Primary consumer delta C-13 and delta N-15 and the trophic position of aquatic consumers

Stable nitrogen isotope signatures (delta(15)N) are increasingly used to in fer the trophic position of consumers in food web studies. Interpreting the delta(15)N of consumers relative to the delta(15)N 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 an d investigate the variation in baseline delta(15)N values in 14 lakes in On tario and Quebec. Values of delta(15)N ranged from -2 to +9 parts per thous and and varied significantly as a function of lake habitat (mean littoral = 1.6 parts per thousand, pelagic = 3.1 parts per thousand, profundal = 5.2 parts per thousand). Stable carbon isotopic signatures (delta(13)C) Of prim ary consumers decreased along this same habitat gradient (mean littoral = - 23.8 parts per thousand, pelagic = -28.4 parts per thousand, profundal = -3 0.5 parts per thousand). Primary consumer delta(13)C and a categorical lake variable explained 72% of the variability in primary consumer delta(15)N. This relationship was corroborated by primary consumer delta(15)N and delta (13)C data from the literature, indicating that habitat-specific variation in baseline delta(15)N and delta(13)C is a widespread phenomenon in freshwa ter systems. We present a method that uses the presented baseline delta(15) N-delta(13)C relationship and the delta(15)N and delta(13)C values of the c onsumer to estimate trophic position; it is a method that corrects for the described variation in baseline delta(15)N. These results emphasize the gen eral importance of accounting for patterns in isotopic signatures character izing the base of the food web when inferring trophic structure using stabl e isotopes.