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-Oléron Bay, France. Suspended particulate organic matter (POM) collected from adjacent marine channels had a mean d¹³C value of -22.2 ± 1.1o/oo (n = 19). Benthic primary producers on Ronce-les-Bains tidal flats had d¹³C values distinct from that of the POM: microphytobenthos (-16.0 ± 0.6o/oo), the macroalgae Enteromorpha compressa (-17.6 ± 2.8o/oo), Fucus serratus (-17.4 ± 2.6o/oo), Fucus vesiculosus (-18.5 ± 1.6o/oo), Phorphyra umbilicalis (-19.7 ± 0.5o/oo), Ulva rigida (-14.7 ± 2.0o/oo) and the seagrass Zostera noltii (-11.1 ± 1.0o/oo). Mean d¹⁵N values of all macroalgae species ranged from 7.6 ± 1.3 to 8.9 ± 1.0o/oo, while those for microphytobenthos, POM and seagrass were 5.3 ± 0.8, 5.0 ± 0.9 and 6.5 ± 1.3o/oo, respectively. The mean d¹³C value of spat (-15.3 ± 0.8o/oo) and juvenile cockles (-15.7 ± 0.7o/oo) varied within a smaller range than those of 1 to 4 yr old adults (-18.2 ± 1.2o/oo). The d¹⁵N values of 0-group cockles (spat and juveniles) and adult cockles are similar (8.0 ± 0.9o/oo for 0-group and 8.4 ± 1.1o/oo 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 d¹³C values reflect a higher dependence of adult cockles 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.

KEY WORDS: Stable isotope ratios · Particulate organic matter · Microphytobenthos · Cerastoderma edule · Suspension feeder

Published in MEPS Vol. 187 (1999) on October 14
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1999