Tracing the sources of dissolved organic carbon occurring in a coastal bay surrounded by heavily industrialized cities using stable carbon isotopes

Abstract. The sources of dissolved organic matter (DOM) in coastal waters are diverse, and they play different roles in biogeochemistry and ecosystems. In this study, we measured dissolved organic carbon (DOC) and nitrogen (DON), δ 13 C-DOC, and fluorescent dissolved organic matter (FDOM) in coastal bay waters surrounded by heavily industrialized cities (Masan Bay, Korea) to determine the different DOM sources in this region. The surface seawater samples were collected in two sampling campaigns (Aug. 2011 and Aug. 2016). The salinities ranged from 10 to 21 in 2011 and from 25.4 to 32 in 2016. In 2011, the excess DOC was observed for higher-salinity waters (16–21), indicating its main source from marine autochthonous production according to the δ 13 C-DOC values of −23.7 ‰ to −20.6 ‰, higher concentrations of protein-like FDOM, and lower DOC / DON (C / N) ratios. By contrast, the high DOC waters in high-salinity waters of 2016 were characterized by low FDOM, more depleted δ 13 C values of −28.8 ‰ to −21.1 ‰, and high C / N ratios, suggesting that the excess DOC is influenced by direct land-seawater interactions. Our results show that multiple DOM tracers such as δ 13 C-DOC, FDOM, and C / N ratios are powerful for discriminating the complicated sources of DOM in coastal waters.