Response of ciliates and Cryptomonas to the spring cohort of a cyclopoid copepod in a shallow hypereutrophic lake

The impact of a cyclopoid copepod population on the protozoa community (two ciliate categories and Cryptomonas) was assessed weekly during the spring cohort of Cyclops vicinus (one month duration) in hypereutrophic Lake Sobygard by in situ gradient experiments with manipulation of ambient zooplankton abundance. As C. vicinus always made up >92% of the zooplankton biomass, the response of protozoa is assumed to be a result of predation by the copepod. Significant effects of copepod biomass on protozoa net population growth rates were obtained in the four experiments. Copepod clearance rates were significantly higher on oligotrichs than on prostomatids and Cryptomonas but declined for all three protozoa categories during the first three weeks of the copepod cohort, probably because of the change in developmental instar composition of the copepod population. Grazing impact on protozoa at ambient copepod abundance was considerable (range, 0.05-0.87 day(-1)) and could, together with the estimated reproductive potential of protozoans (range, -0.2-0.87 day(-1)), account for the decline in abundance and biomass of protozoa during the cohort development. Carbon flow from the protozoa to C. vicinus (range, 2.8-23.5 mu g C l(-1) day(-1)) documents the presence of a trophic link between protozoa and the spring cohort of C. vicinus in Lake Sobygard.