2.2 | Stable isotope analysis
We analyzed fish and invertebrate samples at the Stable Isotope Mass
Spectrometry Laboratory at Kansas State University, Manhattan, KS, USA,
and the Center for Stable Isotopes at the University of New Mexico,
Albuquerque, NM, USA. Isotopic values were standardized between
laboratories using tetramin® fish food for isotopic calibration and we
reported results using the common delta notation (δ ‰) calculated by:
δ ‰ = [(Rsample/Rstandard) – 1)] ×
103.
We pre-processed data prior to analyses by removing obvious outliers
(i.e., δ13C or δ15N values more than
two standard deviations from taxon-specific means). Due to differences
in Colorado Pikeminnow and Channel Catfish tissue types sampled between
time periods (i.e., muscle tissue from historical samples and fin tissue
from contemporary samples), we used mean fin-to-muscle conversions. For
Colorado Pikeminnow (δ13C = -1.29 and
δ15N = +0.56), we used the conversion from Franssenet al . (2016) and for Channel Catfish (δ13C =
-1.08 and δ15N = +0.58), we used Maitland (2020).
Additionally, we corrected museum specimens for the effects of formalin
preservation which depletes 13C by 1.0 ‰ and enriches15N by 0.53 ‰ (Edwards et al ., 2002). Finally,
variable lipid concentration can introduce bias into isotopic analyses
because they are more depleted in 13C than proteins
and carbohydrates (Post et al ., 2007). Therefore, we corrected
δ13C values by the ratio of C:N in samples following
Post et al . (2007) using the formula:
δ13Ccorrected =
δ13Craw - 3.32 + 0.99 × C:N