Identification of nitrogen sources and cycling along freshwater river to estuarine water continuum using multiple stable isotopes

Abstract

Nitrogen (N) transport from terrene to river water is a major source of N in estuarine water, contributing to eutrophication, harmful algal blooms and hypoxia. However, there is a lack of holistic and systematic research on N sources and transformation in the freshwater river-estuarine water continuum. In this study, multiple stable isotope signatures of nitrate (δ15N-NO3−, δ18O-NO3−), ammonium (δ15N-NH4+), and suspended particulate nitrogen (δ15N-PN) were employed to differentiate the sources and transformations of N and calculate the proportional contribution of NO3− sources by Bayesian model in Qiantang River (QTR)-Hangzhou Bay (HZB) during the dry season. The results showed that: (1) Evidences from isotopic signatures suggested the occurrence of N transformation instead of conservation mixing. (2) Negative correlations between the δ15N-NO3− and δ15N-NH4+, the relationships between δ15N-NO3− and NO3−-N concentrations, and smaller δ18O-NO3− values were found in almost all surface water, indicating that nitrification was the dominant N transformation. (3) In addition to the nitrification evidence, significant correlations between δ15N-PN and δ15N-NH4+ revealed that assimilation and nitrification jointly affected the N transformation in the QTR's upstream, midstream and lower tributaries, which are unaffected or less affected by tides. (4) The lack of a relationship between δ15N-NO3− and δ18O-NO3− or ln(NO3−) indicated that denitrification was weakened in all surface waters. (5) Qualitative identification of N pollution sources and quantitative calculation of NO3−-N potential sources revealed that sewage was the dominant source of N in the QTR and the HZB, while the internal nitrification was also important factor in determining N levels. This study provided evidence to further understand the sources, transport, and transformation of N in the river-estuary continuum, which deepens the understanding of the land-ocean integrated management of N contaminant.