동중국해 북부해역 영양염의 계절변동 및 연변동

Abstract

Nutrients, chlorophyll-a (Chl-a), and environmental conditions were extensively investigated in the northern East China Sea (ECS) during seven research cruises from 2003 to 2009. In the eastern part of the study area, surface waters were characterized only by the Tsushima Current Water (TCW) during all cruises. However, the western surface waters changed with season and were characterized by Yellow Sea Cold Water (YSCW) in spring, Changjiang Diluted Water (CDW) in summer, and Yellow Sea Mixed Water (YSMW) in autumn and winter. In spring, autumn, and winter, relatively high concentrations of nitrate and phosphate were observed in the surface waters in the western part of the study area, where vertical mixing brought large supplies of nutrients from deep waters. In summer, however, nutrients were completely depleted in the surface waters due to strong stratification. Changes in wind direction occasionally varied the inflow of the Changjiang plume in summer, clearly causing the annual variation in surface nitrate and phosphate concentrations in summer. In summer, the surface distribution of nitrate and phosphate did not coincide with that of silicate in the study area, which probably resulted from the significant drop in the Si:N ratio in the Changjiang plume since construction of the Three Gorges Dam (TGD). Despite large temporal and spatial variations in surface Chl-a concentrations, depth-integrated Chl-a concentrations exhibited little variation temporally and spatially. In the study area, surface Chl-a concentration did not well reflect the standing stocks of phytoplankton. The vertical distribution of Chl-a showed large temporal and spatial variations, and the main factor controlling the vertical distribution of Chl-a in summer was the availability of nitrate. The thermohaline Front may play an important role for accumulation of phytoplankton biomass in spring and autumn.