SHORT-TERM VARIATIONS OF THE ABUNDANCE AND BIOMASS OF PLANKTONIC CILIATES IN A EUTROPHIC LAKE

Short-time spatio-temporal variations of planktonic ciliates in a eutrophic lake were examined for evidence of diel vertical migration in relation to food supply (bacteria, nanoplankton, and detritus) and physico-chemical parameters. Two campaigns were conducted during successive summers in Lake Aydat, France. Ciliates were less abundant during the first campaign (July 1988; global means 1500 cells/l and 61.0-mu-gC/l) than during the second (July 1989; 5000 cells/l; 110.8-mu-gC/l). On both dates, ciliate abundance decreased from surface to depth, while biomass increased. Each layer (epilimnion, metalimnion, hypolimnion) was inhabited by a separate ciliate community, generally dominated by bacterivores/detritivores. There was no clear evidence of vertical migrations across major physical-chemical boundaries. Circadian variations in each layer occurred independently of light-dark rhythms, partly due to micropatchiness. Temporal variability increased from the vertically mixed epilimnion (C.V. = 32%-41%) to the well stratified deeper zones (C.V. = 41%-100%). Ciliate biomass was negatively and significantly correlated to temperature and dissolved oxygen, and to most principal food parameters. The vertical differences in ciliate biomass were linked to the long-term (seasonal) evolution of the lake system (correlations with temperature, oxygen) and its larger-sized food resources (correlations with chlorophyll, nanoplankton, detritus). By contrast, circadian variations of ciliates were linked to availability of bacteria, an important food resource for many ciliate species of this study.