Characterizing the origin of excess 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 coastal seawater of the Sihwa Lake, South Korea, which is semi-enclosed by a dyke, we measured the stable carbon isotopic ratio of dissolved organic carbon (DOCδC) and optical properties (absorbance and fluorescence) of the DOM in two different seasons (March 2017 10 and September 2018). The concentrations of DOC were generally higher in lower-salinity waters in both periods, while a significant excess of DOC was observed in 2017 in the same salinity range. The main source of DOC, dependent on salinity, was found to be from marine sediments in the freshwater-seawater mixing zone rather than from terrestrial sources based on the DOC-δC values (–20.7±1.2‰) and good correlations among DOC, humic-like fluorescent DOM (FDOMH), and NH4 concentrations. However, the excess DOC observed in 2017 seems to originate from terrestrial sources by direct land15 seawater interactions rather than from in-situ biological production, considering the lower DOC-δC values (–27.8‰ to – 22.6‰) and higher spectral slope ratio (SR) of light absorbance, without increases in FDOMH and NH4 concentrations. This terrestrial DOM source could have been exposed to light and bacterial degradation for a long time, resulting in nonfluorescent and low-molecular-weight DOM, as this study area is surrounded by the reclaimed land. Our results suggest that the combination of these biogeochemical tools can be a powerful tracer of coastal DOM sources. 20


Introduction
Dissolved organic carbon (DOC), a major component of dissolved organic matter (DOM), is one of the most dominant reduced carbon compounds in the ocean (Benner et al., 1992;Raymond and Spencer, 2014). Understanding of sources and characteristics of DOC is important since it plays a significant role in coastal carbon dynamics and biogeochemical cycles (Vetter et al., 2007;Carson and Hansell, 2015). In the coastal oceans, DOM sources are diverse including (1) in-situ 25 biological production , (2) terrestrial sources such as soils and plant matters (Opsahl and Benner, 1997;Bauer and Bianchi, 2011), and (3) anthropogenic sources including industrial and agricultural wastewater (Tedetti et al., 2010;Griffith and Raymond, 2011). https://doi.org/10.5194/bg-2020-272 Preprint. Discussion started: 10 August 2020 c Author(s) 2020. CC BY 4.0 License. Lake water is ~3.3×10 8 m 3 y -1 and the discharge rate is approximately 3.4×10 8 m 3 y -1 (Lee et al., 2014). Sihwa Lake was originally planned to supply agricultural water during the 1980s and 1990s, creating a large artificial lake and agricultural reclaimed land with an area of 173 km 2 (Bae et al., 2010). There are continuous inputs of anthropogenic pollutants from the surrounding industrial complexes (Lee et al., 2017). Freshwater runs through the six small streams into the Sihwa Lake and 65 four waterways connect the lake to the Banwol industrial complex (Fig. 1). Since the lake experienced serious deterioration of water quality, the sluice gates were opened periodically for the water exchange between the lake and the Yellow Sea since 2012.
Water samples were collected in March 2017 and September 2018. The temperature and salinity were measured using a 70 conductivity-temperature-depth (CTD) instrument (Ocean Seven 304, INDONAUT Srl) onboard a ship. In 2018, only surface water samples were collected at shallow stations (station number 1-6) since the water level of the reservoir was lower than in 2017, and the full depth sampling was conducted at stations 12-14. In 2017, sampling was conducted from several depths at all stations. In order to check the effect of industrial wastewater from the industrial complex, an additional sample was collected near the Banwol waterway (station B4) in 2018 ( Fig. 1). 75 Water samples were filtered through a pre-combusted (450 °C for 5h) GF/F filter (pore size: 0.7 µm; Whatman). Samples for DOC and DOC-δ 13 C analyses were acidified with 6M of HCl to avoid any bacterial activities and stored in pre-combusted glass ampoules. Samples for FDOM analysis were stored in pre-combusted amber vials in a refrigerator at 4°C. Samples for total dissolved nitrogen (TDN) and dissolved inorganic nitrogen (DIN) analyses were stored frozen in a polypropylene 80 conical tube.

Chemical analyses
Inorganic and organic nutrients were measured with a nutrient auto-analyzer (QuAAtro39, SEAL analytical). The analytical uncertainties were <5% for the reference materials for NO X (KANTO, Japan). The dissolved oxygen (DO) was determined using the Winkler's method (Carpenter, 1965). DOC concentration was measured using a high temperature catalytic 85 oxidation (HTCO) method using a total organic carbon (TOC) analyzer ( https://doi.org/10.5194/bg-2020-272 Preprint. Discussion started: 10 August 2020 c Author(s) 2020. CC BY 4.0 License.

Optical measurements
Fluorescence and absorbance spectra of the samples were measured using a spectrophotometer (Aqualog, Horiba). For 95 FDOM analyses, the emission and excitation wavelength ranges were set from 240 to 600 nm and from 250 to 500 nm, respectively, with 3 nm scanning intervals (Han et al., 2020). The PARAFAC analysis for the EEM data was performed using the Solo software. The Raman and Rayleigh scattering signals, inner-filter effect, and blank subtraction were corrected indices and parameters of DOM used in this study were prepared as follows. The absorption coefficient was calculated using the following equation: where α is the absorption coefficient (m -1 ), ! is the absorbance, and l is the optical path length of the quartz cuvette (m).
The S R was calculated as the ratio of spectral slope of shorter wavelengths (S 275-295 ) to longer wavelengths (S 350-400 ) (Helms 115 et al., 2008;Han et al., 2020). The spectral slope (S) was calculated using the following equation: where α is the Napierian absorption coefficient (m -1 ), λ is the wavelength, and λ ref is the reference wavelength (Twardowski et al., 2004;Helms et al., 2008).
The concentrations of DO and NH 4 + were in the ranges of 6-11 mg L -1 (average = 8.2±1.6 mg L -1 ) and 0.4-25 µM (average 130 = 13.1±7.9 µM), respectively, in 2018 (Fig. 2). The relatively low salinity and DO concentrations were likely associated with the increased freshwater input in 2018 (Fig. 2). In 2018, the NH 4 + concentrations in the outermost stations were lower than the detection limit (Fig. 2). Although the sharp gradients of DO and NH 4 + concentrations were observed at station 9 in 2017, they occurred near the station 14 in 2018, associated with the expansion of low-salinity water further to outer stations.

Discussion
In 2017 and 2018 sampling periods, higher DOC in low-salinity was associated with lower DO, higher NH 4 + , and higher FDOM H concentrations, indicating larger contribution of DOC either from terrestrial fresh water or by production in the 155 estuarine mixing zone (Figs. 2 and 3). In addition, in 2017, there was significant excess of DOC concentrations were observed together with lower DOC-δ 13 C values and higher S R values, although such excess anomalies were not observed in https://doi.org/10.5194/bg-2020-272 Preprint. Discussion started: 10 August 2020 c Author(s) 2020. CC BY 4.0 License.