Hydrobiologia 440: 317–329, 2000. M.B. Jones, J.M.N. Azevedo, A.I. Neto, A.C. Costa & A.M. Frias Martins (eds), Island, Ocean and Deep-Sea Biology. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 317 Stable isotope evidence of benthic microalgae-based growth and secondary production in the suspension feeder Cerastoderma edule (Mollusca, Bivalvia) in the Marennes-Ol´ eron Bay Pierre-Guy Sauriau 1 & Chang-Keun Kang 2 1 Centre de Recherche en Ecologie Marine et Aquaculture de L’Houmeau (CNRS-IFREMER, UMR-10), BP 5, F-17137 L’Houmeau, France E-mail: Pierre.Guy.Sauriau@ifremer.fr 2 National Fisheries Research & Development Institute (Pohang Regional laboratory), 616 Duhodong, Buk-gu, Pohang 791-110, South Korea Key words: growth, secondary production, Cerastoderma edule, stable isotope ratios, microphytobenthos, intertidal, Marennes-Ol´ eron Bay Abstract The contribution of natural food sources to the growth and secondary production of the suspension feeding bivalve Cerastoderma edule (L.) was estimated under field conditions in the Marennes-Oléron Bay (Atlantic coast, France). Monthly estimates of abundance, biomass and cockle growth were combined with seasonal analyses of δ 13 C and δ 15 N ratios of juvenile and adult cockles, together with their potential food sources [i.e. suspended particulate or- ganic matter (POM), microphytobenthos, macroalgae and seagrass] sampled at mid-tide level in a muddy sandflat. Adult cockles grew mainly in spring, whereas juveniles grew in summer and autumn, following spat recruitment in early summer. Total annual production and elimination of cockles were estimated to be 32.5 and 34.7 g AFDW m −2 yr −1 . Relative contributions of each year class to production were ca 40, 41, 11 and 6% for 0-group, 1-, 3- and 4-yr- old cockles in 1995, respectively. Quantitative assessment of proportions of food sources to the annual secondary production of cockles was obtained by using a simple carbon isotope-mixing model with microphytobenthos (δ 13 C = −16.0±0.6 ‰) and POM (δ 13 C= −22.2±1.1 ‰) as end-members. On average, more than 70% of the total annual cockle production originated from microphytobenthos, with a much higher contribution for the 0-group (88%) than for adult cockles (60%). The between-age difference was induced mainly by changes in the availability of food resources (benthic versus planktonic) during the non-synchronous growing seasons of juvenile and adult cockles. Introduction The edible cockle Cerastoderma edule (L.) is one of the most common benthic suspension feeders on tidal flats along the European Atlantic coast (Tebble, 1966). This species exhibits large year-to-year and between-site fluctuations in larval recruitment success, and of subsequent survival and growth of juveniles (Kristensen, 1957; Dörjes et al., 1986; Ducrotoy et al., 1991). Consequently, relative contributions of each year class to the total annual secondary production of cockle populations are highly variable, as repor- ted previously (Evans, 1977; Möller & Rosenberg, 1983). Amongst the main factors that may induce these variations are abnormal thermal conditions (Or- ton, 1933; Beukema, 1982; Guillou et al., 1992), predation by shore crabs, shrimps and fish (Reise, 1985; Sanchez-Salazar et al., 1987), intraspecific com- petition (André & Rosenberg, 1991; Bachelet et al., 1992; Jensen, 1993), and spatfall transport by near- bottom flows (Baggerman, 1953; de Montaudouin & Bachelet, 1996). However, little attention has been paid to the influence of changes in the diet available for suspension feeding cockles during critical periods of their life cycle. Jensen (1992) hypothesised that mis- match in growing seasons of juvenile and adult cockles may induce a differential use of food resources; this may minimise intraspecific competition within the