Before the analysis of stable isotopes, all samples were dried at 50 °C for 48 h to the constant weight and grounded to a fine homogenous powder. Approximately 0.6 mg of animal samples and 1.5 mg of plant and detritus samples were weighed (at the precision of 0.001 mg) and transferred into tin cups. Stable isotope analyses were performed at the University of Jyväskylä using a Carlo Erba Flash EA 1112 elemental analyser connected to Thermo Finnigan DELTAplus and Advantage continuous-flow isotope ratio mass spectrometer (Thermo Electron Corporation, Waltham, MA, USA).
Vienna Pee Dee belemnite and atmospheric N2 were used as reference standards for carbon and nitrogen, respectively. To control instrument stability, northern pike Esox lucius L., 1758 muscle tissue and birch Betula pendula R. leaves of known isotopic compositions were run after every six samples. Results are expressed using the conventional δ notation as parts per thousand difference from the international standards. Analytical precision was < 0.1 ‰ for δ13C and < 0.3 ‰ for δ15N.
Vienna Pee Dee belemnite and atmospheric N2 were used as reference standards for carbon and nitrogen, respectively. To control instrument stability, northern pike Esox lucius L., 1758 muscle tissue and birch Betula pendula R. leaves of known isotopic compositions were run after every six samples. Results are expressed using the conventional δ notation as parts per thousand difference from the international standards. Analytical precision was < 0.1 ‰ for δ13C and < 0.3 ‰ for δ15N.
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