Stable isotope analyses and calculation of trophic discrimination
factors
We ground oven‐dried tissues samples to a fine powder using a mortar and
pestle, and transferred approximately 1 mg of the powder to tin
capsules. Elemental and stable isotope analyses of carbon and nitrogen
were conducted at the University of California, Davis Stable Isotope
Facility, California, USA, using a PDZ Europa ANCA‐GSL elemental
analyzer coupled to a PDZ Europa 20‐20 isotope ratio mass spectrometer
(Sercon, Cheshire, UK). As in all samples, C/N was low (3.2 ± 0.01;
average ± SD), no lipid normalization was performed (Kiljunen et al.
2006). We express our results using the δ notation, thus, referring the
ratios of samples to the international standards (Vienna Pee Dee
Belemnite and Air for carbon and nitrogen, respectively). Measurement
error was 0.2‰ for 13C and 0.3‰ for15N.
We calculated the TDF (i.e. Δ13C and
Δ15N) for perch as
\({X}_{\text{perch}}=\ \text{δX}_{\text{perch}}-\ \text{δX}_{\text{Chironomid}}\),
using average values for \(\text{δX}_{\text{Chironomid}}\), where \(X\)stands for the heavy isotope of C (13C) and N
(15N).