Jeomshik Hwang

Summer primary productivity and phytoplankton community composition driven by different hydrographic structures in the East/Japan Sea and the Western Subarctic Pacific

The East/Japan Sea (EJS) is a highly productive marginal sea in the northwest Pacific, consisting of three basins (Ulleung Basin: UB, Yamato Basin: YB, and Japan Basin: JB). To find causes of the reportedly high primary productivity in summer in the EJS, especially in the UB, we measured primary productivity, phytoplankton composition, and other environmental variables. The water column was strongly stratified in the EJS compared with the Western Subarctic Pacific (WSP). Integrated primary productivity was two times higher in the EJS (612 mg C m−2 d−1) than in the WSP (291 mg C m−2 d−1). The vertical distributions of physicochemical and biological factors confirmed that production in the subsurface chlorophyll maximum layer in the study regions was an important factor regulating primary productivity within the water column. While picoplankton (<2.7 µm) dominated in the WSP, JB, and YB, micro/nanoplankton (≥2.7 µm) dominated in the UB. Contribution by picoplankton to total biomass and primary productivity in the UB was significantly lower than in the other regions. CHEMTAX analysis using marker pigments showed that diverse phytoplankton groups inhabited the study regions. Cluster and canonical correspondence analyses showed high correlation between the spatial variation in phytoplankton assemblages with the water mass properties mainly represented by water temperature and nitrate concentration. Overall, our results suggest that the hydrographic structure of water column in the study region is an important controlling factor of the biomass and productivity of phytoplankton as well as their diversity in size and taxonomic groups.

Biomagnification of persistent chlorinated and brominated contaminants in food web components of the Yellow Sea

Concentrations of polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs), and polybrominated diphenyl ethers (PBDEs) were measured in 32 species inhabiting the Yellow Sea to assess their bioaccumulation potentials. The concentrations in these samples were lower than those reported for other countries or locations. Relatively high levels of BDE 209 in biota suggest an ongoing source of deca-BDE technical mixing within the Yellow Sea. The accumulation profiles of PCBs were uniform between species, but the concentrations of OCPs and PBDEs varied widely. Pelagic and benthic food-chain components were separated by their δ(13)C values. Significant positive correlations between δ(15)N and PCB 153, PCB 138, p,p-DDE, oxy-chlordane, and trans-nonachlordane were found only for pelagic consumers, indicating that the pelagic food chain is an important bioaccumulation pathway for selected PCB and OCP compounds. The other compounds did not show any biomagnification through benthic and pelagic food chains, suggesting the lower bioaccumulation potentials of these contaminants.

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.

Significant and conservative long-range transport of dissolved organic nutrients in the Changjiang diluted water

The Changjiang River is one of the main nutrient sources in the northwestern Pacific marginal seas. However, most of the previous studies have neglected the long-range transport (>200u2009km) of riverine nutrients since they are rapidly consumed. In this study, we examined the long-range transport (200–800u2009km) of nutrients in the surface layer during the summer of 2017. The plots of nutrients against salinity display that dissolved organic nitrogen (DON) was conservative over ~800u2009km, while more than 99% of the dissolved inorganic nitrogen (DIN) was removed within 200u2009km. As a result, in the study region, DON concentrations (avg. 7.0u2009±u20091.3u2009µM), which are minor in the river water, were much higher than DIN concentrations (avg. 0.28u2009±u20090.26u2009µM). Both nutrients, N and P, showed a similar pattern. Our results suggest that dissolved organic nutrients play a critical role on the long-range transport of riverine nutrients in surface waters and subsequent ecosystem changes.

Natural and Anthropogenic Carbon Cycling

The East Sea (Japan Sea) is experiencing changes in water temperature, oxygen content, and deep water circulation pattern. These changes, in turn, have affected and will continue to affect carbon cycling in the East Sea, directly and indirectly. As the physical dynamics of the East Sea resembles that of the open ocean, studying inorganic and organic carbon cycling in the East Sea may improve our understanding of global carbon cycling, and consequently enable us to predict more accurately the response of the carbon cycle to global climate change. In our review of inorganic carbon cycling, we focus on the uptake of anthropogenic carbon by the East Sea and the shoaling of the saturation depths of the aragonite and calcite caused by acidification. The saturation depths for aragonite and calcite in the East Sea have shoaled by about 80–220 and 500–700 m, respectively, compared to pre-industrial values. Anthropogenic CO2 in the bottom water of the Japan Basin ranged from 15 to 20 μmol kg−1. The largest water-column inventory of anthropogenic CO2, about 80 mol C m−2, was found in the Japan Basin. The uptake rate of anthropogenic CO2 decreased from 0.6 ± 0.4 mol C m−2 year−1 during the 1992–1999 period to 0.3 ± 0.2 mol C m−2 year−1 during the 1999–2007 period, potentially reflecting the slowing of deep water ventilation in the East Sea. In our review of organic carbon cycling, we focus on sinking particulate organic carbon (POC) and dissolved organic carbon (DOC). Sinking POC flux in the East Sea generally reflected conspicuous events in biological production at the surface. Sinking POC flux in the East Sea was higher than in the nearby Kuroshio region. DOC concentration in the Ulleung Basin of the East Sea was higher than in the open oceans. The high content of the lithogenic component and low radiocarbon content in sinking particles in deep waters suggests that the contribution of resuspended sediment, and potentially other allochthonous sources of organic carbon, is considerable in the East Sea.

Evidence of minimal carbon sequestration in the productive Amundsen Sea polynya

The Amundsen Sea polynya (ASP) is reportedly the most productive among the coastal polynyas around Antarctica. However, observational constraints limit our understanding of the biological pump and carbon cycle in the ASP. We measured various carbon-related parameters such as primary production, bacterial production, sinking flux of particulate organic carbon (POC), and accumulation of organic carbon in the sediment as well as hydrographic parameters during field observations and by instrument moorings. Over 95% of the photosynthetically produced POC was converted to suspended POC and/or dissolved carbon forms in the upper ~400xa0m layer. We postulate that most of the carbon transported to the water column by the biological pump in the ASP was flushed out of the shelf without being sequestered for long-term storage in sediments. Lack of bottom water formation due to intrusion of Circumpolar Deep Water in the lower layer reduces carbon sequestration efficiency.