Zooplankton, lipids and stable isotopes: importance of seasonal, latitudinal, and taxonomic differences

We found considerable seasonal, latitudinal, and taxonomic differences in zooplankton lipid content and concurrent δ13C values of zooplankton. We collected cladoceran as well as cyclopoid and calanoid copepod zooplankton from boreal and subarctic lakes throughout a year, allowing us to study zooplankton likely subjected to different isotopic fractionation processes and with highly variable lipid contents. Considerable seasonal variation was observed in the difference between bulk and lipid-extracted zooplankton δ13C values, indicating that seasonally changing lipid content introduced notable variation in zooplankton δ13C values. The difference between bulk and lipid-extracted material was most amplified in lipid-rich subarctic zooplankton in winter, δ13C difference being >5 units. Significant differences were also observed among zooplankton taxa, with copepods showing a greater lipid impact on δ13C than cladocerans. Published lipid correction models failed to produce satisfying fits to our data, and considerable variation was left even after recalibrating the model parameters. This was likely due to taxonomic differences in lipid effects on δ13C values. We therefore produced separate mass balance-based lipid correction models for cladocerans and also cyclopoid and calanoid copepods. We conclude that arithmetic lipid correction models perform well with zooplankton samples, but taxonomic differences need to be considered.