Pseudobalanion planctonicum (Ciliophora, Prostomatida): ecological significance of an algivorous nanociliate in a deep meso-eutrophic lake

Pseudobalanion plancioracum was the most abundant ciliate species in the pelagic zone of Lake Constance, FRG, over a 3 year period. Annual averages accounted for 30, 35 and 43% of total ciliate numbers in the uppermost 20 m of the water column in 1987, 1988 and 1989 respectively. Highest cell numbers were observed in early spring, simultaneously with the first phytoplankton maximum. The small ciliate (mean length 15 μm, mean cell volume 1300 μm 3 ) is a raptorial feeder and predominantly consumes phytoplankton. In laboratory cultures, P planctonicum grew well on a diet of Rhodomonas sp. (Cryptophyceae). Maximum growth rates increased from 0.46 day −1 at 5 5°C to 1.52 day −1 at 18.5° C, while temperatures above 21°C were lethal. Depending on food concentration, 0.2–4.4 Rhodomonas cells were ingested per ciliate and hour.