Chemical hydrography and chlorophyll a distribution in the East China Sea in summer: implications in nutrient dynamics

Abstract

A survey of the chemical hydrography and Chl a distribution in the middle and outer shelves of the East China Sea was carried out in July 1992. Very high standing stock of Chi a (30–110 mg m −2) and supersaturation of oxygen (up to 130%) were observed in the plume of the Changjiang Diluted Water, which was enriched in nitrate and silicate but depleted in phosphate. Beneath the plume, a strong depletion in oxygen and an accompanied enrichment in nitrate as well as phosphate were evident. The rich phosphate in the subsurface water may have been crucial in supporting the rapid phytoplankton growth in the overlying water. The Taiwan Current Warm Water (TCWW) occupied most of the shelf to the south and to the east of the plume. The negative relationships between nitrate and temperature suggest that the nutrient-laden Kuroshio Sub-surface Water from the slope was the main source of nutrients for the TCWW. The input of the slope water to the shelf was probably facilitated by countercurrents associated with the Kuroshio upwelling northeast of Taiwan and the mesoscale eddy at the shelf-break east of the Changjiang river mouth. Yet another source of nutrients for the TCWW was revealed by the positive deviations from the mixing trend. These nutrients that may have originated from local remineralization accounted for up to 30% of the total nutrients in the bottom water on the middle shelf. Compared with the Changjiang river plume, the TCWW was relatively poor in Chi a, except in the upwelling region off northern Taiwan where vertical mixing was strong. The nutrients in the subsurface layer of the TCWW further away from Taiwan were generally not available for phytoplankton uptake in the euphotic zone in summer. However, the reserved nutrients could have supported a phytoplankton bloom subsequent to strong vertical mixing.