Isotope constraints on seasonal dynamics of dissolved and particulate N in the P earl R iver E stuary, south C hina

Isotope measurements were performed on dissolvedNO 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 remineralized soil organic N were the major sources of DIN (NO 3 −and/orNH 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 significantNO 3 −source viaNH 4 +consumption, with N isotope effects of −15.3‰ in summer and −23.7‰ in winter forNH 4 +oxidation. The contribution of nitrification to the totalNO 3 −pool was smaller in summer than in winter, most likely due to freshwater dilution. At midsalinities (3–20), δ 15 N values of PN were similar to those ofNO 3 −andNH 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 toNO 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.