A key issue to understanding the transformations of terrestrial organic carbon in the ocean is to disentangle the latter from marine-produced organic matter. We applied a multiple stable isotope approach using d34S and d13C isotope signatures from estuarine dissolved organic matter (DOM), enabling us to constrain the contribution of terrestrial-derived DOM in an estuarine gradient of the northern Baltic Sea. The stable isotope signatures for dissolved organic sulfur (d34SDOS) have twice the range between terrestrial and marine end members compared to the stable isotope signatures for dissolved organic carbon (d13CDOC); hence, the share of terrestrial DOM in the total estuarine DOM can be calculated more precisely. DOM samples from the water column were collected using ultrafiltration on board the German RV Maria S Merian during a winter cruise, in the Bothnian Bay, Bothnian Sea, and Baltic proper. We calculated the terrestrial fraction of the estuarine DOC (DOCter) from both d13CDOC and d34SDOS signatures and applying fixed C: S ratios for riverine and marine end members to convert S isotope signatures into DOC concentrations. The d34SDOS signature of the riverine end member was +7.02%, and the mean signatures from Bothnian Bay, Bothnian Sea, and Baltic proper were +10.27, +12.51, and +13.67%, respectively, showing an increasing marine signal southwards (d34SDOS marine end member 5 18.1%). These signatures indicate that 87%, 75%, and 67%, respectively, of the water column DOC is of terrestrial origin (DOCter) in these basins. Comparing the fractions of DOCter in each basin—that are still based on few winter values only—with the annual river input of DOC, it appears that the turnover time for DOCter in the Gulf o
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