DIEL PATTERNS OF FEEDING AND VERTICAL MIGRATION IN DAPHNIDS AND DIAPTOMIDS DURING THE CLEAR WATER PHASE IN LAKE GENEVA (FRANCE)

In lacustrine environments, little attention has been paid to small-sc ale interactions between zooplankton diel vertical migration (DVM) and feeding rhythms. Moreover, most of the information on in situ diel fe eding and migratory rhythms is based on low sampling frequencies. The kinetics and the degree of coupling of these processes are thus only r oughly known. Here, we present a study conducted on a diel cycle in La ke Geneva to establish the temporal and spatial relationships between DVM and grazing activity of the dominant planktonic crustaceans. Our m ethodological approach is based on reliable and frequent (every 30 min utes) sampling, and on gut fullness analysis. We test the hypothesis o f temporal and spatial segregation in DVM and feeding activity of symp atric taxa to counteract resource competition. We also evaluate the va riation in DVM and feeding activity between taxa, size and sexes. In L ake Geneva, the Daphnia complex of different species and size (D. hyal ina x galeata) and the diaptomid (Eudiaptomus gracilis) have distinct DVM and diel feeding patterns which lead to temporal and spatial segre gation. Differences arise from the amplitude and kinetics of DVM and d iel feeding rhythms. A strong day/night contrast in depth distribution and feeding activity was observed for the large daphnids while the sm all daphnids and the diaptomids had lower amplitudes of DVM and weaker diel changes in feeding activity. Large Daphnia exhibited a biomodal feeding pattern coupled with dynamic interchange of individuals betwee n the epi- and hypolimnetic layers at dusk and dawn. In contrast, litt le coupling between DVM and feeding patterns was found for the diaptom id. These distinct behaviours can be viewed as specific adaptative str ategies developed by calanids and daphnids to limit interspecific comp etition and to compromise between avoidance of starvation in deep wate rs and avoidance of visual predators in surface layers. Our study supp orts the hypothesis of exogenous control of Daphnia DVM by the relativ e change in light intensity at dusk and dawn, but also suggests that s mall Daphnia (not large ones), are controlled by absolute light variat ions when this major stimulus is lacking. Our results also support the hypothesis that selective predation by fish is responsible for the ob served differences in DVM and diel feeding patterns of sized-daphnids and diaptomids. Other factors explaining the coupling of DVM and feedi ng patterns are hunger, vertical temperature gradient and for daphnids , size. Thus, ecological plasticity in crustacean DVM and feeding patt erns results from the interactive effect of multiple abiotic and bioti c driving forces. Finally, our study also shows that large Daphnia hav e a marked contribution to the acceleration of downward nutrient fluxe s in Lake Geneva, via their diurnal rhythm in feeding and vertical mig ration. Ecological implications of the study for lake management and s ampling design of zooplankton grazing studies are also presented.