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).