Disentangling external loadings, hydrodynamics and biogeochemical controls on the fate of nitrate in a coastal...

Nitrogen, as an essential nutrient, largely contributes to the coastal eutrophication. However, the accurate depiction and evaluation of how external loadings, hydrodynamics, and biogeochemical reactions mediate the occurrence, transport, and transformation of nitrate (NO₃^-) within coastal embayment still pose ongoing challenges to date. In this study, we took advantage of dual isotopes of NO₃^- to track external NO₃^- loadings, radium and dual isotopes of H₂O to characterize the influences of hydrodynamic on NO₃^- transport, δ¹⁸O-NO₃^- and δ¹⁸O-H₂O along with microbial analysis to explore major NO₃^- biogeochemical reactions in Tolo Harbour, Hong Kong. The multiple isotopic evidence showed that NO₃^- in surface harbour water was predominantly contributed by precipitation in wet season and its impact was strengthened by stratification. In dry season, NO₃^- in the surface harbour water became largely influenced by benthic input and biogeochemical reactions due to intensified vertical mixing. Based on NO₃^- mass balance model, biogeochemical reaction, especially nitrification, was found to be the major process to secure the closure of NO₃^- budget and increase NO₃^- inventory from wet to dry season. Hydrodynamics redistributed the external NO₃^- loadings and mediated nitrogen biogeochemical reactions, both of which further synergistically regulated the fate of NO₃^- in the embayment.