The response of phytoplankton development to an increased nutrient input in deep canyon-shaped stratified reservoirs was shown in four case studies. In contrast to lakes, specific features were described: longitudinal succession of phytoplankton development along the valley, transport of algal biomass and the peculiarities of P loading to epilimnion during summer stratification. Long term studies in two reservoirs showed the fluctuations of particular plankton components due to variations in flushing rate on the one hand and fish succession leading to prevalence of planktivorous species on the other.