Stable isotope variations in benthic filter feeders across a large depth gradient on the continental shelf

Spatial variations in carbon and nitrogen stable isotopes (delta C-13 and delta N-15) 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). delta C-13 and delta N-15 values were measured in muscle tissues of four filter-feeding mollusc species (epifaunal: Pecten maxim us, 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 delta C-13 and delta N-15 values with increasing depth. At the Ushant front (similar to 130 m water depth), where there is a decrease in both bottom water chlorophyll a and suspended particulate matter, muscle delta C-13 and delta N-15 values decreased in all species. Although delta C-13 values of infaunal clams initially decreased at 30 m depth, delta C-13 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 delta N-15 values at the deepest stations (similar to 2 parts per thousand) were lower than expected considering the typical trophic enrichment value of 3-4 parts per thousand between prey and consumers. These low delta N-15 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.