Temporal variation in body composition (C : N) helps explain seasonal patterns of zooplankton δ 13 C

Summary 1. The stable carbon isotope ratio δ 13 C is a useful tracer of energy flow in lake food webs, and the zooplankton signature is commonly used to establish a baseline for the pelagic habitat. However, sources of temporal variability in the δ 13 C of different zooplankton taxa are rarely considered. 2. Here, we investigate to what extent temporal variation in the δ 13 C of particulate organic matter (POM) (<41  μ m) and the C : N of zooplankton can explain the temporal variability in δ 13 C of freshwater zooplankton. We compare temporal patterns of δ 13 C and C : N for Daphnia , Hesperodiaptomus franciscanus and Leptodiaptomus tyrelli over a 6‐month period at four sites in two oligotrophic lakes. 3. In all three taxa, seasonal variation in zooplankton C : N explained more of the variation in zooplankton δ 13 C than did the δ 13 C of POM. This suggests that variation in the lipid content of zooplankton can strongly influence temporal variation of δ 13 C in zooplankton. 4. Using these data, we evaluate procedures that estimate the δ 13 C of only the non‐lipid component of zooplankton. If zooplankton lipids are primarily dietary in origin, than extracting lipids or ‘normalising’ δ 13 C based on C : N will exclude a major dietary source, and therefore may be inappropriate. 5. We conclude that temporal variation in body composition (C : N) of zooplankton can significantly influence the temporal variation of zooplankton δ 13 C signatures.