Effect of the Kuroshio intrusion on the chlorophyll distribution in the southern East China Sea during spring 1993

Abstract

In order to assess the effect of the Kuroshio intrusion on phytoplankton biomass in the southern East China Sea north of Taiwan, chlorophyll a distributions observed before and after the recession of the seasonal Kuroshio intrusion were compared. Weekly hydrographie surveys and moored current meter data showed that the Kuroshio intruded onto the shelf under the prevailing northeasterly in early April 1993, and it retreated in early May 1993 shortly after the cessation of the northeasterly. The chlorophyll a distribution showed an eminent increase with a concentric distribution pattern, which was obviously related to the re-emergence of the year-round upwelling at the shelf break northeast of Taiwan. Strengthened upwelling was evidenced by a concomitant increase of nitrate concentration in the subsurface water near the shelf break. The mean value of the euphotic zone integrated chlorophyll a concentration in the normal upwelling area increased from 16 to 36 mg m−2 after the Kuroshio withdrawal. The high chlorophyll a concentration in the upwelling region was shown associated with the high primary productivity measured in the following year, May 1994. The euphotic zone integrated chlorophyll a concentration and primary production in the upwelling water were found to be 33 mg m−2 and 1540 mgC m−2 d−1, respectively. The higher chlorophyll and primary production was apparently the results of ample nutrient supply from the upwelling. At the same time, very high primary production was found in the mainland China coastal water with a value of 1900 mgC m−2 d−1 corresponding to a dinofiagellate bloom with population densities exceeding 1 × 104 cells 1t-1. The Kuroshio water, on the other hand, has the lowest primary production value of 420 mgC m−2 d−1.