Seasonal changes and horizontal distribution of zooplankton in Lake Takkobu, Kushiro Marsh, Hokkaido

Abstract

The occurrence characteristics of zooplankton in Lake Takkobu, a shallow eutrophic lake located in the eastern edge of the Kushiro wetland in Hokkaido, were studies in 2003. In the lake, 17 species of zooplankton were observed. The dominant species were Polyarthra vulgaris in the number of individual and Filinia longiseta in biomass. Seasonal changes in the abundance and composition of zooplankton were studied biweekly from 9 May to 18 November 2003. High predation pressure from planktivorous fish to large crustacean zooplankton is suggested, because small crustacean or rotifers were abundant and large crustaceans were scarce during the period. Horizontal distribution of zooplankton in 23-24 July 2003 was classified into four groups as a result of statistical clustering analysis. The differences among the groups were compared by multiple testing. As a result, Copepods predominated at stations of a dense Trapa japonica community, while rotifers were abundant at stations with sparse macrophytes. These results suggest that copepods seek refuge from planktivorous fish in dense macrophytes community.