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

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, delta 18O-NO3 -and delta 18OH2O 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.