Seasonal and spatial variability of dissolved organic nitrogen concentration and composition in Daya Bay, China.

Dissolved organic nitrogen (DON) has recently been recognized as an important nitrogen source for marine phytoplankton. However, the composition, sources, and biogeochemical cycling of DON in coastal ecosystems remain poorly understood. This study investigates the spatial distribution and seasonal variability of DON in Daya Bay, a subtropical semi-enclosed bay in the northern South China Sea. We measured DON concentrations, the DIN:DIP ratio, and the spectral characteristics of dissolved organic matter (DOM), including a(350), SUVA₂₅₄, and fluorescence components. Our findings reveal clear seasonal differences in the controlling factors for DON distribution: in summer, land-based sources and biological activities dominate, whereas in winter, oceanic circulation and its associated water mass mixing play a predominant role. The combined spectral indexes suggest that the transformation of DON is significantly more active in summer than in winter. Additionally, most stations exhibited low DIN:DIP ratios (<16) and relatively high chlorophyll a concentrations (>2 μg/L) during the summer months, while DIP concentrations in Daya Bay remained generally low (<1 μmol L^-1). This suggests that phytoplankton may assimilate DON, potentially leading to algal blooms and changes in population structure. Overall, these findings highlight the potential role of DON in the coastal nitrogen budget and phytoplankton dynamics, emphasizing the need for further investigation.