Isotopic Correction Factors for Zooplankton Storage

Abstract

Stable isotope analysis is an increasingly popular method of food web monitoring and is leading to an increased understanding of how energy and pollutants move within an ecosystem. This technique involves the use of δ13C and δ15N stable isotopes for specific dietary tracking among trophic levels based on their present ratios; in aquatic ecosystems, zooplankton are frequently used to represent the baseline pelagic trophic level. Upon collection, zooplankton samples are often preserved in ethanol prior to processing and analysis. However, this method has varying effects on isotopic signatures of tissues, leading to a potentially inaccurate isotopic position. In order to determine a correction factor to account for ethanol preservation, zooplankton were collected at 25 sites across five Minnesota lakes. Zooplankton stable isotope data were analyzed using a mixed effects model that showed ethanol preservation leading to significant δ13C enrichment (SE = 0.064, t-value = 17.951) and no significant change in δ15N. The random lake effect had no significant impact on the outcome. 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, and provide a procedure for other correction factors to be determined.