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We measured the concentrations of dissolved inorganic and organic nutrients, dissolved organic carbon (DOC), total hydrolyzable amino acids (THAA), fluorescent dissolved organic matter (FDOM), phytoplankton pigments, and δ 13 C-DOC during the summer of 2019 in the harmful dinoflagellate bloom regions of the southern coast of Korea. In the harmful dinoflagellate bloom region, the concentrations of inorganic nitrogen were depleted, inhibiting the growth of diatoms, while the concentrations of dissolved organic components (nutrients, DOC, FDOM, and amino acids) which fuel dinoflagellates were unusually high. Thus, we attempted to investigate the origins and characteristics of DOM which fuels the harmful dinoflagellate blooms. The δ 13 C-DOC values (− 22.2‰ to − 18.2‰) indicate that the elevated DOC concentrations result from in-situ biological production rather than terrestrial inputs. The enantiomeric (D/L) ratios of THAA indicate that dissolved organic nitrogen was more labile in the early stage of harmful dinoflagellate bloom and became more refractory in the final stage. Our results suggest that the marine production of bioavailable DOM plays an important role in initiating and sustaining harmful dinoflagellate blooms.

Dissolved black carbon (DBC) represents the largest molecularly identifiable slow-cycling organic carbon pool in the ocean. However, its behavior remains debated due to large differences between its radiocarbon ages and residence times based on mass balance estimations, suggesting considerable removal in the source regions. Here, we show that DBC is predominantly derived from riverine sources and behaves conservatively throughout the entire water masses of the northwestern Pacific marginal seas—including the East China Sea, the Yellow Sea, and the East Sea (Japan Sea)—which are characterized by extreme biogeochemical alterations and long water residence times (~100 years). This conservative behavior is evidenced by a strong negative correlation between DBC and salinity, consistent mass balance estimates, and a uniform B6CA/B5CA marker ratio. Thus, we suggest that the discrepancy between DBC ages and residence times in the ocean is more likely due to substantially enhanced contemporary DBC production rather than removal pathways. This study shows that river-derived dissolved black carbon is conservatively transported through the shelf to the deep sea of the northwestern Pacific marginal seas, suggesting efficient preservation of this slow-cycling carbon pool in the ocean.

The East Sea (also known as the Japan Sea) is connected to the Northwest Pacific via shallow straits and has independent deep water circulation, as a model miniature ocean. The radiocarbon age of dissolved organic carbon (DOC) in the East Sea ranged from 2,000 to 3,700 years, exceeding the water turnover time (∼100 years). The oldest DOC was found in the subsurface layer characterized by the Tsushima Warm Water. Comparison of the radiocarbon content and concentration of DOC in the East Sea to those in the ocean suggests that aged DOC was transported conservatively from the Northwest Pacific to the East Sea via the shallow Tsushima Warm Current. The fractions of DOC released by serial‐oxidation of the oldest DOC sample had identical radiocarbon ages, implying that refractory DOC was produced in situ and added to the DOC pool in the East Sea. Plain Language Summary The East Sea (also known as the Japan Sea) is connected to the Northwest Pacific exchanging surface water only through shallow passages. The radiocarbon age of DOC in the East Sea is old, ranging from 2,000 to 3,700 years. This cannot be explained by DOC production and aging in the East Sea. Instead, it appears that old DOC is transported into the East Sea from the Northwest Pacific. The oldest DOC was found in the subsurface layer instead of the deepest layer. We show that this oldest DOC is a mixture of the old DOC from the North Pacific and modern DOC produced in the East Sea however their characteristics in terms of resistance to oxidation were modified. These findings provide clues for understanding the global DOC cycling especially in the surface ocean. Key Points Radiocarbon ages of dissolved organic carbon (DOC) in the East Sea (Japan Sea) ranged from 2,000 to 3,700 years, exceeding the water turnover time The oldest DOC at subsurface depths in the East Sea was transported from the North Pacific via the Tsushima Warm Current The aged DOC from the North Pacific is mixed with younger DOC produced in situ and spreads to the deep waters in the East Sea

Atmospheric brown carbon (BrC) plays significant roles in the light absorption and photochemistry of the atmosphere. Although the sources and occurrences of BrC have been studied extensively, its removal processes and optical characteristics in the atmosphere have been poorly understood. In this study, we examined the seasonal changes in sources and sinks of BrC and water-soluble organic carbon (WSOC) in the atmosphere of Seoul, South Korea. Our results showed that the concentrations of BrC and WSOC decreased by approximately 80 % and 30 %, respectively, from the cold season (October–January) to the warm season (June–September). Excitation–emission matrix (EEM) spectra showed that the humic-like substance (HULIS) was the dominant fraction of BrC as the other components were not measurable. The air mass back trajectories of fire burning practices and the variations in non-crustal potassium (K) and vanadium (V) contents in the water-soluble aerosols during all seasons showed no measurable decrease in input of biomass-burning sources in summer. However, there was a significant shift in photo-resistivity of light-absorbing organic aerosols in the summer, indicating larger removals of ultraviolet (UV) degradable BrC. This trend is supported by laboratory UV radiation experiments on the optical property changes of BrC and WSOC in aerosol samples. Thus, our results suggest that the photodegradation has dominant roles in controlling the quantity and quality of light-absorbing organic aerosols in the different seasons in the midlatitude atmosphere.

We investigated the spatial distribution of prokaryotic heterotrophic production (PHP), its limiting resources, and the metabolic balance between prokaryotic carbon demand (PCD) and primary production (PP) in summer in the central Yellow Sea (YS). Due to the formation of strong stratification, the water column was divided vertically into two water masses: nutrient-poor and high-temperature Yellow Sea Surface Water (YSSW) and nutrient-rich and low-temperature Yellow Sea Cold Bottom Water, and the YSSW was further divided into northern and southern part. In the northern YSSW, where dissolved organic carbon (DOC) concentration was higher, relatively higher PHP and prokaryotic respiration (PR) were observed compared to the southern YSSW. However, despite the high DOC concentration and lability of dissolved organic matter, PHP and PR were relatively lower than those reported in coastal YS. Limiting resource incubation experiments demonstrated that PHP was stimulated in samples amended with P, which suggested that prokaryotic growth was restricted by P deficiency. The P-limited PHP in high DOC condition suggested that P deficiency for prokaryotic growth would inhibit prokaryotic organic carbon uptake, leading to DOC accumulation in the surface layer. The metabolic balance appeared to be autotrophy (PCD:PP < 1) in the mixed layer, and strong stratification might prevent the CO2 accumulated below the thermocline from moving to the surface. Overall results imply that when P deficiency due to the physical structure of the water column limits prokaryotic growth in the central YS in summer, net autotrophy may occur.

Dissolved organic carbon (DOC) and the optical properties of dissolved organic matter (chromophoric- and fluorescent dissolved organic matter; CDOM and FDOM) were measured to determine the distributions and drivers of DOM in the northwestern Pacific marginal seas, including the East China Sea, the Yellow Sea, and the East/Japan Sea, in August 2020. In this study, the concentrations of DOC and CDOM/FDOM in surface water showed good correlations with salinity, indicating a predominant contribution from the Changjiang River. However, significantly high concentrations of DOC and FDOM were also observed in the central Yellow Sea region, which seems to be produced mainly from the continental shelf-water and enriched over the water residence time of the Yellow Sea, while the lowest concentrations of DOC and CDOM/FDOM were found in high-salinity waters near the southern sea of Korea. In addition, the East/Japan Sea showed relatively low DOC concentrations and high FDOM values. To distinguish the physical mixing and biogeochemical processes of DOM, we estimated the water mass fractions using an optimum multi-parameter analysis with hydrological and DOM parameters of the major water masses in this region. Our results showed that five primary external drivers of DOM distribution were intrusions of (1) the Changjiang diluted water and (2) the Kuroshio Current water in the surface water, the mixing of (3) the Yellow Sea bottom cold water and (4) the Kuroshio sub-surface water in the bottom layer, and (5) upwelling of the East/Japan Sea water. We conclude that the distribution of DOM is significantly influenced by the mixing of multiple water masses, and the optical signature of DOM can be an effective proxy for tracing the origins and characteristics of DOM in this region.

The effects of benthic dissolved organic carbon (DOC) flux on the dynamics of DOC in the deep continental margins (200 – 2000 m depth) is poorly understood. We investigated heterotrophic prokaryotes (hereafter bacteria) production (BP) and the bio-reactive properties of sediment-derived dissolved organic matter (SDOM) to elucidate microbially mediated cause-effect relationships regarding the rapid consumption of dissolved oxygen (DO) and accumulation of humic-like fluorescent DOM (FDOM H ) in the deep-water column (750 – 2000 m depth range) of the Ulleung Basin (UB) in the East Sea. BP in the deep water (2.2 μmol C m -3 d -1 ) of the UB was among the highest reported for various deep-sea sites. The high DOC concentration (55 μM) likely supported the high BP seen in the deep-water column of the UB. Concentrations of DOC and C1 component of the FDOM H , which is indicative of microbial metabolic by-products, were 13-fold and 20-fold greater, respectively, in pore water than in the overlying bottom water, indicating that the sediment in the continental margins is a significant source of DOM in the overlying water column. Fine-scale water sampling revealed that BP near the sediment (0 – 30 m above the seafloor; 2.78 μmol C m -3 d -1 ) was 1.67 times higher than that measured in the water column above (30 – 100 m above the seafloor; 1.67 μmol C m -3 d -1 ). In addition, BP increased in the bottom water incubation amended with SDOM-containing pore water (PW). The results demonstrated that SDOM contains bio-reactive forms of DOM that stimulate heterotrophic microbial metabolism at the expense of oxygen in the bottom water layer. The accumulation of C1 component in both PW-amended and unamended bottom water incubation (i.e., without an extra DOM supply from sediment) further indicated that refractory DOM is produced autochthonously in the water column via heterotrophic metabolic activity. This explains in part the microbially mediated accumulation of excess FDOM H in the deep-water column of the UB. Overall results suggest that the benthic release of bio-reactive DOM may be of widespread significance in controlling microbial processes in the deep-water layer of marginal seas.

Fluorescent dissolved organic matter (FDOM) provides crucial information regarding the sources and characteristics of dissolved organic matter (DOM) in oceans. However, results from FDOM measurement can depend on filter blanks, pore sizes, and sample storage. To develop more reliable methods for FDOM measurements, we examined uncertainties associated with different preparation methods for seawater samples. Three primary components were identified from these samples using parallel factor analysis: terrestrial humic-like peak (C peak), marine humic-like peak (M peak), and protein-like peak (T peak). Relatively high blank values were observed when samples were filtered through a pre-combusted glass fiber filter (Whatman, borosilicate, 0.7 µm, 47 mm) and a membrane filter (Whatman, mixed cellulose ester, 0.2 µm, 47 mm) without pre-cleaning. These blank values were negligible when both filters were washed with 5 mL of 0.1 M HCl (â¼0.29 mL cm.sup.-2) or 20 mL of distilled water (â¼1.16 mL cm.sup.-2). The effects of different filter pore sizes were not observed for the C and M peaks, but lower T-peak values were observed for filtered samples relative to unfiltered samples. During storage, C and M peaks showed consistent results for 21 d (8 % ± 3 %) when kept in pre-combusted amber glass vials in a refrigerator or a freezer. In contrast, clear changes were observed in samples stored at room temperature after 5 d. Thus, reliable C and M peaks can be obtained from unfiltered or filtered samples stored in a refrigerator or freezer for up to 3 weeks. However, T-peak intensity decreased rapidly in both filtered (15 %-50 %) and unfiltered samples (10 %-40 %) within 5 d, indicating the influence of significant biological and abiotic processes. Therefore, our results suggest that careful sample filtration, storage, and blank controls are necessary for T-peak measurements.

Fluorescent dissolved organic matter (FDOM) provides crucial information regarding the sources and characteristics of dissolved organic matter (DOM) in oceans. However, results from FDOM measurement can depend on filter blanks, pore sizes, and sample storage. To develop more reliable methods for FDOM measurements, we examined uncertainties associated with different preparation methods for seawater samples. Three primary components were identified from these samples using parallel factor analysis: terrestrial humic-like peak (C peak), marine humic-like peak (M peak), and protein-like peak (T peak). Relatively high blank values were observed when samples were filtered through a pre-combusted glass fiber filter (Whatman, borosilicate, 0.7 µm, 47 mm) and a membrane filter (Whatman, mixed cellulose ester, 0.2 µm, 47 mm) without pre-cleaning. These blank values were negligible when both filters were washed with 5 mL of 0.1 M HCl (∼0.29 mL cm−2) or 20 mL of distilled water (∼1.16 mL cm−2). The effects of different filter pore sizes were not observed for the C and M peaks, but lower T-peak values were observed for filtered samples relative to unfiltered samples. During storage, C and M peaks showed consistent results for 21 d (8 % ± 3 %) when kept in pre-combusted amber glass vials in a refrigerator or a freezer. In contrast, clear changes were observed in samples stored at room temperature after 5 d. Thus, reliable C and M peaks can be obtained from unfiltered or filtered samples stored in a refrigerator or freezer for up to 3 weeks. However, T-peak intensity decreased rapidly in both filtered (15 %–50 %) and unfiltered samples (10 %–40 %) within 5 d, indicating the influence of significant biological and abiotic processes. Therefore, our results suggest that careful sample filtration, storage, and blank controls are necessary for T-peak measurements.

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.