Taiwan Warm Current Water Research Articles

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

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.

On the Taiwan Warm Current Water

With the use of historical data from their 1982–1985 special observation at the source area of the Taiwan Warm Current the authors conducted studies to clarify the temperature and salinity characteristics, variability, and origin of the Taiwan warm Current Water, and its influence on the expanding direction of the Changjiang Diluted Water. The main results are given below.

Discharge of the Changjiang (Yangtze River) into the East China Sea

Hydrographic and current meter data collected in the East China Sea during June 1980, August 1981, and November 1981, have been used to describe the spatial and temporal structure of the Changjiang (Yangtze River) discharge over the inner and mid continental shelf off eastern China. In summer during high runoff, the freshwater discharge near the mouth of the Changjiang exhibited a bimodal distribution, with the freshest water extending in a band to the south along the coast, and a relatively shallow, low salinity plume-like structure extending offshore on average towards the northeast. Relatively warm, saline bottom water characteristic of the Taiwan Current Warm Water flowed into the Changjiang estuary along a submerged relic river valley. Direct current measurements suggest that significant transient offshore fluxes of relatively fresh Changjiang water can also occur from the southward-flowing coastal current south of the Changjiang estuary. In winter, the reduced Changjiang discharge flowed southward in a narrow band confined to the coast. Some possible physical causes for the observed structure of the Changjiang discharge are presented.