Tracing nitrate sources in one of the world’s largest eutrophicated bays (Hangzhou Bay): insights from nitrogen and oxygen isotopes

Eutrophication caused by inputs of excess nitrogen (N) has become a serious environmental problem in Hangzhou Bay (China), but the sources of this nitrogen are not well understood. In this study, the August 2019 distributions of salinity, nutrients [nitrate (NO ⁻₃ ), nitrite, ammonium, and phosphate], and the stable isotopic composition of NO ⁻₃ (δ¹⁵N and δ¹⁸O) were used to investigate sources of dissolved inorganic nitrogen (DIN) to Hangzhou Bay. Spatial distributions of nitrate, salinity, and nitrate δ¹⁸O indicate that the Qiantang River, the Changjiang River, and nearshore coastal waters may all contribute nitrate to the bay. Based on the isotopic compositions of nitrate in these potential source waters and conservative mixing of nitrate in our study area, we suggest that the NO ⁻₃ in Hangzhou Bay was likely derived mainly from soils, synthetic N fertilizer, and manure and sewage. End-member modeling indicates that in the upper half of the bay, the Qiantang River was a very important DIN source, possibly contributing more than 50% of DIN in the bay head area. In the lower half of the bay, DIN was sourced mainly from strongly intruding coastal water. DIN coming directly from the Changjiang River made a relatively small contribution to Hangzhou Bay DIN in August 2019.