Publication Date:
2016-11-03
Description:
Isotope measurements were performed on dissolved NO 3 − , NH 4 + 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 (NO 3 − and/or NH 4 + ) 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 NO 3 − source via NH 4 + consumption, with N isotope effects of −15.3‰ in summer and −23.7‰ in winter for NH 4 + oxidation. The contribution of nitrification to the total NO 3 − pool was smaller in summer than in winter, most likely due to freshwater dilution. At mid-salinities (3–20), δ 15 N values of PN were similar to those of NO 3 − and NH 4 + in summer, reflecting a strong coupling between assimilation and remineralization. In winter, however, higher δ 15 N NH4 but lower δ 15 N NO3 than δ 15 N PN were observed, even though δ 15 N PN 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 δ 18 O NO3 than in δ 15 N NO3 (up to 15.6‰) in winter suggests that atmospheric deposition may contribute to NO 3 − 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.
Print ISSN:
0148-0227
Topics:
Geosciences
,
Physics
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