In order to estimate energetic relationship, microphytic and benthic bivalve levels of a shallow (1.5 m), sandy ecosystem near Marseille (Gulf of Fos, Southern France) were sampled monthly during 1.5 year, in two sites differing by their wave exposure. In the sheltered area, the bivalve Ruditapes decussatus was the dominant species, while another bivalve, Donax trunculus , was found in the open, wave exposed site. Chlorophyll a and primary production of the phytoplankton and the microphytobenthos, as well as population dynamic, growth and biochemical composition of bivalves (lipids, proteins and carbohydrates) were monitored.