Sources of nitrate in Xiangshan Bay (China), as identified using nitrogen and oxygen isotopes

Eutrophication, mainly caused by the oversupply of nitrate (NO3−) and phosphate, has become a serious environmental problem in Xiangshan Bay (east China), but the sources of the nitrate are poorly understood. In this study, the estuarine distributions of dual stable isotopes (δ15N and δ18O) of NO3− measured in May 2012 were used in combination with concurrent measurements of salinity, pH, dissolved oxygen, and nutrients (nitrate, nitrite, ammonium, and phosphate) to investigate NO3− sources and the processes that control NO3− distributions within the bay. In the lower bay, high concentrations of NO3− (up to 60.6 μmol L−1), an N/P ratio of 52, and low values of nitrate δ15N (δ15NNO3 = 5.7‰-6.7‰) and δ18O (δ18ONO3 = 1.8‰-3.5‰) indicate that NO3− in this area was likely from the Changjiang and Qiantangjiang rivers as well as local nitrification-associated inputs. In the upper bay, constituent concentrations and high values of δ15NNO3 (up to 8.8‰) and δ18ONO3 (up to 6.8‰) suggest that NO3− distributions here were controlled mainly by the mixing of coastal and fresh waters, phytoplankton assimilation, and inputs of riverine NO3−, possibly derived from manure and sewage. Spatial variations of Δ(15,18), the nitrate δ15N-to-δ18O anomaly, is possibly resulted from the different water residence times across the bay, which could in turn lead to different degrees of nitrification or variable amounts of NO3− from atmospheric deposition.