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

Nerot, Caroline; Lorrain, Anne; Grall, Jacques; Gillikin, David P.; Munaron, Jean-Marie; Le Bris, Hervé; Paulet, Yves-Marie

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

2012

Spatial variations in carbon and nitrogen stable isotopes (δ¹³C and δ¹⁵N) of benthic filter feeders were investigated on an inshore-offshore gradient (0–250km) 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 220m. All four species showed a decreasing pattern of muscle δ¹³C and δ¹⁵N values with increasing depth. At the Ushant front (∼130m 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 30m depth, δ¹³C values of epifaunal scallops decreased around 120m 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.

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