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.
Highlights
Dissolved organic matter (DOM) plays an important role in biogeochemical cycles and ecosystems of the ocean (Hansell and Carlson, 2002)
We determined the sources of DOM in 2011 and 2016 using the δ13C-dissolved organic carbon (DOC), fluorescent dissolved organic matter (FDOM), and DOC/DON ratios
The main sources were separated into three groups based on DOC concentrations versus salinity plots
Summary
Dissolved organic matter (DOM) plays an important role in biogeochemical cycles (e.g., de-oxygenation, acidification, and photochemistry) and ecosystems of the ocean (Hansell and Carlson, 2002). DOM composition depends on its parent organic matter and subsequent biogeochemical processes. DOM in coastal waters originates from various sources including (1) in situ production by primary production, exudation of aquatic plants, and aquatic plant degradation (Markager et al, 2011; Carlson and Hansell, 2015); (2) terrestrial sources by the degradation of soil and terrestrial plant matter (Opsahl and Benner, 1997; Bauer and Bianchi, 2011); and (3) anthropogenic sources such as industrial, agricultural, and domestic sewage (Griffith and Raymond, 2011). It is still very difficult to determine the sources and characteristics of DOM in coastal waters
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