The strongly lake-type-specific balance between primary production and respiration determines whether a lake acts regionally as a net sink or source of CO2. In the global change context even small changes in photosynthesis and respiration rates and/or in the loadings of organic matter or nutrients (e.g. phosphorus) may change the metabolic type of the lake and turn it from autotrophic to heterotrophic system or vice versa. Present thesis summarizes for Lake Võrtsjärv: the first data on seasonal changes of the ecosystem metabolism based on high frequency measurements; the reconstruction of long term series of phytoplankton primary production by bio-optical modelling; the incoming and outgoing fluxes of carbon, nitrogen, phosphorus, and silicon; and gives an overview of phosphorus retention in Lake Võrtsjärv compared with 54 lakes and reservoirs in different climate regions around the world.