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Abstract

We conducted an enclosure study in Lake Søbygård, a shallow hypertrophic Danish lake, to examine the impact of metazooplankton on the structure of the microbial food web. Here we present results on ciliate abundance, species composition and trophic interactions during 2 consecutive stages of zooplankton succession. Over a 3 wk period, metazooplankton shifted from dominance of rotifers to cyclopoid copepods and thereafter to cladocerans. On 2 different dates with contrasting zooplankton assemblages we performed enclosure experiments where we compared the population dynamics of ciliates in size-fractionated (<50 μm) treatments with enclosures containing in situ densities of metazooplankton. The ciliate community in the lake and in the enclosures was mainly represented (>80% of total abundance) by 3 small-sized taxa: Urotricha spp., Halteria grandinella and Rimostrombidium brachykinetum, which showed different dynamics in response to metazooplankton. In the first experiment, with dominance of rotifers, zooplankton had only a modest predatory impact on the ciliates, and interactions within the ciliate community were probably more important. Larger, raptorial ciliates (e.g. Monodinium sp., Lagynophrya sp.) seemed to have been the main predators of the small ciliates. Different species-specific responses of ciliates within the same size range were observed. In contrast, the second experiment, with dominance of crustacean zooplankton (cladocerans, copepods), demonstrated a dear top-down control of the whole ciliate community by metazooplankton. Predation is probably the dominating regulating mechanism for ciliate abundance, biomass and species composition in Lake Sobygard. In contrast, food limitation is thought to be of minor importance because of generally high concentrations of edible phytoplankton. This view was also supported in our experiments by very high net growth rates of the dominating ciliate species after predator removal (in the range 1.0 to 2.4 d⁻¹). The study revealed 2 characteristics of hypertrophic lakes: first, zooplankton composition and the resulting predation pattern is the decisive factor for the protozoan community structure, and second, the ciliate community is dominated by high densities of a few small-sized species.