Isotope constraints on seasonal dynamics of dissolved and particulate N in the Pearl River Estuary, south China

Abstract

Isotope measurements were performed on dissolved NO3−, NH4+ and suspended particulate total N along a salinity gradient in the Pearl River Estuary (PRE) to investigate seasonal changes in main N sources and its biogeochemical processing under the influence of monsoon climate. Our data revealed that municipal sewage and re-mineralized soil organic N were the major sources of DIN (NO3− and/or NH4+) in freshwater during winter and summer, respectively, whereas phytoplankton biomass was a major component of PN in both seasons. In low salinity waters (<2–3), nitrification was proved to be a significant NO3− source via NH4+ consumption, with N isotope effects of −15.3‰ in summer and −23.7‰ in winter for NH4+ oxidation. The contribution of nitrification to the total NO3− pool was smaller in summer than in winter, most likely due to freshwater dilution. At mid-salinities (3–20), δ15N values of PN were similar to those of NO3− and NH4+ in summer, reflecting a strong coupling between assimilation and remineralization. In winter, however, higher δ15NNH4 but lower δ15NNO3 than δ15NPN were observed, even though δ15NPN was similar between summer and winter. Intense sediment-water interaction and resuspension of sediments during winter appeared largely responsible for the decoupling. At high salinities, the greater enrichment in δ18ONO3 than in δ15NNO3 (up to 15.6‰) in winter suggests that atmospheric deposition may contribute to NO3− delivery during the dry season. Overall, these results show the importance of seasonal variability in physical forcing on biological N sources and its turnover processes in the highly dynamic river-dominated estuary. This article is protected by copyright. All rights reserved.