Tongsup Lee

Summer Hydrographic Features of the East Sea Analyzed by the Optimum Multiparameter Method

CREAHS II carried out an intensive hydrographic survey covering almost entire East Sea in 1999. Hydrographic data from total 203 stations were released to public on the internee. This paper summarized the results of water mass analysis by OHP (Optimum Multiparameter) method that utilizes temperature, salinity, dissolved oxygen, pH, alkalinity, silicate, nitrate, phosphate and location data as an input data-matrix. A total of eight source water types are identified in the East Sea: four in surface waters(North Korea Surface Water, Tatar Surface Cold Water, East Korean Coastal Water, Modified Tsushima Surface Water), two intermediate water types (Tsushima Middle Water, Liman Cold Water), two deep water types (East Sea Intermediate Water, East Sea Proper Water). Of these NKSW, MTSW and TSCW are the newly reported as the source water type. Distribution of each water types reveals several few interesting hydrographic features. A few noteworthy are summarized as follows: The Tsushima Warm Current enter the East Sea as three branches; East Korea Coastal Water propagates north along the coast around then turns to northeastward to and moves eastward. Cold waters of northern origin move southward along the coast at the subsurface, which existence the existence of a circulation cell at the intermediate depth of the East Sea. The estimated volume of each water types inferred from the OMP results show that the deep waters (ESIW + ESPW) fill up ca. 90% of the East Sea basins. Consequently the formation and circulation of deep waters are the key factors controlling environmental condition of the East Sea.

Metagenomic examination of diversity within eukaryotic plankton from the ulleung basin in the East Sea of Korea

We constructed planktonic 18S rDNA clone libraries for micro-organisms in the euphotic zone of the Ulleung Basin in the East Sea of Korea. They revealed the cryptic biodiversity of eukaryotic planktons and demonstrated specific phylogenetic affiliations at certain water depths. Dinoflagellate clones dominated at the surface and 75-m depth. Members of the Syndiniales order (Groups I and II), containing wellknown parasitic dinoflagellates, as well as one their prominent hosts, Polycystinea, were found exclusively at 75 m. These observations suggest that the presence of species from Groups I and II is closely related to the specific water mass. Therefore, their metagenomics information might be useful for delineating both water-mass and physico-chemical properties.

Phosphate vs. Silicate Discontinuity Layer Developed at Mid-Depth in the East Sea

The CREAMS (Circulation Research of the East Asian Marginal Sea) survey in 1999 revealed a sharp mid-depth discontinuity of the phosphate:silicate ratio in all basins of the East/Japan Sea. Incidentally, this discontinuity layer corresponds to the oxygen minimum layer. Directly below the discontinuity layer, oxygen concentration is increased. This increase in oxygen concentration is interpreted as a proof of intermediate water formation. Oxygen minimum indicates that the water parcel is old and stable against mixing. So it seems be an efficient barrier to vertical exchange of materials. This means that, once materials enter the lower domain, they rarely return to the upper domain. Therefore, the biogeochemistry of the East/Japan Sea depends heavily on material input through the Korea Strait, and flux is expected to be sensitive to the climate change. As a result, the East/Japan Sea ecosystem seems vulnerable to tipping (regime shift), which occurred on a decadal time scale.

Dissolved Oxygen and Nutrients

Changing global temperatures may alter oceanic concentrations of both dissolved oxygen (DO), which reflects water quality, and nutrients, which are an essential factor for the growth and distribution of phytoplankton. The East Sea (Japan Sea) is often considered a “miniature ocean” and an ideal natural laboratory to estimate the impact of global climate change on the oceanic biogeochemical cycles, because of the East Sea’s physical dynamics. Thus, the monitoring of nutrients and DO concentrations in the East Sea may help predict the response of biogeochemical cycles in the global ocean to changing climate. We describe and discuss the history of studies on DO in the East Sea, the change in vertical structure of DO profiles, the trend of DO inventory in the bottom layer, and the controversy regarding future projections of DO in the bottom water mass. We also summarize the history of nutrient measurements in the region, the general structure of vertical nutrient profiles, the horizontal nutrient distribution pattern on isobaric surfaces, the seasonal nutrient flux through the Korea Strait, and the recent debate on nutrient dynamics in the East Sea. The results of the studies we examine suggest that the processes controlling the structures and inventories of DO and nutrients in the East Sea are very sensitive to global climate changes.

Re-initiation of bottom water formation in the East Sea (Japan Sea) in a warming world

The East Sea (Japan Sea), a small marginal sea in the northwestern Pacific, is ventilated deeply down to the bottom and sensitive to changing surface conditions. Addressing the response of this marginal sea to the hydrological cycle and atmospheric forcing would be helpful for better understanding present and future environmental changes in oceans at the global and regional scales. Here, we present an analysis of observations revealing a slowdown of the long-term deepening in water boundaries associated with changes of water formation rate. Our results indicate that bottom (central) water formation has been enhanced (reduced) with more (less) oxygen supply to the bottom (central) layer since the 2000s. This paper presents a new projection that allows a three-layered deep structure, which retains bottom water, at least until 2040, contrasting previous results. This projection considers recent increase of slope convections mainly due to the salt supply via air-sea freshwater exchange and sea ice formation and decrease of open-ocean convections evidenced by reduced mixed layer depth in the northern East Sea, resulting in more bottom water and less central water formations. Such vigorous changes in water formation and ventilation provide certain implications on future climate changes.

Eukaryotic Plankton Species Diversity in the Western Channel of the Korea Strait using 18S rDNA Sequences and its Implications for Water Masses

Organisms entering the East Sea (Sea of Japan) through the Korea Strait, together with water, salt, and energy, affect the East Sea ecosystem. In this study, we report on the biodiversity of eukaryotic plankton found in the Western Channel of the Korea Strait for the first time using small subunit ribosomal RNA gene (18S rDNA) sequences. We also discuss the characteristics of water masses and their physicochemical factors. Diverse taxonomic groups were recovered from 18S rDNA clone libraries, including putative novel, higher taxonomic entities affiliated with Cercozoa, Raphidophyceae, Picozoa, and novel marine Stramenopiles. We also found that there was cryptic genetic variation at both the intraspecific and interspecific levels among arthropods, diatoms, and green algae. Specific plankton assemblages were identified at different sampling depths and they may provide useful information that could be used to interpret the origin and the subsequent mixing history of the water masses that contribute to the Tsushima Warm Current waters. Furthermore, the biological information highlighted in this study may help improve our understanding about the complex water mass interactions that were highlighted in the Korea Strait.