Spatiotemporal variability of nitrate sources and transformations in the coastal waters of Wenzhou revealed by dual isotopes.

Anthropogenic nitrogen loading and hydrodynamic exchange jointly regulate nitrate distributions in open coastal waters, complicating source attribution based on concentration or isotope patterns alone. This study investigated nitrate sources and processing in the coastal waters of Wenzhou, East China Sea, using hydrochemical parameters, δ15N-NO₃-, δ18O-NO₃-, δ15N-NH₄+, conservative water-mass mixing analysis, and MixSIAR. A pronounced estuarine-to-offshore environmental gradient was observed. Most nitrate concentration and isotope observations were consistent with conservative mixing among terrestrial, Taiwan Strait Warm Water, and Kuroshio Subsurface Water end-members, indicating first-order hydrodynamic control. Local departures from the mixing relationships were compatible with superimposed biological nitrate processing, although individual transformation pathways could not be uniquely resolved. MixSIAR identified manure and sewage as the largest individual posterior source category, contributing 31-40% in summer and 35-46% in autumn. Strong isotope overlap between soil nitrogen and fertilizer limited their independent discrimination, and their combined contribution was more robust than their individual posterior proportions. Nitrate dynamics in the Wenzhou coastal waters therefore reflect the combined effects of terrestrial nitrogen inputs, water-mass mixing, and spatially heterogeneous biogeochemical processing.