Food sources of the infaunal suspension-feeding bivalve Cerastoderma edule in a muddy sandflat of Marennes-Oleron Bay, as determined by analyses of carbon and nitrogen stable isotopes

Abstract

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 delta(13)C value of -22.2 +/- 1.1 parts per thousand (n = 19). Benthic primary producers on Ronceles-Bains tidal flats had delta(13)C values distinct from that of the POM: microphytobenthos (-16.0 +/- 0.6 parts per thousand), the macroalgae Enteromorpha compressa (-17.6 +/- 2.8 parts per thousand), Fucus serratus (-17.4 +/- 2.6 parts per thousand), Fucus vesiculosus (-18.5 +/- 1.6 parts per thousand), Phorphyra umbilicalis (-19.7 +/- 0.5 parts per thousand), Ulva rigida (-14.7 +/- 2.0 parts per thousand) and the seagrass Zostera noltii (-11.1 +/- 1.0 parts per thousand). Mean delta(15)N values of all macroalgae species ranged from 7.6 +/- 1.3 to 8.9 +/- 1.0 parts per thousand, while those for microphytobenthos, POM and seagrass were 5.3 +/- 0.8, 5.0 +/- 0.9 and 6.5 +/- 1.3 parts per thousand, respectively. The mean delta(13)C value of spat (-15.3 +/- 0.8 parts per thousand) and juvenile cockles (-15.7 +/- 0.7 parts per thousand) varied within a smaller range than those of 1 to 4 yr old adults (-18.2 +/- 1.2 parts per thousand). The delta(15)N values of 0-group cockles (spat and juveniles) and adult cockles are similar (8.0 +/- 0.9 parts per thousand for 0-group and 8.4 +/- 1.1 parts per thousand 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 delta(13)C values reflect a higher dependence of adult cockies on POM variability 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.