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). 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‰ for 15N. As C/N in muscle tissue was low (3.2 ± 0.01; average ± SD), no lipid normalization was performed (Kiljunen et al. 2006). However, C/N in liver tissue (4.4 ± 0.6 average ± SD) exceeded the recommended ratio of 3.4 and we therefore used a mathematical lipid normalization on the δ13C values to account for the bias of carbon fractionation occurring during lipid synthesis (Kiljunen et al. 2006, Sweeting et al. 2006, Skinner et al. 2016). As recommended by Skinner et al. (2016), we used the percent lipid model introduced by Post et al. (2007), together with the normalization model described by Kiljunen et al. (2006).
We calculated 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).