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We conducted comprehensive field investigations to characterise the trophic status and food web structure of Großer Vätersee (GV) and Kleiner Vätersee (KV), two stratified lakes in the Baltic lake region of northeastern Germany. The lakes will be used as manipulation (GV) and reference (KV) ecosystems in food web studies. Both lakes are mesotrophic to slightly eutrophic with mean epilimnetic total phosphorus (P) concentrations of 24 and 21 μg L–1 in GV and KV, respectively. Mean phytoplankton biomass was 0.9 and 0.5 cm3 m–3 in GV and KV. During summer, the algal community of both lakes was dominated by cyanobacterial picoplankton and Ceratium spp. Filamentous cyanobacteria (mostly Planktothrix sp.) prevailed in a deep chlorophyll maximum found within the chemocline of both lakes. Bacterial biomass in the epilimnion of GV usually ranged between 1 and 3 cm3 m–3. Mean ciliate abundance was 13 ind. mL –1 in GV and 14 ind. mL–1 in KV. While Oligotricha dominated in GV, Scuticociliata were found most frequently in KV. Mean annual crustacean biomass was high in both lakes (GV 2.5 cm3 m–3, KV 1.2 cm3 m–3); Eudiaptomus gracilis, E. graciloides and Daphnia spp. were the most prominent species. Total fish biomass of both lakes was dominated by roach (GV 52%; KV 57%) and perch (GV 24%; KV 34%) Vaucheria dichotoma and Chara spp. dominated the macrophyte community in GV; Chara spp. dominated in KV. The slightly more eutrophic character of GV was most likely a result of greater P accumulation in the sediments due to external loading, which resulted in greater hypolimnetic P concentrations throughout the summer in GV. However, on an annual basis these differences were only occasionally reflected at higher trophic levels. The differences in fish biomass, however, might have been a consequence of higher structural diversity in GV provided by submersed macrophytes rather than its more eutrophic character. We conclude that the trophic characteristics and food web structure of Großer Vätersee and Kleiner Vätersee are similar enough to be suitable as manipulation and reference lakes in food web experiments.

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The predatory impact of Chaoborus flavicans on zooplankton, especially on daphniids, is often regarded as a crucial factor in the propagation or repeal of top-down effects in lakes. Therefore consumption rates and prey selectivity of C. flavicans were determined in mesotrophic Lake Großer Väter-see, where cladocerans were the dominant zooplankton prey and the predatory impact of fish towards C. flavicans and cladocerans was low. Two generations of the main predatory instar IV of C. flavicans appeared in the pelagial, the first in autumn and the second in spring after having over wintered in the sediment. In discrepancy to the abundances, only the autumn generation showed high peaks of consumption. C. flavicans fed preferentially on medium-sized cladocerans, but mostly disregarded daphniids and rotifers. Copepods were only selected when cladocerans were not present. The highest losses due to Chaoborus predation were found in Bosmina longirostris, Bosmina coregoni and Chydorus sphaericus. There was no evidence that the predatory impact of C. flavicans on daphniids substantially influenced lower trophic levels. IntroductionVertebrate and invertebrate predation are important control mechanisms within zooplankton communities (HRBÁCEK et al., 1961;ELSER et al., 1987;HANAZATO and YASUNO, 1989). Predators can structure entire zooplankton communities by size-selective feeding. Fish, as visual foraging predators, can remove larger zooplankton (ZARET and KERFOOT, 1975) and lead to a predominance of small-bodied zooplankton. Higher abundances of large-bodied zooplankton are more likely to occur in the absence of fish predation. Although there is variation in prey preference among species (MOORE et al., 1994), invertebrate predators like Chaoborus are gape-limited (SELL, 2000) and, therefore, preferentially consume smaller zooplankton. YAN et al. (1991) showed that Chaoborus is capable of controlling small sized cladocerans like Bosmina. Large zooplankton species were able to reach a size refuge, i.e., a body size exceeding the size range that Chaoborus was able to ingest. Consequently, these species were released from the pressure of invertebrate predation and they dominated the zooplankton community (YAN et al., 1991). 192 I. S. JÄGER et al.

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