Stable isotope variability of meso-zooplankton along a gradient of dissolved organic carbon

1. The delta C-13 and delta N-15 signatures of zooplankton vary with dissolved organic carbon (DOC), but inconsistent and limited taxonomic resolution of previous studies have masked differences that may exist among orders, genera or species and are attributable to dietary and/or habitat differences. Here we investigate differences among the isotopic signatures of five zooplankton taxa (Daphnia, Holopedium, large Calanoida, small Calanoida and Cyclopoida) in Precambrian shield lakes with a sixfold range of DOC concentration. 2. delta C-13 signatures of Daphnia, small calanoids and large calanoids became more depleted with increasing lake DOC, whereas Holopedium and cyclopoid delta C-13 became enriched with increasing DOC concentration. 3. The variability of delta C-13 and delta N-15 isotopic signatures among zooplankton groups was reduced in high-DOC, compared to low-DOC lakes, especially for delta C-13. Differences in delta C-13 and POM-corrected delta N-15 accounted for up to 33.7% and 19.5% of the variance, respectively, among lakes of varying DOC concentration. 4. The narrow range of signatures found in higher DOC lakes suggests that different taxa have similar food sources and/or habitats. In contrast, the wide range of signatures in low-DOC lakes suggests that different taxa are exploiting different food sources and/or habitats. Together with the variable trends in zooplankton isotopic signatures along our DOC gradient, these results suggest that food web dynamics within the zooplankton community of temperate lakes will change as climate and lake DOC concentrations change.