Modelling food chain structure and contaminant bioaccumulation using stable nitrogen isotopes

Abstract

THE nitrogen pools of animals are enriched in ¹⁵N relative to their food¹, with the top predators having the highest concentrations of this stable isotope². The use of δ¹⁵N to indicate trophic position depends on the degree to which it reflects variation in the underly-ing food-web structure, rather than variable fractionation along the food chain. Here we compare adult lake trout, a top pelagic predator, from a series of lakes, and find that δ¹⁵N values vary from 7.5 to 17.5%o, a surprisingly wide range for one species. The length of the food chain can explain this variation, supporting the idea that δ¹⁵N is a food-web descriptor. Food-chain length was measured by the presence or absence of two intermediate trophic levels, pelagic forage fish and the macrozooplankter, Mysis relicta, each of which when present contributes about three δ¹⁵N units to the trout signature. We find that δ¹⁵N can be used as a continuous, integrative measure of trophic position, which is supported by its correlation to mercury levels in lake trout.