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Coupling experimental and field-based approaches to decipher carbon sources in the shell of the great scallop, Pecten maximus (L.)
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International audience. This research investigated how the carbon isotopic composition of food source (d13Cfood) and dissolved inorganic carbon(d13CDIC) influences the carbon isotopic composition of Pecten maximus shells (d13Cshell) under both experimental and naturalconditions. The objectives are to better understand the relationship between P. maximus and its environment, and to specificallydistinguish conditions under which calcification is influenced by respired CO2 derived from food sources versus conditions inwhich calcification uses inorganic carbon from seawater. Laboratory experiment investigated carbon incorporation into shellcarbonates by maintaining scallops under conditions where the stable carbon isotopic composition of food sources was considerablydepleted (-54 per mille), relative to values observed in the natural environment (-21 per mille). Laboratory experiment ran for 78 daysunder three temperature conditions, 15 °C, 21 °C and 25 °C. A survey of the environmental parameters and stable carbon isotopiccomposition into shell carbonate of natural population of P. maximus was also realized during the same year in the Bay ofBrest, France. Data collected from both laboratory experiment and the natural environment confirmed that both d13CDIC andd13Cfood influence d13Cshell values and that organic carbon incorporation (CM) averages about 10% (4.3–6.8% under experimentalconditions and 1.9–16.6% in the natural environment). The shift in stable carbon isotopic composition from the uptake ofdepleted food sources under experimental conditions realized a marked divergence in the predicted equilibrium between calciumcarbonate and ambient bicarbonate, relative to the natural environment. This offset was 1.7 ± 0.6 per mille for scallops in their naturalenvironment and 2.5 ± 0.5 and 3.2 ± 0.9 per mille for scallops under experimental conditions at water temperatures of 15 °Cand 21 °C,respectively. The offset of 3 per mille for scallops subjected to laboratory experiment could not be explained in light of growth rate butmay be related to food supply and/or temperature. Food source and temperature effects may also explain the annual variationobserved in CM values measured from scallops in their natural environment. CM estimation from the natural population ofP. maximus varied seasonally from around 2% at the end of winter, to 12% in summer. The seasonal variation resemblesvariability in the carbon isotopic composition of the food sources throughout the year with an exception at the end of winter.
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