Impact of metazooplankton on the composition and population dynamics of planktonic ciliates in a shallow, hypertrophic lake

We conducted an enclosure study in Lake SmbygBrd, 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 shlfted 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 pm) treatments with enclosures containing in situ densities of metazooplankton. The cihate cornmunlty 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 brachylanetum, which showed Mferent dynamics in response to inetazooplankton. In the first experiment, with dominance of rotifers, zooplankton had only a modest predatory impact on the ciliates, and interactions within the ciliate conlmunity were probably more important. Larger, raptonal ciliates (e.g. Monodinium sp., Lagynophrya sp.) seemed to have been the main predators of the small chates. Dlfferent species-specific responses of cihates within the same size range were observed. In contrast, the second expenment, w t h dominance of crustacean zooplankton (cladocerans, copepods), demonstrated a clear 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 Subygbrd. In contrast, food limitation is thought to be of minor ~rnportance 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 clliate species after predator removal [in the range 1.0 to 2.4 d-'). The study revealed 2 characteristics of hypertrophic lakes-first, zooplankton con~position and the resulting predation pattern is the decisive factor for the protozoan community structure, and second, the clliate community is dominated by high densities of a few small-sized species.