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

TLDR

Stable nitrogen isotope signatures (δ15N) are increasingly used to infer the trophic position of consumers, with interpretation relative to the base of the food web providing a time‑integrated measure of trophic level. The study investigates how baseline δ15N values vary among 14 lakes in Ontario and Quebec by using primary consumers (trophic level 2) as indicator organisms. It presents a method that employs the observed δ15N–δ13C relationship of primary consumers and the consumer’s own δ15N and δ13C values to estimate trophic position while correcting for baseline variation. δ15N values ranged from –2 to +9 ‰ and varied with lake habitat, while δ13C decreased along the same gradient; the δ15N–δ13C relationship explained 72 % of δ15N variability, a pattern confirmed by literature and underscoring the need to account for baseline isotopic variation when inferring trophic structure.

Abstract

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.