This thesis investigates the past and present zooplankton composition, taxa richness, and the present genetic diversity of Daphnia in Swiss Alpine lakes and in particular the influence of environmental factors (food, predation, UVR) on the Diel Vertical Migration (DVM) behaviour of Daphnia.
It is generally accepted that DVM of zooplankton is a predator avoidance strategy in which species trade-off reduced reproductive output for lower light-dependent mortality. DVM behaviour of D. galeata was investigated in a Swiss high-mountain lake (Oberer Arosasee, 1734 m a.s.l.) with a deep food maximum and a weak temperature gradient. Seasonal migration patterns of D. galeata in the Oberer Arosasee showed that both adults and juveniles exhibit DVM behaviour almost throughout the year. Daphnids did not increase migration amplitude when fish biomass or kairomone concentration was altered, neither in the natural fish stocking experiment nor in an in situ enclosure experiment. However, in a large laboratory experiment (‘Plankton Towers’) that mimicked lake summer conditions, daphnids increased mean day depth in the presence of fish kairomones, but did not migrate without fish chemical cues. This behaviour indicates that the alpine D. galeata population responds to the presence of fish and migrates into deeper colder water.
Growth rates of D. galeata were higher on seston of the deeper strata, even after temperature correction. The results suggest that the major benefit of night-time movement into surface waters of daphnids in the lake was an acceleration of embryo development due to warmer temperatures, but not a feeding gain. Evidence of a further study suggests that zooplankton of the high-mountain lake may avoid surface waters during the day because of the high UVR. Whereas the day depths are determined when the combination of food and temperature is optimal, UVR may determined the timing of migration, since the time spent in surface and deeper strata is strongly synchronised with light intensity.