Compositional and interlake variability of zooplankton affect baseline stable isotope signatures

Abstract

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 δ13C and d15N for different taxa of pelagic zooplankton. We measured the δ13C and δ15N 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 δ15N and 41.7% of the variance in δ13C. Among all lakes, the δ15N of calanoid copepods was on average 2.55‰ and 2.44‰ higher than Daphnia or Holopedium, respectively, whereas the δ13C 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 δ15N suggest that calanoid copepods either feed at a higher trophic position in the food web or they have a consistently higher baseline δ15N signature than Daphnia or Holopedium among lakes. Differences in δ13C 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.