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).