Abstract
The diurnal vertical distribution of zooplankton have been compared on a sunny and on a cloudy day in two shallow, oligotrophic and fishless alpine lakes, exploring the role of cloud cover in shaping zooplankton movements. UV ray avoidance is considered the ultimate reason for zooplankton vertical migration in the lakes, which are naturally devoid of zooplanktivorous fish. Thus, as a thick cloud cover reduces the visible and UV radiation, a shallower diurnal distribution was expected on cloudy days, since migrant zooplankton should balance the amplitude of migration with the light intensity. However, we found very little field evidence that zooplankton change their vertical distribution according to daytime light conditions.
