Stable carbon and nitrogen isotope ratios were used to define the trophic base of the infaunal suspension-feeding bivalve Cerastoderma edule (L.) living on an intertidal muddy sandflat in Marennes-Oleron Bay, France. Suspended particulate organic matter (POM) collected from adjacent marine channels had a mean 6I3C value of -22 2 * 1.1 %O (n = 19). Benthic primary producers on Ronceles-Bains tidal flats had 613C values distinct from that of the POM: microphytobenthos (-16.0 * 0.6%). the macroalgae Enteromorpha compressa (-17.6 + 2.8%), Fucus serratus (-17.4 + 2.6%0), Fucus vesiculosus(-18.5 * 1.6%), Phorphyra umbilicalis (-19.7 t 0.5%0), Ulva rigida (-14.7 + 2 0%) and the seagrass Zostera noltii (-11.1 + 1.0%). Mean 615N values of all macroalgae species ranged from 7.6 + 1.3 to 8.9 * 1.0%, while those for microphytobenthos, POM and seagrass were 5.3 + 0.8, 5.0 * 0 9 and 6.5 r 1.3%0, respectively. The mean 613C value of spat (-15.3 + 0.8%0) and juvenile cockles (-15.7 * 0.7%0) varied within a smaller range than those of 1 to 4 yr old adults (-18.2 + 1.2%). The 6I5N values of 0-group cockles (spat and juveniles) and adult cockles are similar (8.0 * 0.9%0 for 0-group and 8.4 * 1.1% for adults). The results suggest, based on the average trophic enrichment found in the literature for C and N, and the relative abundance of each food source, that there are 2 major sources of organic matter assimilated by cockles of the studied flats: microphytobenthos and POM. Seasonal variations in 613C values reflect a higher dependence of adult cockles on POM vanability while spat and juveniles are more closely Linked to microphytobenthos. The isotopic shifts indicate that the relative importance of the 2 major food sources depends on the age of the cockles and the season.
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