PRIMARY CONSUMER δ13C AND δ15N AND THE TROPHIC POSITION OF AQUATIC CONSUMERS

Stable nitrogen isotope signatures (δ15N) are increasingly used to infer the trophic position of consumers in food web studies. Interpreting the δ15N of consumers relative to the δ15N characterizing the base of the food web provides a time-integrated measure of trophic position. We use primary consumers (trophic level 2) as baseline indicator organisms and investigate the variation in baseline δ15N values in 14 lakes in Ontario and Quebec. Values of δ15N ranged from −2 to +9‰ and varied significantly as a function of lake habitat (mean littoral = 1.6‰, pelagic = 3.1‰, profundal = 5.2 ‰). Stable carbon isotopic signatures (δ13C) of primary consumers decreased along this same habitat gradient (mean littoral = −23.8‰, pelagic = −28.4‰, profundal = −30.5‰). Primary consumer δ13C and a categorical lake variable explained 72% of the variability in primary consumer δ15N. This relationship was corroborated by primary consumer δ15N and δ13C data from the literature, indicating that habitat-specific variation in baseline δ15N and δ13C is a widespread phenomenon in freshwater systems. We present a method that uses the presented baseline δ15N–δ13C relationship and the δ15N and δ13C values of the consumer to estimate trophic position; it is a method that corrects for the described variation in baseline δ15N. These results emphasize the general importance of accounting for patterns in isotopic signatures characterizing the base of the food web when inferring trophic structure using stable isotopes.