NANOCILIATES OF THE ORDER PROSTOMATIDA - THEIR RELEVANCE IN THE MICROBIAL FOOD-WEB OF A MESOTROPHIC LAKE - DEDICATED TO PECHLANER,ROLAND FOR HIS 60TH BIRTHDAY

Two nanociliates of the order Prostomatida, Pseudobalanion planctonicu m (12-18 mum) and Urotricha furcata (12-21 mum), were found to be the most abundant ciliates in the epilimnion of Piburger See, a small meso trophic lake. Temperature and food availability were the main factors controlling the sudden increase in abundance, which reached a maximum of 101 cell ml-1 at the beginning of summer. During their exponential development in numbers and biomass, a strong decrease in chlorophyll a and in the abundance of phytoplankton, especially Rhodomonas, was obs erved. We assume, therefore, that Pseudobalanion planctonicum and Urot richa furcata together with rotifers, mainly Polyarthra dolichoptera, are able to reduce the phytoplankton biomass in the upper epilimnion t o the same level as found during the clear-water phase. Preliminary re sults of grazing on bacteria suggest that these nanociliates are omniv orous, although their impact on bacterial assemblages was low (3.1 % o f the standing stock grazed per day). Feeding on the base of the food web combined with their high abudance at certain times makes them an i mportant link for higher trophic levels. This study constitutes the se cond report on Pseudobalanion as an important component of the microbi al food web in lakes.