Seasonal Variations in Euphotic Zone and Upper Mixed Layer

Values of PAR (Fig. 2A) ranged from 3.3 to 67.3 E/m2/day with large variations among stations. PAR in Oyashio-transition waters was lower than that in the other areas throughout the year. PAR was high in May but low from October to March in all waters. The depth of the euphotic zone (Fig. 2B) ranged from 15 to 109 m with large differences among stations. A common feature of the seasonal variation among different waters was a decrease in the depth of the euphotic zone during April and May.

SST (Fig. 2C) varied widely, from 0.0°C in the Okhotsk Sea to 30.3°C in Kuroshio waters. The monthly mean temperatures in the Oyashio-transi-tion waters were 5.3 to 20.2°C, showing both the lowest absolute SST and the largest seasonal variations we observed. Higher temperatures and smaller seasonal variations were recorded in Kuroshio waters, Tosa Bay, and ECS continental shelf waters, where temperatures ranged from 14 to 30°C. The depth of the upper mixed layer shoaled into the euphotic zone in all waters from April to October (Fig. 2D). In summer, the upper mixed layer reached less than 10% light depth in Oyashio-transition waters, and about 20% light depth in the other waters.

Oyashio-transition

Kuroshio

Tosa Bay

Oyashio-transition

Kuroshio

Tosa Bay

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Figure 2: Seasonal change in (A) total daily incident photosynthetically available radiation (PAR, E/m2/day), (B) euphotic zone depth in 1% light depth (m), (C) sea surface temperature (°C), (D) upper mixed layer depth in relative light (%) in the euphotic zone of Oyashio-transition waters, Kuroshio waters, Tosa Bay, and continental shelf waters of the East China Sea. Solid lines connect monthly means with standard deviations.

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