Quantifying the effects of ethanol storage on zooplankton stable isotopes.
Stable isotope analysis is a popular method of food web monitoring and is leading to an increased understanding of how energy and pollutants move within ecosystems. This technique involves the use of δ13C and δ15N stable isotopes for dietary tracking among trophic levels based on their present ratios; in aquatic ecosystems, zooplankton are frequently used to represent a basal trophic level for the offshore environment. Upon collection, zooplankton samples are frequently preserved in ethanol prior to processing. However, this method has varying effects on isotopic signature of tissues, leading to a potentially inaccurate isotopic position. In order to determine an enrichment factor for zooplankton preserved in ethanol, 25 sites were sampled across five Minnesota lakes using vertical tows with a 80μm mesh zooplankton net. Samples were stored in lake water for at least three hours to allow for gut clearance. Samples were then filtered down to 120μm to remove excess debris and either frozen or placed in 70% ethanol for comparison between treatments. Zooplankton stable isotope data were analyzed by a hierarchical linear regression model with random lake effects in order to determine a correction factor that was applied to preserved sample data. Fixed effects of ethanol preservation led to significant carbon enrichment (slope = 1.15, Rsq. = .973) and had no effect on nitrogen. The random lake effect was not significant. This correction factor will enhance the accuracy and efficacy of stable isotope analysis for freshwater food webs by providing more reliable baseline isotope values upon which these studies rely.