Publication
Biogeochemical Dynamics in a Eutrophic Tidal Estuary Revealed by Isotopic Compositions of Multiple Nitrogen Species
Publisher:
American Geophysical Union (AGU)
Date:
07-2019
DOI:
10.1029/2018JG004959
Abstract: Estuaries 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: NO 3 − , nitrite: NO 2 − , ammonium: NH 4 + , 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 NO 3 − followed conservative two end‐member mixing with dual isotopes varying in a narrow range ( –2‰) due to low microbial preferences. Moreover, δ 15 N‐NO 2 − 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 NH 4 + removal and was accompanied by NO 2 − accumulation. In the middle‐lower estuary, NH 4 + uptake became dominant, with phytoplankton showing strong preference for it over NO 3 − and NO 2 − . Based on measured NH 4 + 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 ( 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.