Flexible diet and trophic position of dreissenid mussels as inferred from stable isotopes of carbon and nitrogen

In this study, the trophic position and food-web impacts of invading zebra mussels (Dreissena polymorpha) were investigated by sampling mussels, seston (= phytoplankton), macrophytes, zooplankton, and surficial sediment in two small inland lakes (1999-2002) and similarly in western Lake Erie (1999 only). Tissues from quagga mussels (Dreissena bugensis) from Lake Erie were also analyzed. Stable-isotope ratios (N-15/N-14 and C-13/C-12) were used to identify likely food sources and estimate relative trophic position. For Lake Erie, stable-isotope ratios indicated no diet differences between the two mussel species. For all lakes, zooplankton delta C-13 was indicative of phytoplanktivory. The C-13 stable isotope ratios indicated that seston comprised similar to 50% of food sources for mussels in Lake Erie, but 73%-97% and 52%-100% of the diet of mussel populations in Lake Wawasee and Clark Lake, respectively. Stable nitrogen isotope ratios placed zooplankton at trophic levels equal to or higher than those of mussels in seven of eight comparisons. Dreissena polymorpha and D. bugensis are able to exploit suspended detritus as a significant energy source, as well as compete directly with zooplankton for seston as a food source and with each other in areas of sympatry.