Isotope constraints on seasonal dynamics of nitrogen in Zhanjiang Bay, a typical mariculture bay in South China

Eutrophication in coastal waters has been increasing remarkably, severely impacting the water quality in mariculture bays. In this study, we conducted multiple isotopic measurements on suspended particulate nitrogen (δ¹⁵N-PN) and dissolved nitrate (δ¹⁵N-NO ₃ ⁻ and δ¹⁸O-NO ₃ ⁻ ) in Zhanjiang Bay, a typical mariculture bay with a high level of eutrophication in South China, to investigate the changes in nitrogen sources and their cycling between the rainy and dry seasons. During the rainy season, the study found no significant relation between δ¹⁵N-PN and δ¹⁵N-NO ₃ ⁻ due to the impact of heavy rainfall and terrestrial erosion. In the upper bay, a slight nitrate loss and slightly higher δ¹⁵N-NO ₃ ⁻ and δ¹⁸O-NO ₃ ⁻ values were observed, attributed to intense physical sediment-water interactions. Despite some fluctuations, nitrate concentrations in the lower bay mainly aligned with the theoretical mixing line during the rainy season, suggesting that nitrate was primarily influenced by terrestrial erosion and that nitrate isotopes resembled the source. Consequently, the isotopic values of nitrate can be used for source apportionment in the rainy season. The results indicated that soil nitrogen (36%) and manure and sewage (33%) were the predominant nitrogen sources contributing to nitrogen loads during this period. In contrast, the dry season saw a deficient ammonium concentration (<0.2 µmol/L) in the bay, due to nearly complete consumption by phytoplankton during the red tide period. Additionally, the significant loss of nitrate and simultaneous increase in the stable isotopes of dissolved and particulate nitrogen suggest a strong coupling of assimilation and mineralization during the dry season. More active biogeochemical processes during the dry season may be related to decreased runoff and increased water retention time. Overall, our study illustrated the major seasonal nitrogen sources and their dynamics in Zhanjiang Bay, providing valuable insights for formulating effective policies to mitigate eutrophication in mariculture bays.