Temporal variation in body composition (C:N) helps explain seasonal patterns of zooplankton δ13C

1. The stable carbon isotope ratio delta(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 delta(13)C of different zooplankton taxa are rarely considered. 2. Here, we investigate to what extent temporal variation in the delta(13)C of particulate organic matter (POM) (<41 mum) and the C : N of zooplankton can explain the temporal variability in delta(13)C of freshwater zooplankton. We compare temporal patterns of delta(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 delta(13)C than did the delta(13)C of POM. This suggests that variation in the lipid content of zooplankton can strongly influence temporal variation of delta(13)C in zooplankton. 4. Using these data, we evaluate procedures that estimate the delta(13)C of only the non-lipid component of zooplankton. If zooplankton lipids are primarily dietary in origin, than extracting lipids or 'normalising'delta(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 delta(13)C signatures.