The Sources and Transformations of Dissolved Organic Matter in the Pearl River Estuary, China, as Revealed by Stable Isotopes

To investigate the sources and transformation processes of bulk marine dissolved organic matter (DOM), seasonally collected water samples from the Pearl River Estuary (PRE), south China, were analyzed for concentration and stable isotopic ratios (C-13 and N-15) of DOM. The C-13 values clearly indicated a shift of dissolved organic carbon (DOC) sources from terrigenous material in the river to predominantly plankton in the lower estuary. The distribution of C-13 during the estuarine mixing, in combination with the notable deviation of DOC concentration, provided strong evidence for a substantial DOC removal in the salinity range of 5 to 22. In contrast to C-13, the N-15 of DOM exhibits a strong seasonality, implying a clear decoupling of estuarine DOC and dissolved organic nitrogen (DON) dynamics. During dry periods with low river discharge, sewage-derived DON contributed significantly to the riverine loading. While freshwater-saltwater mixing is important in regulating the distribution of DON, the distribution of N-15 along the salinity gradient provided strong evidence for active sediment-water interaction and adsorption/desorption processes at the middle to high salinities. During wet periods, severe soil loss and erosion, and/or in situ biological production, adsorption onto suspended particles dominate the riverine DON fluxes, whereas the DON dynamics might be governed by different biogeochemical processes between spring and summer, as reflected by distinct (DON)-D-15 patterns in the estuarine zone. In spring, highly degraded DON originating from soil experienced little further biological alternation during estuarine mixing, whereas there was a tight coupling of DON production and consumption in summer. Isotopic evidence shows a clear decoupling of DOC and DON dynamics in the Pearl River Estuary, south China. DOC undergoes substantial removal in the salinity range of 5 to 22. Estuarine DON dynamics are influenced by both biotic and abiotic processes.