Comparing trophic position of freshwater fish calculated using stable nitrogen isotope ratios (δ¹⁵N) and literature dietary data

Abstract

Stable nitrogen isotope ratios ( delta 15 N) are commonly used to represent the trophic structure of aquatic systems, yet the ability of delta 15 N to indicate the trophic position of aquatic consumers remains untested using traditional dietary methods. Interpreting the delta 15 N of aquatic consumers relative to large, long-lived primary consumers such as unionid mussels provides a continuous measure of an organism's trophic position that adjusts for among-system variation in baseline delta 15 N. We used this method to estimate the trophic position of eight littoral fish species from 36 lakes in Ontario and Quebec. We validated these delta 15 N measures of trophic position by compiling literature dietary data from 342 populations of these same fish species and calculated a continuous measure of trophic position for each population. Mean dietary trophic position estimates corresponded closely to 15 N estimates, with mean trophic position ranging from 3.3 for pumpkinseed (Lepomis gobbosus) to 4.4 for walleye (Stizostedion vitreum). Both methods indicated approximately one trophic level of variation among populations of a species. This study confirms the ability of baseline-adjusted delta 15 N to represent the trophic position of aquatic consumers.