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The shipboard measurements over approximately 55 years in the southwestern part of the East Sea (Sea of Japan) demonstrate a remarkable basin-wide, interannual-interdecadal variability in the temperature-based thickness of the East Sea Intermediate Water (ESIW) whose temporal variability shows strong correlation with the density-based thickness (r = 0.97). Relevant to the long-term variability of the ESIW thickness, clear changes in horizonal and vertical features have been observed at the intermediate layer in the mid-1990s, such as 1) increases in vertical temperature gradient in the thermocline by shoaling of 2°C–5°C isotherms, 2) relatively high correlations among isotherms in the interdecadal timescale, 3) appearance of zonal phase difference in the ESIW thickness variability after the mid-1990s, and 4) correlation phase change between the Arctic Oscillation Index and the ESIW thickness. The ESIW thickness could be smaller when its formation is weaker and when the formation of deep-water mass below it becomes stronger. Based on the features observed, we hypothesized on the regime shift concerning the East Sea meridional overturning circulation; before the mid-1990s, active deep-water formation mainly controlled the ESIW layer variability, but after the mid-1990s, the ESIW formation rate predominantly affected its own thickness variability.

The concentration and structure of nutrients play a key role in sustaining marine ecosystems. In recent decades, the change in nutrient input from land has led to significant changes in nutrient concentration and structure in the marginal sea and has had an important impact on the coastal ecosystem. Based on historical data from 1984 to 2016, the long-term variations in nutrients and their potential influencing factors, were analyzed in the Changjiang River Estuary. The concentration of dissolved inorganic nitrogen (DIN) increased gradually from 1984 to 2003, but after 2003 the concentration began to decrease. Compared with DIN, the change in dissolved inorganic phosphate (DIP) had a delayed effect and has exhibited a slow downward trend since 2005. The decreases in DIN and DIP are mainly due to the improvement of wastewater treatment efforts and chemical fertilizer use efficiency in recent years. The retention of nutrients by the Three Gorges Reservoir (TGR) has been another important factor since its completion in 2003, especially because the large amount of particulate phosphorus retained by the TGR has led to a significant decrease in the total phosphorus flux into the sea, which is the main reason for the decrease in DIP. The vatiations of DIP and DIN have been instrumental in causing the observed changes of phytoplankton community in the Changjiang River Estuary.