Riverine and Submarine Groundwater Discharge Jointly Shape Dissolved Black Carbon Budgets in the Northern Beibu Gulf

Abstract Dissolved black carbon (DBC) is a major refractory component of marine dissolved organic carbon, yet its sources and transport pathways in coastal systems remain incompletely constrained. Here, we investigate the seasonal dynamics of DBC in the northern Beibu Gulf, a monsoon‐influenced marginal sea. DBC concentrations (0.7–2.4 μmol L −1 ) showed strong negative correlations with salinity, indicating that freshwater‐seawater mixing is a dominant control on DBC distributions. Riverine DBC concentrations of the major rivers were reconstructed using a 254 ‐based relationships, yielding a discharge‐weighted riverine DBC endmember of ∼10 μmol L −1 (95% bootstrap confidence interval (CI): 9.2–10.9 μmol L −1 ) in the wet season. This value accounted for most of the wet‐season effective DBC (DBC eff ) derived from the DBC–salinity relationship (∼11.9 μmol L −1 ; 95% CI: 10.3–13.7 μmol L −1 ), with a difference of 1.9 μmol L −1 (95% CI: 0.09–3.8 μmol L −1 ), reflecting potential non‐riverine contributions. Wet‐season flux estimates indicate that riverine discharge contributes ∼8.0 t day −1 of DBC, whereas non‐riverine sources, primarily submarine groundwater discharge (SGD), contribute ∼1.7 t day −1 , highlighting the contribution of non‐riverine pathways to coastal DBC transport. Compared with the dry season, the wet season had higher DBC concentrations and DBC eff , reflecting stronger terrestrial influence. Furthermore, DBC concentrations were lower in surface than bottom waters at similar salinities in the wet season, potentially due to photodegradation and sedimentary inputs. These results highlight the dominant role of rivers in transporting DBC to coastal seas while demonstrating the need to incorporate SGD and other coastal pathways into regional carbon budgets.