This study on the functioning of freshwater pelagic food webs was based on a set of mesocom experiments conducted in situ. Several aspects of the impact of fish on the functioning of aquatic ecosystems were considered : (1) role of their feeding selectivity on the structure of planktonic communities, (2) cascading effects from fish to primary producers, (3) role in nutrient recycling, (4) interaction effects between fish and the physical structure of the systems. The mesocosm study was supplemented by in vitro experiments and interlake comparative analyses.

The results confirm the importance of fish zooplanktivory on the regulation of phytoplankton biomass and water clarity. Even at high levels of nutrient loading, the reduction of fish stock contributed to regulating phytoplankton through the grazing of zooplankton. These results clarify the potential role of fish on the eutrophication process in freshwaters.They are dicussed in the light of current theories on food web structure and ecosystem functions and compared to the results of whole-lake biomanipulations.

This study shows also the effects of spatial heterogeneity and predator selectivity on the dynamics of zooplankton populations, and on the coexistence of zooplankton-planktivorous fish systems.

The role of light as a limiting factor was pointed out by using enclosures of different depths. The importance of depth as a key factor for the understanding of light-limited systems is discussed within the framework of recent theories.