Abstract
Stable isotope analysis is an important tool for characterising food web structure; however, interpretation of isotope data can often be flawed. For instance, lipid normalisation and trophic fractionation values are often assumed to be constant, but can vary considerably between ecosystems, species and tissues. Here, previously determined lipid normalisation equations and trophic fractionation values were re-evaluated using freshwater fish species from three rivers in the Upper Zambezian floodplain ecoregion in southern Africa. The parameters commonly used in lipid normalisation equations were not correct for the 18 model species (new D and I parameters were estimated as D = 4.46‰ [95% CI: 2.62, 4.85] and constant I = 0 [95% CI: 0, 0.17]). We suggest that future isotopic analyses on freshwater fishes use our new values if the species under consideration do not have a high lipid content in their white muscle tissue. Nitrogen fractionation values varied between species and river basin; however, the average value closely matched that calculated in previous studies on other species (δ15N fractionation factor of 3.37 ± 1.30 ‰). Here we have highlighted the need to treat stable isotope data correctly in food web studies to avoid misinterpretation of the data.
Highlights
Stable isotope analysis is a popular tool for analysing the trophic ecology of individuals, populations and communities [1]
The results presented here demonstrate that, for lipid normalisation, the standard use of the McConnaughey and McRoy [22] formulae may be inaccurate without the re-estimation of the parameter D and constant I
These differences may be attributed to the ecosystems sampled, since freshwater fishes were sampled in this study, while marine organisms were sampled by McConnaughey and McRoy [22], and brackish water fishes were sampled by Kiljunen et al [24]
Summary
Stable isotope analysis is a popular tool for analysing the trophic ecology of individuals, populations and communities [1]. Stable carbon (δ13C) and nitrogen (δ15N) isotopic composition reflects the assimilated food intake of an organism over a given time and can be used to describe food web structure [1,2,3,4,5,6]. Despite recent advances in the field, there are certain assumptions which need to be met when applying isotope tools where information is still lacking. Defining a fractionation factor, the changes in δ13C and δ15N between prey and predator, is essential for tracing energy flows and sources, determining trophic position. Nitrogen fractionation and lipid correction foundation of South Africa (UID: 74015; 77444). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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