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Abstract. In this study we investigated the effects of experimental biomanipulation on community structure, ecosystem metabolism, carbon biogeochemistry and stable isotope composition of a shallow eutrophic lake in the Netherlands. Three different biomanipulation treatments were applied. In two parts of the lake, isolated from the rest, fish was removed and one part was used as a reference treatment in which no biomanipulation was applied. Stable isotopes have proved useful to trace trophic interactions at higher food web levels but until now methodological limitations have restricted species specific isotope analysis in the plankton community. We applied a new approach based on the combination of fluorescence activated cell sorting (FACS) and isotope ratio mass spectrometry (IRMS) to trace carbon flow through the planktonic food web. With this method we aimed at obtaining group specific δ 13 C signatures of phytoplankton and to trace possible shifts in δ 13 C resulting from fish removal.Biomanipulation led to an increase in transparency and macrophyte biomass and decrease in phytoplankton abundance, but zooplankton numbers did not increase. Fish removal also resulted in high pH, high O 2 , low CO 2 and more negative δ 13 C DIC values than expected, which is attributed to chemical enhanced diffusion with large negative fractionation. Despite high temporal variation we detected differences between the isotopic signatures of the primary producers and between the different treatments. The fractionation values of green algae (∼21‰) and diatoms (∼23‰) were similar and independent of treatment, while fractionation factors of filamentous cyanobacteria were variable between the treatments that differed in CO 2 availability. 13 C-labeling of the phytoplankton groups showed that biomanipulation led to increased growth rates of green algae and diatoms at the expense of cyanobacteria. Finally, consumers seemed generalists to the available food sources.

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We studied the functional response of the freshwater unionid bivalve Anodonta anatina, feeding on five phytoplankton strains differing in food quality: the small green alga Scenedesmus obliquus, a toxic and a non-toxic strain of the filamentous cyanobacterium Planktothrix agardhii and a toxic and a non-toxic strain of the coccoid cyanobacterium Microcystis aeruginosa. On S. obliquus, A. anatina had a type II functional response with a maximum mass-specific ingestion rate (IR max ) of 5.24 mg C g DW -1 h -1 and a maximum mass-specific clearance rate (CR max ) of 492 (±38) ml g DW -1 h -1 , the highest values for all the phytoplankton strains that were investigated. On toxic and non-toxic P. agardhii filaments, A. anatina also had a type II functional response, but IR max and CR max were considerably lower (IR max 1.90 and 1.56 mg C g DW -1 h -1 ; CR max 387 (±97) and 429 (±71) ml g DW -1 h -1 , respectively) than on S. obliquus. Toxicity of P. agardhii had no effect on the filtration rate of the mussels. On the non-toxic M. aeruginosa (small coccoid cells), we also observed a type II functional response, although a type I functional response fitted almost as good to these data. For the colonial and toxic M. aeruginosa, a type I functional response fitted best to the data: IR increased linearly with food concentration and CR remained constant. CR max and IR max values for the (colonial) toxic M. aeruginosa (383 (±40) ml g DW -1 h -1 ; 3.7 mg C g DW -1 h -1 ) demonstrated that A. anatina filtered and ingested this cyanobacterium as good as the other cyanobacterial strains. However, on the non-toxic M. aeruginosa we observed the lowest CR max of all phytoplankters (246 (±23) ml g DW -1 h -1 , whereas IR max was similar to that on toxic M. aeruginosa. The high maximum ingestion rates on S. obliquus and M. aeruginosa indicate a short handling time of these phytoplankton species. The high clearance rates on S. obliquus, toxic M. aeruginosa and P. agardhii reflect a high effort of the mussels to filter these particles out of the water 123 Hydrobiologia (2007) 584:191-204 DOI 10.1007 column at low concentrations. The low clearance rates on non-toxic M. aeruginosa may be explained by the small size and coccoid form of this cyanobacterium, which may have impaired A. anatina to efficiently capture the cells. Although A. anatina had relatively high maximum clearance rates on non-toxic and toxic P. agardhii, this cyanobacterium does not seem to be a good food source, because of the observed high rates of pseudofaeces production and hence low ingestion rates.