Determination of diet in Manila clams by spatial analysis of stable isotopes

Spatial changes in the dietary regime ofRuditapes philippinarumfrom intertidal sediments of Arcachon Bay were depicted by analyzing stable isotope ratios in both its adductor muscles and potential trophic sources. Manila clams were collected from 50 sites in May to June 2006. Dietary reconstruction was based on the IsoSource mixing model, considering trophic enrichments of 3.5‰ for carbon and 3.0‰ for nitrogen, which were determined experimentally. δ13C and δ15N values, respectively, averaged –28.2 and 5.2‰ for riverine particulate organic matter (POM), –20.6 and 4.7‰ for inner bay phytoplankton, –21.0 and 5.6‰ for outer bay phytoplankton, –16.8 and 4.8‰ for microphytobenthos, –18.4 and 3.9‰ for sedimentary organic matter (SOM) and 11.8 and 4.0‰ forZostera noltii. Clam muscle δ13C and δ15N values ranged from –20.5 to –16.4‰, and from 7.6 to 9.3‰, respectively. δ15N significantly decreased from southern to northern parts of the bay, while δ13C values increased with tidal elevation. Outer bay phytoplankton was the major diet component of clams. Clams from the southeast mainly incorporated outer bay phytoplankton despite the proximity of the Leyre River, whereas clams from the northwest incorporated approximately equal parts of inner and outer bay phytoplankton, riverine POM, microphytobenthos and SOM. These patterns were consistent with spatial gradients driven by the dominant role of tidal hydrodynamics within the bay and land-use characteristics of the catchment.