We investigated the taxonomic composition of the heterotrophic nanoflagellate (HNF) assemblage and its taxon-specific bacterial grazing rates in Lake Constance (Germany) over the course of 1 yr. Bacterial grazing rates were measured using natural fluorescently labelled bacteria (FLB) and compared to bacterial production estimated by the uptake of 14 C-leucine incorporation. Glutaraldehyde-fixed, DAPI-stained flagellates were counted using epifluorescence microscopy. Based on annual averages, small species such as Spumella sp. (2 to 6 µm) were the most numerous HNF and the dominant bacterivores. Larger flagellates such as Kathablepharis sp. contributed significantly to total HNF biomass, in particular during spring, but were relatively unimportant as bacterial grazers. The HNF community structure changed during the transition from the phytoplankton spring bloom to the clearwater phase, with small flagellates such as heterokonts, kinetoplastids and choanoflagellates becoming increasingly abundant. The flagellate community composition was more diverse during summer and autumn than in spring. Per capita ingestion rates ranged from 0 to 31 bacteria HNF -1 h -1 and changed seasonally up to 10-fold within a given taxon. Mixotrophic species contributed little to total bacterivory. We provide evidence that the relative significance of bacterial ingestion by a given flagellate taxon may change seasonally. Based upon our experimental results, we discuss potential shortcomings inherent in the FLB technique.KEY WORDS: Heterotrophic nanoflagellates (HNF) · Seasonal succession · FLB · Grazing rate · Negative binomial distribution · Lake ConstanceResale or republication not permitted without written consent of the publisher

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