The selection and analysis of fatty acid ratios: A new approach for the univariate and multivariate analysis of fatty acid trophic markers in marine pelagic organisms

Martin Graeve,Michael J. Greenacre

doi:10.1002/lom3.10360

Abstract

AbstractFatty acid (FA) compositions provide insights about storage and feeding modes of marine organisms, characterizing trophic relationships in the marine food web. Such compositional data, which are normalized to sum to 1, have values—and thus derived statistics as well—that depend on the particular mix of components that constitute the composition. In FA studies, if the set of FAs under investigation is different in two separate studies, all the summary statistics and relationships between the FAs that are common to the two studies are artificially changed due to the normalization, and thus incomparable. Ratios of FAs, however, are invariant to the particular choice of FAs under consideration—they are said to be subcompositionally coherent. Here, we document the collaboration between a biochemist (M.G.) and a statistician (M.J.G.) to determine a suitable small set of FA ratios that effectively replaces the original data set for the purposes of univariate and multivariate analysis. This strategy is applied to two FA data sets, on copepods and amphipods, respectively, and is widely applicable in other contexts. The selection of ratios is performed in such a way as to satisfy substantive requirements in the context of the respective data set, namely to explain phenomena of interest relevant to the particular species, as well as the statistical requirement to explain as much variance in the FA data set as possible. Benefits of this new approach are (1) univariate statistics that can be validly compared between different studies, and (2) a simplified multivariate analysis of the reduced set of ratios, giving practically the same results as the analysis of the full FA data set.

Highlights

D Additional Supporting Information, including a video abstract, may be found in the online version of this article
18:4(n−3) is used as FATM for dinoflagellates and 16:0 is a ubiquitous Fatty acid (FA) mostly connected to membrane lipids
The statistical criteria are based on considering the complete set of ratios of the compositional data set, where the ratios are logarithmically transformed, and identifying those that maximally explain the total variance of these logratios

Summary

Introduction

D Additional Supporting Information, including a video abstract, may be found in the online version of this article. These essential FAs or fatty acid trophic markers (FATMs) are transferred unchanged through the food chain from planktonic microalgae to higher trophic levels (Dalsgaard et al 2003), such as fish, whales, and seals. Dinoflagellates (Dinophyceae) have high proportions of the 22:6(n−3) FA and C18 PUFAs (Graeve et al 1994a, 1994b; Dalsgaard et al 2003) These FATMs are incorporated unchanged in storage and membrane lipids of marine zooplankton and are rapidly transferred through the food web, supplying higher trophic levels with the required energy (Falk-Petersen et al 1990). There are other studies providing information on the transfer of FAs in higher trophic level organisms such as seals and whales (Budge et al 2008; Falk-Petersen et al 2009)

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Conclusion