Abstract
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 δ 34 S and δ 13 C 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 (δ 34 S DOS ) have twice the range between terrestrial and marine end members compared to the stable isotope signatures for dissolved organic carbon (δ 13 C DOC ); 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 (DOC ter ) from both δ 13 C DOC and δ 34 S DOS signatures and applying fixed C: S ratios for riverine and marine end members to convert S isotope signatures into DOC concentrations. The δ 34 S DOS 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 (δ 34 S DOS marine end member 5 18.1‰). These signatures indicate that 87‰, 75‰, and 67‰, respectively, of the water column DOC is of terrestrial origin (DOC ter ) in these basins. Comparing the fractions of DOC ter 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 of Bothnia is much shorter than the hydraulic turnover time, suggesting that high‐latitude estuaries might be efficient sinks for DOC ter .