Characterization of chromophoric dissolved organic matter and UV-VIS spectra of Almada River blackwaters, Northeastern Brazil.

Blackwater rivers draining forested areas rich in organic soils are globally relevant because they mobilize large quantities of terrestrial dissolved organic carbon (DOC) and colored dissolved organic matter (CDOM) downstream, and export disproportionately large amounts of these compounds by drainage area to the ocean. This study investigated the optical properties and sources of CDOM in the lower portion of the Almada River watershed, located in the Eastern Brazil Basin between latitudes 14° 35'S and 14° 46'S and longitudes 39° 17'W and 39° 03'W, a region increasingly affected by deforestation. Water samples were collected during six campaigns from June 2022 (dry season) to January 2023 (rainy season) at seven sites along the river. Fluorescent CDOM (FDOM) and UV-VIS absorption spectra were analyzed to infer CDOM characteristics and composition. DOC and specific absorbance at 254 nm (SUVA) were also estimated. Results revealed that humic-like DOM predominates throughout the basin. Lower concentrations of FDOM, DOC, and aromatic compounds downstream evidence the microbial degradation and photooxidation along the river course. Elevated FDOM and α near a tributary confluence point to distinct CDOM sources likely associated with a lagoon-connected channel, a large riparian wetland, and possibly a landfill leachate. Seasonal variations showed increased CDOM quantity and aromaticity during the rainy season, attributed to enhanced leaching from forest soils, despite ongoing deforestation. This work is the first study conducted to evaluate CDOM characteristics in the region and contributes to a better understanding of DOM dynamics in tropical blackwater rivers, which are recognized as major contributors to global DOC fluxes.