DISTRIBUTION OF SUMMER DIATOM ASSEMBLAGES IN AND AROUND A LOCAL UPWELLING IN THE EAST CHINA SEA NORTHEAST OF TAIWAN

Distributions of diatom assemblages in and around a persistent upwelli ng area off northeastern Taiwan were investigated during summer, 1993. Two water types and two diatom assemblages were defined by principal component analyses. The two water types represent the nutrient-deplete d surface water and the nutrient-laden upwelled subsurface Kuroshio Wa ter. During the period of this study, the latter water type did not ou tcrop but stayed in the subsurface layer, probably resulting from the outflowing of the warm nutrient-poor Taiwan Strait Water. Diatom assem blages indicated the interplay between the two water types. One assemb lage, composed of cosmopolitan species in low density, represented the Background Assemblage, which was widely distributed in most of the su rface water and also in the underlying upwelling water. The other asse mblage, which was observed in the surface water in contact with the un derlying upwelling water over the shelf break, represented an Enhanced Assemblage. Some species existing in the Background Assemblage were l ikely to be enhanced when they were brought into contact with the nutr ient-rich upwelled subsurface Kuroshio Water and, subsequently formed a new assemblage. When the water parcel containing the new assemblage flowed away from the nutrient-rich water, this Enhanced Assemblage som etimes switched back into the Background Assemblage a few days later.