Marine Water-Quality Characteristics in Surface and Bottom Waters West of Jeju Island, Korea, in the Northern East China Sea During Summer 2025: Implications for Sustainable Marine Monitoring

Surface and bottom water samples collected at 20 sampling points west of Jeju Island, Korea, in the northern East China Sea in July 2025 were analyzed using Temperature–Salinity diagrams and multivariate analyses to characterize water-quality features. A thermocline and halocline were observed within the upper 40 m throughout the study area, together with a west-to-east salinity gradient. In surface waters, Changjiang Diluted Water (CDW) was identified in the western and central parts of the study area, whereas the Taiwan Warm Current was identified in the eastern part. In contrast, bottom-water masses were not distinctly separated, and Yellow Sea Cold Water and Kuroshio-origin Water overlapped in the central part of the study area. Multivariate analyses showed that the southwestern part of the study area was characterized by potential CDW influence in surface water. The results highlight layer-specific clustering patterns, with surface water clustering mainly in relation to water-mass characteristics, whereas bottom waters clustered primarily along a water-depth gradient. CDW may also transport nutrient-imbalanced water characterized by excess nitrogen and relative phosphorus deficiency. This study delineates the summer extent of CDW influence west of Jeju Island and provides a basis for distinguishing potentially CDW-influenced zones in the study area under summer hydrographic conditions. These findings provide an observation-based basis for sustainable marine monitoring in the northern East China Sea, with implications for assessing nutrient imbalance and contaminant transport under summer hydrographic conditions.