Weak coupling between heterotrophic nanoflagellates and bacteria in a eutrophic freshwater environment

In a study on the dynamics and trophic role of the heterotrophic nanoflagel late (HNAN) assemblage in the microbial food web of a eutrophic oxbow lake abundances, biomass, and production rates of HNAN and their potential prey organisms, namely heterotrophic bacteria and autotrophic picoplankton, were monitored for a period of 2 years. No coupling between HNAN abundance and biomass and the abundance and biomass of their picoplanktonic prey was obse rved for the investigation period. The ratio of heterotrophic bacterial to HNAN abundance ranged from 2.2 x 10(3) to 8.6 x 10(3) (mean: 4.2 x 10(3) 1. 8 x 10(3)). HNAN carbon consumption could account for only 10% to 40% of ba cterial secondary production. The lack of coupling between HNAN and their p otential prey and the low HNAN abundance relative to bacterial abundance su ggested (a) that HNAN grazing was an insignificant factor in the regulation of bacterial abundance and (b) that HNAN abundance was regulated by predat ion rather than by prey abundance. This hypothesis was supported by the fac t that HNAN growth rates were high (in the range of 0.45 d(-1) to 1.00 d(-1 ) during spring and summer, yearly mean: 0.52 d(-1)), and only weakly corre lated with prey abundance and biomass. The results indicated strong top-dow n control of HNAN and consequently a weak coupling of HNAN and picoplankton in the investigated eutrophic freshwater environment.