PSEUDOBALANION-PLANCTONICUM (CILIOPHORA, PROSTOMATIDA) - ECOLOGICAL SIGNIFICANCE OF AN ALGIVOROUS-NANOCILIATE IN A DEEP MESOEUTROPHIC LAKE

Pseudobalanion planctonicum 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-mu-m, mean cell volume 1300-mu-m3) 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-degrees-C to 1.52 day-1 at 18.5-degrees-C, while temperatures above 21-degrees-C were lethal. Depending on food concentration, 0.2-4.4 Rhodomonas cells were ingested per ciliate and hour.