ABSTRACT: We examined the simultaneous effect of climate warming and light availability on the phytoplankton spring bloom using 1400 l (1 m depth) indoor mesocosms. The timing of the spring bloom was advanced both by warming and higher light intensity, but the influence of temperature on the phytoplankton community was stronger than the light effect. Warming affected phytoplankton directly and indirectly via enhanced grazing pressure at higher temperatures. Warming resulted in markedly lower phytoplankton biomass and a shift towards smaller cell sizes. It also led to changes in the community structure of phytoplankton and zooplankton. Among phytoplankton, large-celled diatoms were most negatively affected by warming. Overwintering zooplankton species (Oithona, Pseudocalanus) remained dominant in the cold treatments, while they were replaced by late spring or summer species (Acartia, Centropages, Temora) in the warmed treatments. Our results show that understanding food web interactions might be very important to the study of the effects of climate warming on pelagic ecosystems.