The effects of preservation methods, dyes and acidification on the isotopic values (δ15N and δ13C) of two zooplankton species from the KwaZulu‐Natal Bight, South Africa

Stable isotope measurements are an important tool for ecosystem trophic linkage studies. Ideally, fresh samples should be used for isotopic analysis, but in many cases organisms must be preserved and analysed later. In some cases dyes must be used to help distinguish organisms from detritus. Since preservatives and dyes are carbon-based, their addition could influence isotopic readings. This study aims to improve understanding of the effects of sample storage method, dye addition and acidification on the δ15N and δ13C values of zooplankton (Euphasia frigida and Undinula vulgaris). Zooplankton was collected and preserved by freezing, or by the addition of 5% formalin, 70% ethanol, or 5% formalin with added Phloxine B or Rose Bengal, and stored for 1 month before processing. Samples in 5% formalin and 70% ethanol were also kept and processed after 3 and 9 months to study changes over time. Formalin caused the largest enrichment for δ13C and a slight enrichment for δ15N, while ethanol produced a slight depletion for δ13C, and different effects on δ15N depending on the species. In formalin, dyes depleted the δ13C values, but had variable effects on δ15N, relative to formalin alone. Acidification had no significant effect on δ15N or δ13C for either species. Long-term storage showed that the effects of the preservatives were species-dependent. Although the effects on δ15N varied, a relative enrichment in 13C of samples occurred with time. This can have important consequences for the understanding of the organic flow within a food web and for trophic studies. Copyright © 2011 John Wiley & Sons, Ltd.