Multiple isotopic compositions reveal complex nitrogen cycling in a subtropical estuary

Abstract Nitrogen (N) pollution and the resulting eutrophication can have deleterious consequences on estuaries, such as hypoxia, fish kills, and loss of biotic diversity. An understanding of N sources and cycling in estuaries is fundamental to determining how to effectively manage these ecologically and commercially important areas. We applied a multiple-isotopic approach to examine the transformations and sources of the N pools in the Pearl River Estuary (PRE) during winter. The surface water in the West PRE was characterized by low salinity and high NO3−, while that in the east had high salinity and low NO3−. The NO3− in the West PRE was largely regulated by a conservative mixing process. In contrast, assimilation and nitrification dominated in the East PRE, which was attributed to the long water-residence time. For the first time, the source contributions of NO3− and NH4+ were estimated by isotope mixing models. Our results suggest that river discharge and nitrification contributed 81% and 12% to the NO3− pool, respectively. A major portion (68%) of the NH4+ was from river discharge, with the remainder likely from sewage and the aquitard-aquifer system. Our study demonstrates that internal nitrification can potentially be of pivotal importance in determining the NO3− level in an estuary and its export to coastal waters.