An evaluation of stable nitrogen isotopes and polychlorinated biphenyls as bioenergetic tracers in aquatic systems

Abstract

This study investigated the relationship between stable nitrogen isotopes (δ15N) and polychlorinated biphenyl (PCB) concentrations in rock bass (Ambloplites rupestris) and bluegill sunfish (Lepomis macrochirus) with respect to age, size, and diet. δ15N signatures in both species exhibited enrichment with increasing size and approached steady state with respect to dietary δ15N values by the second year of growth. Young-of-the-year fish, however, exhibited lower nitrogen isotope enrichment over the diet, indicating that the commonly held trophic enrichment factor of 3.4‰ is more suitable for older, slow-growing individuals. PCB accumulation in both species progressed from being dominated by uptake from water in small (<100 g) fish to dietary uptake in larger individuals as a function of bioenergetic constraints such as food energy conversion and contaminant assimilation efficiencies. Significant increases in PCB accumulation were attributed to decreased specific growth rates such that ≤60% of body mass was gained on an annual basis. This effect was most noted in bluegills where higher PCB biomagnification factors were a consequence of increased foraging costs associated with an invertebrate diet. It is concluded that growth-related changes in species bioenergetics regulate both contaminant accumulation and δ15N dynamics.