Stable isotope evidence of benthic microalgae-based growth and secondary production in the suspension feeder Cerastoderma edule (Mollusca, Bivalvia) in the Marennes-Oleron Bay

The contribution of natural food sources to the growth and secondary produc tion of the suspension feeding bivalve Cerastoderma edule (L.) was estimate d under field conditions in the Marennes-Oleron Bay (Atlantic coast, France ). Monthly estimates of abundance, biomass and cockle growth were combined with seasonal analyses of delta C-13 and delta N-15 ratios of juvenile and adult cockles, together with their potential food sources [i.e. suspended p articular organic matter (POM), microphytobenthos, macroalgae and seagrass] sampled at mid-tide level in a muddy sandflat. Adult cockles grew mainly i n spring. whereas juveniles grew in summer and autumn, following spat recru itment 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 contributi ons 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 cockl es was obtained by using a simple carbon isotope-mixing model with microphy tobenthos (delta C-13 = -16.0+/-0.6 parts per thousand) and POM (delta C-13 = -22.2+/-1.1 parts per thousand) 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 cockle s (60%). The between-age difference was induced mainly by changes in the av ailability of food resources (benthic versus planktonic) during the non-syn chronous growing seasons of juvenile and adult cockles.