AME 10:255-263 (1996)  -  doi:10.3354/ame010255

Protistan size-selective grazing on aquatic bacteria has frequently been demonstrated in the laboratory, but its postulated effects on the size distribution of natural bacterioplankton communities remain uninvestigated. For a period of 5 wk we followed the spring development of the epilimnetic microbial community in an oligo-mesotrophic lake. Changes in bacterial size distributions and growth parameters were quantified and compared with the abundance, biomass and bacterivory of the heterotrophic nanoflagellate (HNF) community. We observed pronounced shifts within 2 distinct size ranges of the bacterioplankton. Short, rod-shaped cells between 0.4 and 1.2 um cell length formed between 30 to 70% of total bacterial biomass; cells >2.4 um were responsible for another 20 to 50%. High HNF community grazing rates (>100% of newly produced cells) were reflected in a biomass drop of bacteria between 0.4 and 1.2 um cell length, whereas large cells (>2.4 um) were almost half of total bacterial biomass during this period. When HNF abundance and bacterivory decreased, the opposite pattern was observed. [³H]Thymidine and [¹⁴C]leucine uptake per respiring bacterial cell were maximal during the period of rapid increase of bacterial cells between 0.4 and 1.2 um. Both flagellate total grazing and individual clearance rates were negatively correlated with growth per active bacterial cell. The biomass of 'grazing-resistant' bacteria was correlated with flagellate bacterivory and biomass, and size-selective protistan grazing influenced the size structure of bacterioplankton as well as the mean per-cell growth of the active bacterial fraction. We suggest a model that divides freshwater bacterioplankton into 4 size ranges of different vulnerability to size-selective protistan grazing.

Bacterioplankton size structure . Bacterial biomass . Image analysis . Grazing-resistant bacteria . Protistan size selectivity . Cell-specific activity