Estimating ethanol correction factors for δ¹³C and δ¹⁵N isotopic signatures of freshwater zooplankton from multiple lakes

In freshwater systems, δ 13 C and δ 15 N stable isotopes can be used to differentiate between pelagic and littoral energy sources and to quantify trophic position. In these ecosystems, crustacean zooplankton are frequently used to characterize the pelagic baseline. Zooplankton samples are often preserved prior to processing and analysis, which can affect isotopic signatures. Variability in preservation effects across studies make it difficult to determine if and how to correct for preservation effects. Here, we develop a correction factor for ethanol preservation and present a flexible statistical method that can be updated with additional data to increase its applicability. We collected zooplankton from five lakes in Minnesota, USA encompassing wide isotopic ranges (δ 13 C from −37.23‰ to −23.96‰; δ 15 N from 3.07‰ to 14.44‰). Changes in zooplankton δ 13 C and δ 15 N signatures were quantified using a Bayesian hierarchical model predicting fresh values from ethanol‐preserved values. Ethanol preservation increased δ 13 C by a factor of 1.158 (95% CI 0.866–1.441) and had a negligible effect on δ 15 N (slope = 1.077; 95% CI 0.833–1.359). Lake‐specific values did not differ from the overall relationship. K‐fold and leave‐one‐out cross validation tests verified that both models were accurate; RMSE of predicted δ 13 C = 0.701 and RMSE of predicted δ 15 N = 0.590. Our correction factors could be applied to other systems in which baseline δ 13 C and δ 15 N values fall within the range of our study, and this approach also enables the inclusion of data from additional lakes to estimate new corrections.