To determine the distribution of dissolved organic matter (DOM) in the East China Sea (ECS) during the summer, we measured the dissolved organic carbon (DOC) and nitrogen (DON), fluorescent dissolved organic matter (FDOM), and chlorophyll a (Chl. a) in the upper 100-m layer of this region during July and September 2015. The DOC (r² = 0.72 and 0.78 in July and September, respectively) and DON (r² = 0.43 and 0.33) were significantly correlated with salinity, suggesting that the river is the primary origin of DOM. However, we found that at a DOC “pulse” under a salinity ranging from 24 to 35, the extrapolating DOC values (304 ± 11 μM) were twice higher than those with a salinity of close to 0, as found in a previous study. The excess DOC concentration seemed to be attributed to the microbial metabolism during transport from the estuary based on the good relationships between DOC and marine humic-like FDOM (r² = 0.42 and 0.47), as well as the fluorescence, humification, and biological indexes, but showed no correlation with Chl. a. Thus, the results of our study indicate that microbial activities can be a significant factor controlling the distribution of DOM in the ECS during summer.

South Korea recently shifted its assessment indicator for organic matter in terrestrial environments from chemical oxygen demand (COD) to total organic carbon (TOC) due to the increase in refractory organic carbon levels. However, in the marine environment, where the inflow of refractory organic matter is also on the rise, COD is still used in some instances to assess organic pollution in contaminated areas. Our findings reveal that the main source of dissolved organic carbon (DOC) is terrestrial-derived refractory organic carbon, which enters through nearby wastewater treatment plant (WWPT) outlets. The low oxidation efficiency of COD to TOC (approximately 4 %) prevents it from being an accurate measure of terrestrial-derived refractory DOC. Contrasting results were observed when comparing the organic pollution index (OPI), which we calculated using TOC, with the currently employed water quality index (WQI) for ocean water quality evaluation, particularly in areas influenced by WWPTs. This discrepancy arises because the WQI primarily evaluates autochthonous organic carbon through chlorophyll measurements, whereas the OPI incorporates both autochthonous and allochthonous organic carbon through TOC measurements. Our findings demonstrate that TOC can effectively replace COD as an organic pollution indicator in marine environments.

To determine the origins of dissolved organic carbon (DOC) in a bay of volcanic Jeju Island, where the discharge of fresh groundwater (FGW) is dominant, we measured fluorescent dissolved organic matter (FDOM) and implemented a parallel factor analysis (PARAFAC). The PARAFAC model identified three humic-like components (FDOM_H) and one protein-like component (FDOM_P). DOC was extremely deficient in the FGW (35 ± 13 μM) and positively correlated with salinity in the coastal environment, indicating oceanic DOC contribution. The FDOM_P pattern was similar to that of DOC, suggesting that marine biological production is a primary DOC source in this region. Particularly, significant FDOM_P correlations in the coastal waters with the fluxes of dissolved inorganic phosphorus (DIP; R² = 0.31) and dissolved silicon (R² = 0.46) from the FGW demonstrated that in situ biological production is facilitated by FGW-borne nutrient addition. However, the absence of a correlation between the fluxes of dissolved inorganic nitrogen (DIN) and FDOM_P (R² <0.01) indicated that anthropogenic DIN is not essential for DOC production under the P-limited nutrient conditions and diatom-dominant conditions prevailing on the coastal Jeju Island. Here, we calculated the potential capacity of carbon fixation by marine biological activity based on the Redfield ratio of carbon and phosphorus with DIP fluxes. The flux accounts for approximately 2% of the terrestrial carbon uptake in South Korea. Therefore, optical properties of FDOM may be good indicators of coastal DOC origin, and nutrient speciation may be linked to the carbon cycle.

Vertical and horizontal distributions of total dissolved amino acids (TDAAs), dissolved organic carbon (DOC), and dissolved organic nitrogen (DON) were measured in the East/Japan Sea (EJS). The euphotic zone of this sea is N-limited, and the N : P ratio is ∼ 13 below 200 m depth. Elevated TDAA concentrations (137 ± 34 nM) and DOC-normalized yields (0.8 ± 0.2 % of DOC) were observed in deep waters ( ≥ 1000 m) of the EJS and compared with those in the deep North Pacific Ocean. Significantly high TDAA concentrations and yields were observed in a region of deep-water formation, indicating the convection of margin-derived bioavailable dissolved organic matter (DOM) to deep waters. Declining TDAA concentrations (36 ± 12 %) and yields (33 ± 13 %) were observed between 1000 and 3000 m throughout the EJS, indicating the utilization of bioavailable DOM in deep waters. Concentrations of the D-enantiomers of amino acids (Ala, Glx, Asx, and Ser) were relatively high in deep waters of the EJS, indicating substantial bacterial contributions to DOM from surface and upper mesopelagic waters. Climate warming during the past few decades in the EJS is weakening deep convection during the winter, and one consequence of this reduction in deep convection is a decline in the supply of bioavailable DOM from surface waters.

We examined the residence time, seepage rate, and submarine groundwater discharge (SGD)-driven dissolved nutrients and organic matter in Hwasun Bay, Jeju Island, Korea during the occurrence of a typhoon, Kong-rey, using a humic fluorescent dissolved organic matter (FDOM_H)-Si mass balance model. The study period spanned October 4–10, 2018. One day after the typhoon, the residence time and seepage rate were calculated to be 1 day and 0.51 m day⁻¹, respectively, and the highest SGD-driven fluxes of chemical constituents were estimated (1.7 × 10⁶ mol day⁻¹ for dissolved inorganic nitrogen, 0.1 × 10⁶ mol day⁻¹ for dissolved inorganic phosphorus (DIP), 1.1 × 10⁶ mol day⁻¹ for dissolved silicon, 0.5 × 10⁶ mol day⁻¹ for dissolved organic carbon, 1.6 × 10⁶ mol day⁻¹ for dissolved organic nitrogen, 0.4 × 10⁶ mol day⁻¹ for particulate organic carbon, and 38 × 10⁶ g QS day⁻¹ for FDOM_H). SGD-driven fluxes of dissolved nutrient and organic matter were over 90% of the total input fluxes in Hwasun Bay. Our results highlight the potential of using the FDOM_H-Si mass balance model to effectively measure SGD within a specific area (i.e., volcanic islands) under specific weather conditions (i.e., typhoon/storm). In oligotrophic oceanic regions, SGD-driven chemical fluxes from highly permeable islands considerably contribute to coastal nutrient budgets and coastal biological production.