Biogeochemical Dynamics in a Eutrophic Tidal Estuary Revealed by Isotopic Compositions of Multiple Nitrogen Species

Abstract

AbstractEstuaries sit at the land‐ocean boundary and act as natural reactors for nitrogen (N) transformations among numerous forms. Superimposing onto physical mixing, biological processes lead to nonconservative N behavior in estuaries. Under the coinfluences of biological alteration and multiple end‐members, however, some N species can exhibit apparent conservation. To explore N dynamics and potential mechanisms that modulate the mixing behaviors, we measured concentrations and natural isotopic compositions of multiple N species (nitrate: NO3−, nitrite: NO2−, ammonium: NH4+, and particulate nitrogen) in a typical eutrophic estuary in southern China. Additionally, N uptake and oxidation rates were measured by using isotope labelling techniques to evaluate processes potentially offsetting the conservative mixing of specific N pools. We found that NO3− followed conservative two end‐member mixing with dual isotopes varying in a narrow range (<1–2‰) due to low microbial preferences. Moreover, δ15N‐NO2− revealed a nonconservative pattern with an involvement of multiple end‐members. The dominant N transformation processes shifted downstream. In the upper estuary, ammonia oxidation (~20 μmol L−1 day−1) dominated NH4+ removal and was accompanied by NO2− accumulation. In the middle‐lower estuary, NH4+ uptake became dominant, with phytoplankton showing strong preference for it over NO3− and NO2−. Based on measured NH4+ uptake rates (9.1–12.5 μmol L−1 day−1 in the light and 0.9 μmol L−1 day−1 in the dark), short water residence time (<1 day) was required to maintain the conservative mixing. Coupling N isotopes in multiple nitrogen species with measurements of N uptake and nitrification, we successfully uncovered N dynamics and distinguished biological processes from physical mixing.