture as well as nutrient dilution due to physical mixing of river and seawater during winter and summer.