Physiological processes involved in energy acquisition by the filter-feeding bivalve Cerastoderma edule (L.) (Mollusca: Bivalvia) were quantified under naturally fluctuating feeding conditions imposed by tidal cycles in the Bay of Marennes-Oleron. Physiological measurements were performed during two neap and two spring tidal cycles in order to cover a wide range of seston concentrations (TPM = 15-95 mg l super(-1)). The main effect exerted by tides on the food supply was the resuspension of bottom sediments of low organic content, leading to a strong 'dilution' of suspended organic matter. Although filtration rate was found to increase with seston concentration, ingestion rate was strictly regulated by means of pseudofaeces production. Selection efficiencies for chlorophyll a (SE sub(chl)), overall organic matter (SE sub(o)), carbon (SE sub(c)) and nitrogen (SE sub(N)) were estimated and related to dietary descriptors. The following ranking was found for the efficiency with which different substrates were selected: SE sub(chl)>SE sub(N)>SE sub(o)>SE sub(c). Absorption efficiency was found to depend on the organic content of ingested matter according to an exponential saturating function. Observed differences between carbon and nitrogen absorption efficiency were not statistically significant. Absorption rate was kept fairly constant through the wide range of seston concentrations and qualities. Sediment resuspension processes may exert important and variable effects on energy acquisition processes depending on the intensity of resuspension events and the quality of the resuspended matter.