Compositional and interlake variability of zooplankton affect baseline stable isotope signatures

Zooplankton are commonly used to establish a baseline isotopic signature for pelagic production in lakes. Our objective was to evaluate this approach by quantifying among‐lake and within‐lake variability of δ 13 C and d15N for different taxa of pelagic zooplankton. We measured the δ 13 C and δ 15 N of Daphnia , Holopedium , and calanoid copepods from four lakes sampled from June to November 2001 and from eight additional lakes sampled once in midsummer. In the four lakes with temporal sampling, within‐lake differences due to taxonomic grouping accounted for 36.7% of the variance in δ 15 N and 41.7% of the variance in δ 13 C. Among all lakes, the δ 15 N of calanoid copepods was on average 2.55‰ and 2.44‰ higher than Daphnia or Holopedium , respectively, whereas the δ 13 C of calanoid copepods was 2.19‰ and 2.23‰ lower than Daphnia or Holopedium , respectively. If 15N fractionation is similar among species, the differences in δ 15 N suggest that calanoid copepods either feed at a higher trophic position in the food web or they have a consistently higher baseline δ 15 N signature than Daphnia or Holopedium among lakes. Differences in δ 13 C suggest that zooplankton taxa in the pelagia of lakes have different food sources. We conclude that species composition and feeding behaviors of the zooplankton community should be considered before making among‐lake comparisons of food web structure. We show that Daphnia is a useful isotopic baseline for organisms that rely on primary production in lakes.