Tracing terrestrial versus marine sources of dissolved organic carbon in a coastal bay using stable carbon isotopes

Abstract

Abstract. The sources of dissolved organic matter (DOM) in coastal waters are diverse, and they play different roles in the biogeochemistry and ecosystems of the ocean. In this study, we measured dissolved organic carbon (DOC) and nitrogen (DON), the stable carbon isotopic composition of dissolved organic carbon (δ13C-DOC), and fluorescent dissolved organic matter (FDOM) in coastal bay waters surrounded by large cities (Masan Bay, Republic of Korea) to determine the different DOM sources in this region. The surface seawater samples were collected in two sampling campaigns (August 2011 and August 2016). The salinities were in the range of 10–21 in 2011 and 25–32 in 2016. In 2011, excess DOC was observed in high-salinity (16–21) waters; the excess DOC source was found to be mainly from marine autochthonous production according to the δ13C-DOC values (−23.7 ‰ to −20.6 ‰), the higher concentrations of protein-like FDOM, and the lower DOC∕DON (C∕N) ratios (8–15). In contrast, excess DOC observed in high-salinity waters in 2016 was characterized by low FDOM, more depleted δ13C values (−28.8 ‰ to −21.1 ‰), and high C∕N ratios (13–45), suggesting that the source of excess DOC is terrestrial C3 plants by direct land–seawater interactions. Our results show that multiple DOM tracers such as δ13C-DOC, FDOM, and C∕N ratios are powerful for determining different sources of DOM occurring in coastal waters.

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

“…4c). The S R values were relatively low and constant (average: 0.86±0.1) at all stations in 2018, indicating no significant changes in the input sources or the effects of photochemical and bacterial degradation (Moran et al, 2000;Helms et al, 2008) (Fig. 4d).…”

NAPTRAM - Plastiktransportmechanismen, Senken und Interaktionen mit Biota im Nordatlantik / NAPTRAM - North Atlantic plastic transport mechanisms, sinks, and interactions with biota, Cruise No. SO279, Emden (Germany) – Emden (Germany), 04.12.2020 – 05.01.2021

“…Before collecting the samples into glass ampoules, care was taken to wash the filter in place and filter holder with seawater sample. Then samples were immediately preserved by acidifying with HCl (preferably 100 µL of 6M HCL) (Lee et al, 2020) to reduce the pH<2 (Sharp et al, 1993) using a BioHit M200 20-200 µL pipette and were then sealed tightly and stored at 4 ºC in the dark environment.…”

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

“…The stable carbon isotopic composition of DOC (DOC-δ 13 C) has been used to differentiate terrestrial versus marine DOC (Gearing, 1988;Wang et al, 2004;Lee and Kim, 2018;Lee et al, 2020). In general, δ 13 C values of terrestrial sources such as C 3 and C 4 plants are in the range of -23‰ to -34‰ and -9‰ to -17‰, respectively, while those derived from marine phytoplankton are in the range of -18‰ to -22‰ (Gearing, 1998).…”

“…Since δ 13 C values are likely to overlap within the isotopic ranges, it is hard to differentiate specific carbon source itself. Thus, many of studies used δ 13 C values combined with other 50 parameters such as optical properties and DOC to dissolved organic nitrogen (DON) ratios to identify different sources of DOM in coastal environments (Lee and Kim, 2018;Han et al, 2020;Lee et al, 2020).…”

“…3). This implies 185 that the excess DOC of Group 2 may have been introduced from terrestrial sources, which is not fluorescent, through direct interaction between the artificial reclaimed land and seawater (Lee et al, 2020).…”