Stable isotope analysis
Analyses of stable isotopes of hydrogen (δ2H) of fish muscle tissue (bleak: N = 27, perch: N = 13, roach:N = 26), seston (N = 6), and terrestrial insects (N= 12) samples were conducted at the Leibniz-Institute for Zoo-and Wildlife Research Berlin. To remove contaminants, each insect sample was washed using a 2:1 chloroform: methanol solution for 24 h. Fish muscle samples were fat-extracted using a Soxtherm extractor (C. Gerhardt GmbH & Co. KG, Königswinter, Germany). After being dried in an oven at 50℃ for 72 hours, sample aliquots of 0.35 ± 0.10 mg were placed into silver capsules (IVA Analysentechnik, Meerbusch, Germany). Non-exchangeable hydrogen isotope ratios were measured using a High Temperature Conversion Elemental Analyzer (Thermo Fischer Scientific Inc., Waltham, USA) connected to an online temperature-controlled vacuum-equilibration autosampler Uni-Prep (EuroVector, Pavia, Italy) and coupled to a continuous‐flow isotope‐ratio mass spectrometer (Delta V Advantage; Thermo Fischer Scientific Inc.). Measurements were determined using a comparative equilibration method (Wassenaar and Hobson 2003). Samples and standards were loaded into the autosampler at 60℃. After flushing with helium and evacuating the carousel, 20 μl of water of known isotopic composition was injected trough the Uni-Prep septum for equilibration (1 h). To ensure that a similar H exchangeability among standards and samples, samples were measured together with in-house standards, previously calibrated against international reference materials. Stable hydrogen isotope ratios were expressed as deviations from the international standard V-SMOW. Precision of δ2H measurements was always better than 1.1‰ (1 SD).