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

Abstract

Abstract. 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-δ13C) 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-δ13C values (-20.7±1.2 ‰) and good correlations among the DOC, humic-like fluorescent DOM (FDOMH), and NH4+ 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-δ13C values (−19.1 ‰ to −20.5 ‰) and the other mainly from terrestrial sources by land–seawater interactions based on its depleted DOC-δ13C values (−21.5 ‰ to −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 (SR) of light absorbance and no concurrent increases in the FDOMH and NH4+ concentrations. Our study demonstrates that the combination of these biogeochemical tools can be a powerful tracer of DOM sources and characteristics in coastal environments.

Distribution of nutrients and dissolved organic matter in a eutrophic equatorial estuary, the Johor River and East Johor Strait

“…SR also correlates 315 with DOM apparent molecular weight, and has been described as a suitable proxy to distinguish terrestrial and marine CDOM, with marine-like CDOM having SR >1 (Helms et al, 2008;Helms et al, 2014). However, Han et al (2021) found that in a temperate estuary system in Korea, SR was actually elevated (~1.3) in samples richer in terrigenous DOM, which they attributed to prior decomposition. While this indicates that caution is needed when interpreting DOM optical properties as source indicators, we also note that the relatively low concentrations of chl-a in the Johor River suggest that autochthonous production 320 is probably not a major source of DOM.…”

Distribution of nutrients and dissolved organic matter in a eutrophic equatorial estuary: the Johor River and the East Johor Strait

“…S R also correlates with the apparent molecular weight of DOM and has been described as a suitable proxy for distinguishing terrestrial and marine CDOM, with marine-like CDOM having S R > 1 (Helms et al, 2008(Helms et al, , 2014. However, Han et al (2021) found that in a temperate estuary system in Korea, S R was actually elevated (∼ 1.3) in samples richer in terrigenous DOM, which they attributed to prior decomposition. While this indicates that caution is needed when interpreting optical properties of DOM as source indicators, we note that the relatively low concentrations of chl-a in the Johor River (Fig.…”

Soil organic carbon fraction accumulation and bacterial characteristics in curtilage soil: Effects of land conversion and land use

“…Samples were air-dried, ground, and sieved through a 0.9 mm sieve. Subsequently, 10 g of each soil sample was weighed and combined with 40 mL of a 1.85-gmL -1 sodium iodide solution for the soil stratification [ 24 ]. The light fraction (LF) was afloat and the heavy fraction (HF) was deposited [ 25 ].…”