Diel patterns of feeding and vertical migration in daphnids and diaptomids during the clear water phase in Lake Geneva (France)

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In lacustrine environments, little attention has been paid to small-scale interactions between zooplankton diel vertical migration (DVM) and feeding rhythms. Moreover, most of the information on in situ diel feeding and migratory rhythms is based on low sampling frequencies. The kinetics and the degree of coupling of these processes are thus only roughly known. Here, we present a study conducted on a diel cycle in Lake Geneva to establish the temporal and spatial relationships between DVM and grazing activity of the dominant planktonic crustaceans. Our methodological approach is based on reliable and frequent (every 30 minutes) sampling, and on gut fullness analysis. We test the hypothesis of temporal and spatial segregation in DVM and feeding activity of sympatric taxa to counteract resource competition. We also evaluate the variation in DVM and feeding activity between taxa, size and sexes. In Lake Geneva, the Daphnia complex of different species and size (D. hyalina x galeata) and the diaptomid (Eudiaptomus gracilis) have distinct DVM and diel feeding patterns which lead to temporal and spatial segregation. Differences arise from the amplitude and kinetics of DVM and diel feeding rhythms. A strong day/night contrast in depth distribution and feeding activity was observed for the large daphnids while the small daphnids and the diaptomids had lower amplitudes of DVM and weaker diel changes in feeding activity. Large Daphnia exhibited a bimodal feeding pattern coupled with dynamic interchange of individuals between the epi- and hypolimnetic layers at dusk and dawn. In contrast, little coupling between DVM and feeding patterns was found for the diaptomid. These distinct behaviours can be viewed as specific adaptative strategies developed by calanids and daphnids to limit interspecific competition and to compromise between avoidance of starvation in deep waters and avoidance of visual predators in surface layers. Our study supports the hypothesis of exogenous control of Daphnia DVM by the relative change in light intensity at dusk and dawn, but also suggests that small Daphnia (not large ones), are controlled by absolute light variations when this major stimulus is lacking. Our results also support the hypothesis that selective predation by fish is responsible for the observed differences in DVM and diel feeding patterns of sized-daphnids and diaptomids. Other factors explaining the coupling of DVM and feeding patterns are hunger, vertical temperature gradient and for daphnids, size. Thus, ecological plasticity in crustacean DVM and feeding patterns results from the interactive effect of multiple abiotic and biotic driving forces. Finally, our study also shows that large Daphnia have a marked contribution to the acceleration of downward nutrient fluxes in Lake Geneva, via their diurnal rhythm in feeding and vertical migration. Ecological implications of the study for lake management and sampling design of zooplankton grazing studies are also presented.