Can Introduced Species Alter Trophic Energy Flow in Reservoirs?

Introduced species are known to alter the trophic structure of aquatic communities in various ways, such as changing the energy flow within a system by increasing or decreasing the amount of nearshore-offshore energy transfer (benthic-pelagic coupling).   If introduced species are exclusively pelagic, for example, more energy may become available in the pelagic zone, whereas the addition of a primarily littoral species could alter nearshore benthic production.   Reservoirs are particularly susceptible to species introductions and are, therefore, optimal systems to quantify the effects of introduced species on trophic interactions.  Lake Norman, the largest reservoir in North Carolina, has experienced multiple species introductions since its impoundment in 1963; however impacts of these introductions are poorly understood.    We examined diet composition and isotopic signature of five species regularly collected in both the littoral and pelagic zones, two introduced (alewife, a planktivore, and white perch, an omnivore) and three established (threadfin shad, a planktivore, black crappie, an omnivore, and striped bass, a  piscivore), to determine the influence of successful invaders on benthic-pelagic coupling in the reservoir.   Muscle tissue from twenty individuals of each species were collected for nitrogen and carbon stable isotope analysis.   Stable isotope analysis revealed that fish collected offshore were more depleted of carbon than conspecifics collected nearshore, suggesting trophically segregated populations.  Of the two introduced species, alewife appear to have greater potential for shifting trophic interactions as they are far more dependent on pelagic resources than all native species sampled.  Further, comprehensive diet analysis revealed alewife consumption of newly emerging Diptera pupae from the offshore profundal zone, a diet item rarely observed in the stomachs of other fishes; consumption of this prey item may increase the amount of energy transferred from the pelagic profundal zone into the littoral zone, as alewife are found in both areas but appear to forage exclusively pelagically.  White perch, the other introduced species, showed nearly identical isotopic signatures to other native species, suggesting that this introduction may have little effect on benthic-pelagic coupling in Lake Norman, a pattern confirmed by diet analysis.    This ongoing research is designed to elucidate patterns and consequences of introduced species on trophic interactions in an established reservoir food web with the goal of assisting managers with the development of mitigation or management strategies.