IMPORTANCE OF CILIATED PROTOZOA IN RELATION TO THE BACTERIAL AND PHYTOPLANKTONIC BIOMASS IN AN OLIGO-MESOTROPHIC LAKE, DURING THE SPRING DIATOM BLOOM

The abundance and the biomass of bacterial, phytoplanktonic, and cilia te communities were estimated at different depths during the spring pl anktonic development in an oligo-mesotrophic lake (the Pavin lake). Th e bacterial population, which consists mainly of free bacteria (94% of the total bacterial abundance), displays only low cell densities (0.6 to 7 x 10(5) cells . ml-1) and represents low biomass values (0.9 to 11.5 mugC.1-1). The bacteria represent from 0.9 to 23.8% (M = 9.7%) of the ''microplanktonic biomass'' (with the exclusion of heterotrophic nanoflagellates, i.e. bacteria + phytoplankton + ciliates, size range 0.2-160 mum). The abundance of the phytoplankton varies between 0.5 an d 1.8 x 10(6) cells . l-1, and the biomass values between 12 and 118 m ugC.l-1. The phytoplankton population constitutes the largest part of the microplanktonic biomass (51.9 to 96.6%, M = 80.6%), and the diatom Melosira italica subsp. subarctica is the largely dominant species of this community. The population of ciliates, essentially Oligotrichida and Scuticociliatida, displays densities between 1. 3 and 38.3 x 10(3 ) cells . l-1 (M = 6.7 x 10(3) cells . l-1), and biomass values vary f rom 0. 1 0 to 16.30 mugC.l-1 (M = 6.01 mugC.l-1). The ciliates constit ute thus from 0.1 to 26.4% (M = 9.8%) of the microplanktonic biomass. Whereas the oligotrichs are best represented in the euphotic zone, the small-sized scuticociliates dominate in the hypolimnion. Besides, spe cies having symbionts and considered to be mixotrophic (Strobilidium g yrans, Strombidium viride, Stokesia vernalis) develop preferentially i n the epilimnion and constitute more than 50% of the total ciliate bio mass.