Selection of food sources by Ruditapes philippinarum and Mactra veneriformis (Bivalva: Mollusca) determined from stable isotope analysis

Carbon and nitrogen isotopic ratios in the body of the bivalves Ruditapes philippinarum and Mactra veneriformis were measured to estimate the role of the bivalves in the ecosystem in an estuary. The isotopic signatures were compared with those obtained from particulate organic matter (POM) in the middle reach of the river, in the estuary, and off the estuary. Both the bivalves showed constant delta(13)C and delta(15)N values of approximately -17parts per thousand. and 10parts per thousand. that were significantly enriched compared with those for POM in the habitat (-25parts per thousand. and 4parts per thousand.). The relative contributions of terrestrial organic matter to the POM in the habitat and to the bivalve diet were estimated: the former was approximately 90% while the latter approximately 10%. This indicates that the bivalves select marine POM from the organic matter available in their habitat, and play a minor role in the direct removal of terrestrial particulate matter. Temporal changes in delta(13)C and delta(15)N for both bivalves were small, but a distinct decrease in delta(13)C was observed following a period of heavy rain. During rainfall the concentration of terrestrial material increased in the habitat, and the bivalves would then have to change their food sources temporarily.