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

Abstract Eutrophication, mainly caused by the oversupply of nitrate (NO 3 − ) 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 (δ 15 N and δ 18 O) of NO 3 − 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 NO 3 − sources and the processes that control NO 3 − distributions within the bay. In the lower bay, high concentrations of NO 3 − (up to 60.6 μmol L −1 ), an N/P ratio of 52, and low values of nitrate δ 15 N (δ 15 N NO3  = 5.7‰–6.7‰) and δ 18 O (δ 18 O NO3  = 1.8‰–3.5‰) indicate that NO 3 − 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 δ 15 N NO3 (up to 8.8‰) and δ 18 O NO3 (up to 6.8‰) suggest that NO 3 − distributions here were controlled mainly by the mixing of coastal and fresh waters, phytoplankton assimilation, and inputs of riverine NO 3 − , possibly derived from manure and sewage. Spatial variations of Δ(15,18), the nitrate δ 15 N-to-δ 18 O 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 NO 3 − from atmospheric deposition.