Humans are currently altering aquatic ecosystems through overfishing and nutrient loading. This is altering ecosystems through top-down and bottom-up processes. In the Baltic Sea, the removal of top predators has increased the density of mesopredators, while eutrophication has boosted primary production. Using a mesocosm experiment, we show that a high density of a mesopredator-the threespine stickleback Gasterosteus aculeatus-increases the biomass of primary production not transferred to higher trophic levels under simplified ecosystem conditions. However, no effect of stickleback density on algae biomass is detected in the field, although stickleback gut inspection reveals that stickleback are feeding on amphipods, and stickleback density tends to correlate negatively with amphipod density in the field. This suggests that trophic cascades induced by the mesopredator release are attenuated in the field and do not reach primary producers. This is probably caused by the complexity of the ecosystem where many processes regulate the food web, such as bottom-up effects, the presence of alternative prey and density-dependent predation and distribution. However, the degree to which the ecosystem will be able to buffer further changes, if human disturbances continue, is unknown. Trophic cascades that reach primary producers could have drastic consequences for the ecosystem by promoting the accumulation of drifting algal mats that alter the benthic habitat.