Eutrophication-driven biogeochemical processes influencing dissolved organic matter dynamics in a tropical coastal bay: Insights from Lianzhou Bay, northwestern South China Sea

The global understanding of coastal carbon cycling is increasingly recognising the disproportionate importance of small estuary-coastal bay systems. This study analysed dissolved organic carbon (DOC) and nitrogen (DON), coloured and fluorescent dissolved organic matter (DOM, CDOM and FDOM), along with environmental parameters in Lianzhou Bay, northwestern South China Sea, during summer 2022. A two-endmember mixing model was applied to quantify the non-conservative (Δ) dissolved inorganic nitrogen (DIN), phosphorus (SRP), and DOM. DIN decreased linearly with salinity, suggesting conservative mixing, while SRP showed no salinity dependence and was elevated nearshore, suggesting anthropogenic inputs. Non-conservative additions of DOC (44 %), CDOM (31–44 %), and protein-like FDOM (FDOM P , 46 %) were accompanied by substantial DON removal (−83 %). ΔDOC and ΔCDOM were negatively correlated with chlorophyll-a, suggesting their origin from the early microbial transformation of phytoplankton-derived particles. Conversely, the positive correlation between ΔDON and chlorophyll-a, together with a negative ΔDON–ΔDIN correlation of differing magnitudes, suggests partial microbial utilisation of N-rich compounds, with nitrogen not fully retained as DIN. Regionally, CDOM accumulated in the middle bay with minimal ΔDOC, driven by microbial processing of biogenic particles; the southern bay showed zero ΔDOC but positive ΔFDOM P , suggesting coupled production and degradation; in the north, excess primary production led to DOC buildup and DON removal, with labile DOM actively utilized. Among Chinese bays, the relatively low FDOM P proportion in Lianzhou Bay, despite high DOM levels, reflects limited direct anthropogenic inputs and substantial microbial humification, highlighting the complex interplay of natural and human-driven processes shaping coastal DOM composition. • DIN showed conservative mixing, while SRP reflected strong anthropogenic influence. • FDOM H mixed conservatively, while DOC, CDOM, and FDOM P showed significant additions. • DON showed strong removal via microbial use, with N not fully retained as DIN. • Middle, southern, and northern bays exhibited contrasting DOM processing patterns. • Lianzhou Bay had the highest DOC but low FDOM P proportion among Chinese bays.