Seasonal and spatial variability of dissolved black carbon in the Red River delta (North Vietnam)

Dissolved black carbon (DBC) is a key component of the global carbon cycle, yet its seasonal dynamics and river-to-sea transport remain poorly understood, particularly in Southeast Asia where anthropogenic pressures are intense. This study investigates the spatial and seasonal variability of DBC along with dissolved organic matter (DOM) in the main branch of the Red River (North Vietnam) based on three sampling campaigns conducted in March (dry season), June (early wet season), and September 2023 (late wet season). DBC concentrations increased from 29 μg C L⁻1 in March to 66 μg C L⁻1 in September, following rainfall-driven inputs. This seasonal pattern was accompanied by changes in DOM quality, as inferred from optical indices: higher SUVA₂₅₄ (specific UV absorbance at 254 nm), aCDOM(350) (absorption coefficient of chromophoric DOM at 350 nm), and HIX (humification index) in September indicated more terrestrial and humified material, while higher BIX (biological index) in March suggested a higher contribution of fresher, autochthonous DOM. Spatial trends showed a downstream decrease in DBC in June, likely due to abiotic degradation (particularly photodegradation) and dilution. This contrasted with the increasing concentrations from Hanoi to the estuary in March and September, which may be linked to local inputs during dry-season groundwater dynamics and rainfall. DOM optical indices support a contribution of low-DBC groundwater near Hanoi in March. Estimated DBC fluxes at the estuary reached up to 20.7 Gg yr⁻1, representing 0.11% of the global riverine DBC flux to the ocean during the wet season. These results emphasize the role of tropical rivers as dynamic conveyors of combustion-derived carbon, where seasonality and local processes, such as rainfall, photodegradation, and groundwater inflow, strongly shape DBC transport from land to sea.