Dongseon Kim

Variations and Controls of Sulfate Reduction in the Continental Slope and Rise of the Ulleung Basin off the Southeast Korean Upwelling System in the East Sea

Despite its importance in carbon mineralization pathways, relatively little is known about sulfate reduction along a continental slope and rise. We investigated the temporal and spatial variations and controls of sulfate reduction rates (SRRs) in the continental margin sediment of the Ulleung basin (UB), off the southeast Korean upwelling system in the East Sea. SRRs ranged from 1.22 to 8.07 mmol m−2 d−1 at the slope sites and from 0.69 to 3.18 mmol m−2 d−1 at the basin sites. These values were exceptionally higher than those observed within other marginal seas and were comparable to SRR values at the same depth range in the Peruvian and Chilean upwelling systems. The high sulfate reduction in the UB was attributable to enhanced primary production in the water column associated with coastal upwelling and a high export flux of large organic particles into the basin via the Ulleung warm eddy. The depth integrated SRR was approximately 4 times higher in the highly productive spring (4.91 ± 2.55 mmol m−2 d−1) than during the summer, which exhibited a stratified water column (1.28 ± 0.48 mmol m−2 d−1). In the meantime, despite the high organic carbon content (> 2.5% dry wt.) in the UB, the SRR was consistently suppressed in the Mn oxide-enriched (174 μ mol cm−3) surface sediments of the continental rise. Overall, these results indicate that the production of organic carbon in the water column and its subsequent deposition on the seafloor is the primary source controlling the temporal variability of sulfate reduction, whereas the Mn oxides that were enriched in the basin are responsible for the spatial variability of the SRR in the UB. Carbon oxidation in the UB accounted for approximately 30% of primary production and approximately 60% of the export flux, indicating that the continental slope and rise of the UB is a region of rapid organic carbon turnover and nutrient regeneration.

Seasonal variations in nutrients and chlorophyll-a concentrations in the Northern east China sea

Nutrients, chlorophyll-a, particulate organic carbon (POC), and environmental conditions were extensively investigated in the northern East China Sea (ECS) near Cheju Island during three seasonal cruises from 2003 to 2005. In spring and autumn, relatively high concentrations of nitrate (2.6~12.4 μmol kg-1) and phosphate (0.17~0.61 μmol kg-1) were observed in the surface waters in the western part of the study area because of the large supply of nutrients from deep waters by vertical mixing. The surface concentrations of nitrate and phosphate in summer were much lower than those in spring and autumn, which is ascribed to a reduced nutrient supply from the deep waters in summer because of surface layer stratification. While previous studies indicate that upwellings of the Kuroshio Current and the Changjiang (Yangtze River) are main sources of nutrients in the ECS, these two inputs seem not to have contributed significantly to the build-up of nutrients in the northern ECS during the time of this study. The lower nitrate:phosphate (N:P) ratio in the surface waters and the positive correlation between the surface N:P ratio and nitrate concentration indicate that nitrate acts as a main nutrient limiting phytoplankton growth in the northern ECS, contrary to previous reports of phosphate-limited phytoplankton growth in the ECS. This difference arises because most surface water nutrients are supplied by vertical mixing from deep waters with low N:P ratios and are not directly influenced by the Changjiang, which has a high N:P ratio. Surface chlorophyll-a levels showed large seasonal variation, with high concentrations (0.38~4.14 mg m-3) in spring and autumn and low concentrations (0.22~1.05 mg m-3) in summer. The surface distribution of chlorophyll-a coincided fairly well with that of nitrate in the northern ECS, implying that nitrate is an important nutrient controlling phytoplankton biomass. The POC:chlorophyll-a ratio was 4~6 times higher in summer than in spring and autumn, presumably because of the high summer phytoplankton death rate caused by nutrient depletion in the surface waters.

Spatial and Temporal Trends in Water Quality in Response to Sewage Discharge in Masan and Hangam Bays, Korea

Abstract This study investigated the spatial and seasonal variations of nutrients, chlorophyll-a (Chl-a), particulate organic carbon, and dissolved oxygen at four stations around Masan Bay from February 2004 to February 2006. Nutrients showed large spatial and seasonal variations in the study area. Dissolved inorganic nitrogen and dissolved silicate concentrations were highest at inner Masan Bay, which was ascribed to the largest inputs of both nutrients from the lands among four stations. Dissolved inorganic phosphorus concentrations are highest at Hangam Bay, where a fertilizer plant caused serious soil pollution with the by-products of phosphate rocks. The extremely high Chl-a concentrations (>60 µg L−1) were observed at inner Masan Bay in July and attributed to the plentiful nutrients, high water stability, and longer residence time at this area during summer. Surface Chl-a concentrations displayed large seasonal variations at all stations, with high values in summer and low values in fall and winter. The lower Chl-a concentrations in fall and winter may be due to strong vertical mixing, which could transport phytoplankton below the euphotic depth, and as a result, constrained phytoplankton growth. Particulate organic carbon–to–Chl-a ratios varied from 59.8 to 121, with large spatial variation and small seasonal change. Hypoxia (<2 mg L−1) was observed at inner and outer Masan and Hangam bays during summer. Outer Masan Bay showed more severe hypoxia than inner Masan Bay in summer 2004 and 2005, which was probably because of the higher organic contents at the bottom sediments and deeper water depth. Despite construction of the Masan and Jinhae wastewater treatment plants, Masan and Hangam bays still received high loading of nutrients, and serious eutrophication was still observed at these bays.

Seasonal and spatial variability of sediment oxygen fluxes in the Beobsan intertidal flat of Taean Bay, mid-western Korean Peninsula

Core incubation and oxygen microelectrode measurements were performed at two stations in the Beobsan intertidal flat of Taean Bay on the mid-western Korean Peninsula. Total and diffusive fluxes were determined for four different seasons (August and October 2006 and February and May 2007). Diffusive oxygen fluxes ranged from 7.62 to 11.4 mmol m−2 d−1 at station S1 and 8.85 to 14.3 mmol m−2 d-1 at station S2. Total oxygen fluxes varied from 20.2 to 27.6 mmol m−2 d−1 at station S1 and from 18.7 to 31.7 mmol m−2 d−1 at station S2. Total oxygen fluxes were 2∶3 times higher than diffusive oxygen fluxes, a result that agrees well with previous studies of the coastal and continental shelf area. The difference between the total and diffusive fluxes is caused by the bio-irrigation activities of macrofauna living in the sediments, which enhance the ventilation of the sediment and introduce oxygenated water to the deeper, anoxic sediment layer. The total oxygen flux at station S1 exhibited small seasonal variation while that at station 2 showed large seasonal variation. Organic carbon contents in the surface sediments correlated well with total oxygen flux, suggesting that the input flux of organic matter is a major factor controlling the seasonal variation in total oxygen flux. Annual benthic aerobic respiration rates were determined to be 8.8 and 9.1 mol C m−2 yr−1 at stations S1 and S2, respectively; these values are very similar to estimations for Aarhus Bay, Denmark and the southwestern lagoon of New Caledonia.

Human and riverine impacts on the dynamics of biogeochemical parameters in Kwangyang Bay, South Korea revealed by time-series data and multivariate statistics.

The successful management of sustainable coastal environments that are beneficial to both humans and marine ecosystems requires knowledge about factors that are harmful to such environments. Here, we investigated seawater nutrient and carbon parameters between 2010 and 2012 in Kwangyang Bay, Korea, a coastal environment that has been exposed to intensive anthropogenic activities. The data were analyzed using cluster and factor analysis. We found that the biogeochemical cycles of nutrients and carbon were determined by river discharge into the bay and biological activity. However, the impacts of these factors varied both spatially and seasonally. During the past 10 years, nutrient loads from the river and industrial complexes to the bay have decreased. The impacts of this decrease are visible in the phosphate concentration, which has fallen to a third of its initial value. We also examined the potential role of atmospheric nitrogen deposition in nitrogen cycling in the study area.

Assessing the ecosystem health status of Korea Gwangyang and Jinhae bays based on a planktonic index of biotic integrity (P-IBI)

To assess the responses of planktonic organisms to pollutants in the coastal ecosystems of Gwangyang and Jinhae bays in Korea, we investigated seasonal changes in various biological factors during the period from 2010 to 2012. Based on the characteristics of nutrient uptake by planktonic organisms under the coastal pollution conditions, we focused on four major parameters: total phytoplankton, harmful algal bloom (HAB) species, heterotrophic bacteria (HB) and Escherichia coli to develop a planktonic index of biotic integrity (P-IBI). The threshold values for abundances of total phytoplankton, HAB species and HB were based on the zero Z-score following normal distribution of the data for each parameter during the three years. Based on this approach, the threshold values were: 1.2 × 106 cells L−1 for total phytoplankton; 3.3 × 104 cells L−1 for HAB species; and 1.7 × 106 cells mL−1for HB. Five grade levels for the P-IBI were established, based on the zero Z-scores. The threshold value for E. coli not to be normalized was based on the USEPA and the Korean Ministry of Oceans and Fisheries guidelines. Validity of the grades and threshold values for each parameter established using the field data were tested by algal bioassays in a mesocosm enclosure, which supported the threshold values obtained in the field. In Gwangyang Bay, the annual integrated score for the P-IBI in 2010 was better than in the other years of the study. In Jinhae Bay, the P-IBI for the inner area of Masan Bay was Grade IV–V, indicating unhealthy conditions relative to the central and western outer areas, where the P-IBI varied from Grade II to III. The P-IBI values for Gwangyang and Jinhae bays were mostly rated as “Good (Grade II) or Fair (Grade III)”, except for a few stations in the semi-enclosed areas of the inner part of Jinhae Bay. From an overall view based on the P-IBI developed in this study, the coastal ecosystem health status in Gwangyang Bay was in a better condition than Jinhae Bay. The P-IBI indicated also a change to an unhealthy status during the rainy periods of spring and summer, whereas during winter and autumn the index indicated healthy conditions.

Biologically Mediated Seasonality of Aragonite Saturation States in Jinhae Bay, Korea

ABSTRACT Kim, D.; Choi, S.-H.; Yang, E.-J.; Kim, K.-H.; Jeong, J.-H., and Kim, Y.O., 2013. Biologically mediated seasonality of aragonite saturation states in Jinhae Bay, Korea. Aragonite saturation states (Ωarag) were estimated for the surface and bottom waters of Jinhae Bay over four seasons to assess the major controlling factors. The surface and bottom waters of Jinhae Bay were highly supersaturated with respect to aragonite in spring, autumn, and winter. In summer, however, most of the bottom waters were undersaturated, even though the surface waters were supersaturated. This can be explained by the phytoplankton–carbonate saturation state interaction; high primary production increases Ωarag in surface waters, whereas vertical export and the subsequent remineralization of organic matter decreases Ωarag in subsurface waters. A strong linear correlation between the Ωarag values and dissolved oxygen concentrations supported the idea that biological activity is a major factor controlling Ωarag in Jinhae Bay. The ocean acidification induced by the invasion of anthropogenic CO2 also considerably affected the aragonite saturation state in all seasons, particularly in winter, when the Ωarag values have been reduced by 32–35% in the surface waters and 33–37% in the bottom waters since the preindustrial era. In Jinhae Bay, the undersaturation with respect to aragonite in summer can have large impacts on the calcifying benthic organisms; mussel calcification may no longer occur, and the oyster calcification rate is probably reduced by 50%. In addition, the pH and Ωarag decreases associated with the anthropogenic CO2 increase may reduce the calcification rates of oyster and mussel in all seasons, particularly in winter.

Long-Term Changes in Water Quality of Masan Bay, Korea

Abstract Kim, D.; Lee, C.-W.; Choi, S.-H., and Kim, Y.O., 2012. Long-term changes in water quality of Masan Bay, Korea. This study investigated the annual variation in nutrients, chemical oxygen demand (COD), and dissolved oxygen (DO) at three stations in Masan Bay from 1995 to 2005. The dissolved inorganic nitrogen (DIN) concentration decreased significantly at all stations after 2001, whereas the dissolved inorganic phosphorus (DIP) concentration showed no distinct annual variation. The DIN/DIP molar ratio also decreased tremendously at all stations after 2001. The COD decreased at all stations after 2001, whereas the DO concentration increased slightly. The decreases in DIN and COD and the increase in DO may have been associated with the almost doubling of the treatment capacity of the Masan wastewater treatment plant, which considerably improved the water quality in the bay.

Evaluation of Resuspended Sediments to Sinking Particles by Benthic Disturbance in the Clarion-Clipperton Nodule Fields

The geochemical properties of sinking particles and sediments in the Clarion-Clipperton Zone were examined to develop a quantitative indicator with which to evaluate the contribution of sediment resuspended by nodule mining activity to sinking particles. The ratio of lithogenic material to organic carbon varies from ∼3 in sinking particles to ∼211 in sediments. This ratio is easily measured and is not easily affected by degradation and/or dissolution in the water column. A mixing model indicates that the ratio may be used as a potential proxy for estimating the contribution of resuspended sediment derived from mining operations to sinking particles.

The reduction in the biomass of cyanobacterial N 2 fixer and the biological pump in the Northwestern Pacific Ocean

The comparison of sediment trap data with physical and biogeochemical variables in the surface water column of the Tropical Northwestern Pacific Ocean (TNWPO) indicated that the magnitude of the springtime biological pump has reduced with time due to a corresponding decrease in the biomass of cyanobacterial N2 fixer. The decrease in the biomass of N2 fixer likely resulted from a reduction in phosphate concentrations in response to surface water warming and consequent shoaling of the mixed layer depth during the study period (2009−2014). The same reduction in biological pump was also observed during summer. However, the cause of the summer reduction remains uncertain and is worth assessing in future studies. Our findings have major implications for predicting future trends of the biological pump in the TNWPO, where significant warming has occurred.