Compositional and inter-lake variability of zooplankton affect baseline stable isotope signatures
Matthews, Blake, and Asit Mazumder
Limnol. Oceanogr., 48(5), 2003, 1977–1987

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 ¹³C and d15N for different taxa of pelagic zooplankton. We measured the ¹³C and ¹⁵N 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 ¹⁵N and 41.7% of the variance in ¹³C. Among all lakes, the ¹⁵N of calanoid copepods was on average 2.55‰ and 2.44‰ higher than Daphnia or Holopedium, respectively, whereas the ¹³C 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 ¹⁵N suggest that calanoid copepods either feed at a higher trophic position in the food web or they have a consistently higher baseline ¹⁵N signature than Daphnia or Holopedium among lakes. Differences in ¹³C 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.