The aim of this study was to examine the impact of light and temperature on the spring phytoplankton bloom and disentangle direct and indirect effects of warming on phytoplankton. I conducted two indoor mesocosm experiments with the natural winter plankton and performed a metaanalysis on the effects of warming on primary productivity and diversity-productivity relationship. I found a general direct positive temperature effect on the carbon specific primary productivity. I observed a decline of phytoplankton standing biomass and a decline in phytoplankton size with warming, which effects were related to increased grazing pressure under higher temperature. Higher consumer activity changed community composition and dominance of phytoplankton species and increased phytoplankton diversity. I concluded that the primary productivity in marine pelagic ecosystem depends on the relative strength between direct and indirect temperature effects and on the consumer-producer interactions
