Ontogenetic changes in the trophic position of a freshwater Unionidae mussel

Freshwater mussels have often been used as indicators of a trophic baseline (i.e., primary consumers) in the food web. To assess the utility of a large filter-feeding mussel, Cristaria plicata, as an isotopic indicator, we compared the carbon (delta C-13) and nitrogen (delta N-15) stable isotope ratios of the mussel with those of its potential food sources, particulate organic matter (POM) and sediment organic matter (SOM). The delta C-13 values of large mussels (shell length > 140 mm, -31 parts per thousand to -29.2 parts per thousand) were similar to those of POM (-30.2 parts per thousand) rather than those of SOM (-27.6 parts per thousand), indicating that mussel carbon was derived mainly from POM (mainly phytoplankton). In contrast, the mussels exhibited 6.3 parts per thousand to 9.0 parts per thousand higher delta N-15 values than did POM. Assuming a previously reported trophic enrichment factor (+3.4 parts per thousand), the trophic level estimate of the mussel ranged from 2.9 to 3.6, indicating that they functioned as secondary rather than primary consumers. Our results also revealed positive correlations between shell lengths and delta N-15 values, suggesting that the mussels changed their trophic position from primary consumer (shell length 140 mm) with growth. A significant inter-annual difference was found in the y-intercepts of the regression lines between shell length and delta N-15, indicating that mussels can reflect inter-annual changes in the isotopic baseline (i.e., primary producers).