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Abstract: The nutrients from the East China Sea (ECS) through the Tsushima/Korea Strait (TS) strongly impact the ecosystem of the Japan Sea (JS). The complex origins of the Tsushima Warm Current and the various nutrient sources in the ECS result in complex spatial-temporal variations in nutrients in the TS. Using a physical-biological model with a tracking technique, we studied the effects of nutrient sources from the ECS on the TS. Among all the nutrient sources, the Kuroshio has the highest nutrient concentrations in the TS. Its maximum concentration occurs at the bottom, while those of rivers and atmospheric deposition occur at the surface, and that of the Taiwan Strait occurs in the middle layer. The nutrient transport through the TS exhibits similar seasonal variations, as does the volume transport. The transport of nutrients from the Kuroshio accounts for more than 85% of the total. The transport of nutrients from the Taiwan Strait is greater during autumn and winter. The transport of dissolved inorganic nitrogen (DIN) from both rivers and atmospheric deposition through the TS peak in August. Nutrient transport cannot be equated with volume transport. The DIN in the less saline zone originates not only from rivers but also from atmospheric deposition and the Kuroshio. The transport of nutrients from the Taiwan Strait is not as significant as its volume transport in the TS.
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Figure 1. Model bathymetry in the East China Sea and location of the Tsushima Strait section (TS), the Taiwan Strait section (TAS), and the 200-m isobath (200 m) (a). b. The dashed-line encircled area in Fig. 1a is displayed. The red pentagram marks the boundary between the eastern (ETS) and western (WTS) channels. The locations of the ETS and WTS are described previously by Takikawa et al. (2005). c. Monthly variations in volume transport through the TS. Red and blue represent the model and observation results, respectively.
Figure 2. Vertical distributions of the velocity normal to the section (cm/s) (a, b), the temperature (℃) (c, d), the salinity (e, f), the CHL concentration (mg/m3) (g, h), the DIN concentration (mmol/m3) (i, j) and the DIP concentration (mmol/m3) (k, l) in the TS section in August and October from the model results. A positive velocity indicates transport in the direction from the ECS to the JS.
Figure 3. Vertical distributions of DIN (mmol/m3) in the TS from rivers (DINR), the atmosphere (DINA), the Kuroshio (DINK), the Taiwan Strait (DINT), and the sum of the four sources (DINW) in four seasons. The proportion of DIN from a single source to DINW is shown by the black contour lines in the first four rows. The black dash lines indicate 5% and 20%. The thin black line represents 90%. The thick black line represents 50%, which suggests the dominant role of the DIN source. The blue contours in the last row denote an N/P ratio of 16. The letters “H” and “L” represent N/P ratios higher than 16 and lower than 16, respectively.
Figure 4. Vertical distributions of DIP (mmol/m3) in the TS from rivers (DIPR), the atmosphere (DIPA), the Kuroshio (DIPK), the Taiwan Strait (DIPT), and the sum of the four sources (DIPW) in four seasons. The proportion of DIP from a single source to DIPW is shown by the black contour lines in the first four rows. The black dash lines with 5 and 20 indicate 5% and 20%. The thin black line with 90 represents 90%. The thick black line with 50 represents 50%, which suggests the dominant role of the DIP source. The blue contours in the last row denote an N/P ratio of 16. The letters “H” and “L” represent N/P ratios higher than 16 and lower than 16, respectively.
Figure 7. DIN (a) and DIP (b) transport across the TS. R, A, T, K, and W represent nutrients from the river, atmospheric deposition, the Taiwan Strait, the Kuroshio, and total nutrients, respectively. The annual mean of each transport is shown behind these letters in the legend. The transports of nutrients from the Kuroshio and the total are indicated by the dashed line and correspond to the right y-axis.
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