Dissolved organic carbon (DOC) flux evaluated by Fick's first law of diffusion. The values were evaluated using the pore water dissolved organic matter, average weighted apparent molecular weight of DOM measured in samples collected by Rhizon samplers from the water 2 cm above the sediment and the top 10 cm of the sediment cores at 2 cm intervals. The porosity was evaluated as sediment water content was obtained from sediment samples collected via slicing of the upper 10 cm of sediment cores over 2 cm intervals. The sediment cores were recovered using GeMax sediment corer. These samples were collected between 2020 and 2021 during cruises onboard RV Oceania (Institute of Oceanology of the Polish Academy of Sciences), which took place in spring, autumn, and winter, to the Central and Southern Baltic Sea, i.e., Eastern Gotland Basin, Bornholm Basin, and Gdansk Basin. The reported DOC sediment fluxes are the final product.

DOC diffusive flux calculations, DOC concentrations in the near-sediment waters and pore water DOC concentrations, porosity and average weighted molecular weight (AMW) of the dissolved organic matter from the near bottom waters (~2cm above sediment surface) and the uppermost slice of the sediment core (0 to 2 cm depth) were used. The fluxes (JDOC) were evaluated according to the Fick's first law of diffusion: J_DOC=-ϕ×Ds×ⅆC/ⅆz; where ϕ is the sediment porosity, ⅆC/ⅆz is the gradient of DOC concentration over the investigated depth interval (0 to 2 cm) and Ds is a bulk sediment diffusion coefficient. Ds was previously demonstrated to be dependent on the sediment formation resistivity factor (F). Ds was calculated using F which equals ϕ^(−3). AMW was calculated from Size Exclusion chromatograms for each sample, and the AMW evaluated for the pore water at 0-2 cm at each station was incorporated into Ds:\nDs=1.1×10^(-3) AMW^(-0.81)/ϕF.