The Marennes-Oléron Bay is subject to chronic pollution by cadmium (Cd) from the Gironde watershed. An ecosystem approach was used to study the fate of cadmium in the different biological compartments. The median Cd concentration was 0.4 µg g–1 dry weight for the 63 benthic species measured. When combined with the respective biomasses for the different species studied, we estimated that 7 kg of Cd is partitioned into the soft tissues of the benthic species in the bay. The majority of this cadmium was distributed between primary producers, mainly microphytobenthos (40%), and suspension-feeders, mainly oysters (40%). All other benthic species measured were associated with negligible masses of Cd. Two trophic levels contained 98% of the Cd: 3 kg was partitioned into primary producers (of which 77% is associated with the microphytobenthos) and 3.2 kg of Cd was distributed among all suspension feeders. The carnivores, including scavengers, concentrated less than 0.2 kg of Cd, suggesting an absence of biomagnification of Cd in the trophic food web of the bay. The microphytobenthic compartment was estimated to control the largest quantity of Cd (ca. 188 kg yr–1), suggesting an important role for the microphytobenthos in the biogeochemical cycle of Cd in the bay. The quantities of Cd associated with annual biological production in other biological compartments were low: 2 kg yr–1 for eelgrass which could represent a vector of Cd transfer to winter populations of Brent geese and 1.4 kg yr–1 for all suspension feeders, principally the cultivated Pacific oysters (64%) representing a vector of Cd transfer to humans. An ecosystem-wide budget for the quantities of Cd present in all the biological and physical compartments showed that the majority of Cd (1 t) is trapped in the upper 5 cm of the bay sediments, representing a potential risk for the oyster and shellfish cultivation in the bay.