Enrichment of coastal zooplankton communities by drifting zooplankton patches from the Kuroshio front

Abstract

The community structure of copepods and chaetognaths in a Kuroshio-associated coastal area in the south of Japan was investigated in order to consider what forces (biological and/or physical) potentially produce drift zooplankton patches, an essential prey resource of planktivorous reef fish at the study site. On the basis of bimonthly samples of the patches and routine collections from October 1987 to August 1988, we compared the con~munity structures within and outside the patches. The patches consisted largely of a great variety of copepods and chaetognaths and exhibited high prey densities compared with routine samples. Within months, similarity values between patches and routine samples were relatively low, as were those between patches and the cumulative routme samples. In contrast, similarity values of patches compared with one another within months were high, as were those among months, suggesting that the patches depended on a common water mass retained through the entire year. Useful indicator species of the Kuroshio water and oceanic species dominated the patches in percentages and species numbers, while neritic species dominated routine samples. The neritic species composition of the patches was characterized by only 2 copepod species, Oncaea media and Microsetella norvegica, whereas the routine samples were numerically balanced by various neritic species. These results show that the zooplankton community structure in the patches is dominated by species which are usually found in the Kuroshio front formed in open sea environments. It may play an important role in supplying reef-dwelling planktivores with their main prey resource.