Characterizing the origins of dissolved organic carbon in coastal seawater using stable carbon isotope and light absorption characteristics

In order to determine the origins of dissolved organic matter (DOM) occurring in the seawater of Sihwa Lake, we measured the stable carbon isotope ratios of dissolved organic carbon (DOC-inline-formulaδ¹³C) and the optical properties (absorbance and fluorescence) of DOM in two different seasons (March 2017 and September 2018). Sihwa Lake is enclosed by a dike along the western coast of South Korea, and the water is exchanged with the Yellow Sea twice a day through the sluice gates. The DOC concentrations were generally higher in lower-salinity waters in both periods, and excess of DOC was also observed in 2017 in high-salinity waters. Here, the excess DOC represents any DOC concentrations higher than those in the incoming open-ocean seawater. The excess DOC occurring in the lower-salinity waters originated mainly from marine sediments of tidal flats, based on the DOC-inline-formulaδ¹³C values (inline-formula $M3inlinescrollmathml-\mathrm{normal\; 20.7}\pm \mathrm{normal\; 1.2}$ 58pt10ptsvg-formulamathimg1d72abcf6d66ff54473613b71b21263c bg-18-1793-2021-ie00001.svg58pt10ptbg-18-1793-2021-ie00001.png  ‰) and good correlations among the DOC, humic-like fluorescent DOM (inline-formulaFDOM_H), and inline-formula $M5inlinescrollmathmlchem{\mathrm{normal\; NH}}_{\mathrm{normal\; 4}}^{+}$ 24pt15ptsvg-formulamathimg68d940fa21d9c6691de36bd82f3e56d8 bg-18-1793-2021-ie00002.svg24pt15ptbg-18-1793-2021-ie00002.png concentrations. However, the origins of the excess DOC observed in 2017 appear to be from two different sources: one mainly from marine sources such as biological production based on the DOC-inline-formulaδ¹³C values (inline-formula−19.1 ‰ to inline-formula−20.5 ‰) and the other mainly from terrestrial sources by land–seawater interactions based on its depleted DOC-inline-formulaδ¹³C values (inline-formula−21.5 ‰ to inline-formula−27.8 ‰). This terrestrial DOM source observed in 2017 was likely associated with DOM on the reclaimed land, which experienced extended exposure to light and bacterial degradation as indicated by the higher spectral slope ratio (inline-formulaS_R) of light absorbance and no concurrent increases in the inline-formulaFDOM_H and inline-formula $M14inlinescrollmathmlchem{\mathrm{normal\; NH}}_{\mathrm{normal\; 4}}^{+}$ 24pt15ptsvg-formulamathimgcb88f58b3b25b473f7c5a29ace587a7f bg-18-1793-2021-ie00003.svg24pt15ptbg-18-1793-2021-ie00003.png concentrations. Our study demonstrates that the combination of these biogeochemical tools can be a powerful tracer of DOM sources and characteristics in coastal environments.