Species identity matters when interpreting trophic markers in aquatic food webs

Abstract

In aquatic systems, food web linkages are often assessed using diet contents, stable isotope ratios, and, increasingly, fatty acid composition of organisms. Some correlations between different trophic metrics are assumed to be well-supported; for example, particular stable isotope ratios and fatty acids seem to reflect reliance on benthic or pelagic energy pathways. However, understanding whether the assumed correlations between different trophic metrics are coherent and consistent across species represents a key step toward their effective use in food web studies. To assess links among trophic markers, we compared relationships between major diet components, fatty acids, and stable isotope ratios in three fishes: yellow perch (Perca flavescens), round goby (Neogobius melanostomus), and spottail shiner (Notropis hudsonius) collected from nearshore Lake Michigan. Yellow perch and spottail shiner are native in this system, while round goby are a relatively recent invader. We found some evidence for agreement between different trophic metrics, especially between diet components, n-3:n-6 fatty acid ratios, and stable isotope ratios (δ13C and δ15N). However, we also observed significant variation in observed relationships among markers and species, potentially due to taxonomic variation in the specific diet items consumed (e.g., chydorid microcrustaceans and Dreissena mussels) and species-specific biochemical processes. In many of these latter cases, the invasive species differed from the native species. Understanding the effects of taxonomic variation on prey and predator signatures could significantly improve the usefulness of fatty acids in food web studies, whereas diet contents and stable isotopes appear to be reliable indicators of trophic niche in aquatic food webs.

Bidirectional energy subsidies for fish in river mouths of a large lake as revealed by stable isotopes and fatty acids

“…The relative concentrations of different fatty acids varied by species and generally agreed with other Lake Michigan studies (Czesny et al 2011; Feiner et al 2018). Importantly, across species, fatty acid concentrations, especially PUFAs, and DHA, varied by habitats.…”

“…Muscle lipids were transesterified to fatty acid methyl esters (FAMEs) with methanol and a 3% concentrated sulfuric acid solution (Budge et al 2006). Overall, 43 individual FAMEs were detected using a Shimadzu gas chromatograph model GC-2010 with flame ionization detector and autoinjector with EZStart software version 7.4 (Shimadzu Corp.; Feiner et al 2018). The proportions of individual fatty acids were expressed as a mass percentage of the total fatty acid content.…”

Individual and spatial variation are as important as species‐level variation to the trophic complexity of a lentic food web

“…We developed a standardised set of 17 prey items from the taxa present in the guts of both species for use in comparing between‐species patterns in gut content diversity and composition (Table S1). We classified prey items in terms of taxonomic (primarily family or order), functional (e.g., trophic position, metabolism) and behavioural (e.g., feeding, habitat use, position in the water column) traits as accurately as possible given limitations in identifying often partially digested organisms (Czesny et al, ; Feiner, Foley et al, ). For example, we distinguished between Chironomidae larvae and pupae because of evidence that metabolic processes during metamorphosis lead to distinct fatty acid and isotopic signatures in each (Doi, Kikuchi, Takagi, & Shikano, ; Hanson, Cummins, Cargill, & Lowry, ).…”

“…Darkened area represents the extent of Lake Michigan watershed the guts of both species for use in comparing between-species patterns in gut content diversity and composition (Table S1). We classified prey items in terms of taxonomic (primarily family or order), functional (e.g., trophic position, metabolism) and behavioural (e.g., feeding, habitat use, position in the water column) traits as accurately as possible given limitations in identifying often partially digested organisms (Czesny et al, 2011;Feiner, Foley et al, 2018a).…”

State of art and best practices for fatty acid analysis in aquatic sciences

“…When doing so, it is important to remember that there are no ubiquitous FA biomarkers and that their applicability will depend on the studied system or organism (e.g. Feiner et al, 2018). To avoid interpretation biases, biomarker compounds should be chosen carefully with respect to the ecological context and scientific question of interest.…”