Spatial variations in carbon and nitrogen stable isotopes (δ13C and δ15N) of benthic filter feeders were investigated on an inshore-offshore gradient (0-250 km) along the continental shelf of the northern Bay of Biscay (NE Atlantic Ocean). δ13C and δ15N values were measured in muscle tissues of four filter-feeding mollusc species (epifaunal: Pecten maximus, Aequipecten opercularis; infaunal: Glycymeris glycymeris, Venus casina) and in benthic particulate organic matter along a corresponding water-depth gradient from 6 to 220 m. All four species showed a decreasing pattern of muscle δ13C and δ15N values with increasing depth. At the Ushant front (∼130 m water depth), where there is a decrease in both bottom water chlorophyll a and suspended particulate matter, muscle δ13C and δ15N values decreased in all species. Although δ13C values of infaunal clams initially decreased at 30 m depth, δ13C values of epifaunal scallops decreased around 120 m depth, far below the expected depth reduction in microphytobenthic production suggesting that carbon isotopes might not simply track microphytobenthic utilization. The difference between infaunal and epifaunal bivalve stable isotope values may reflect differences in feeding strategies. Muscle δ15N values at the deepest stations (∼2‰) were lower than expected considering the typical trophic enrichment value of 3-4‰ between prey and consumers. These low δ15N values may result from low metabolic rates and suggest the classic trophic enrichment may not hold true in species inhabiting deep waters. Stable isotopes in benthic filter feeders can reveal much information regarding their ecology and environment, but are not straightforward recorders of stable isotope baseline variations as is often assumed.