Taxonomic composition and biomass of heterotrophic flagellates in relation to lake trophy and season

1. Taxonomic composition and abundance of heterotrophic flagellates (HF) were studied in 55 lakes with different trophy in northern Germany using a live-counting technique. 2. Mean abundances and biomasses of HF ranged from 169 cells L-1 and 22 mug L-1 in mesotrophic lakes to 2439 cells mL(-1) and 475 mug L-1 in hypertrophic lakes, respectively. Highest values were generally observed in spring, but mesotrophic lakes showed maximum values in early summer. 3. The taxonomic composition of HF was not significantly influenced by lake trophy and season. The major fraction of HF consisted of chrysomonads and Protista. incertae sedis; other important groups were choanoflagellates and bicosoecids. The size distribution of HF changed with lake trophy and season, with a higher proportion of large HF (> 10 mum) in hypertrophic lakes and in spring, respectively. 4. Correlation analyses revealed a strong negative impact of cladocerans on total HF biomass and especially on large HF. Ciliates and large bacteria (> 10 Lm) were strongly positively correlated with HF biomass; small bacteria (< 2 m) showed a weak positive correlation. 5. Analyses at the level of species and genera revealed distinct distribution patterns of some taxa. Paraphysomonas, Aulacomonas and Quadricilia as large-bodied HF showed highest abundance in hypertrophic lakes and in spring. Attached taxa (e.g. Monosiga, Salpingoeca amphoridium) were highly abundant in late summer and autumn, whereas Spumella and Kathablepharis occurred frequently in most samples.