Effects of changes in isotopic baselines on the evaluation of food web structure using isotopic functional indices.

Simon Belle,Gilbert Cabana

doi:10.7717/peerj.9999

Abstract

BackgroundThis study aimed to assess whether ecological inferences from isotopic functional indices (IFIs) are impacted by changes in isotopic baselines in aquatic food webs. We used sudden CO2-outgassing and associated shifts in DIC-δ13C brought by waterfalls as an excellent natural experimental set-up to quantify impacts of changes in algal isotopic baselines on ecological inferences from IFIs.MethodsCarbon (δ13C) and nitrogen (δ15N) stable isotopic ratios of invertebrate communities sharing similar structure were measured at above- and below-waterfall sampling sites from five rivers and streams in Southern Quebec (Canada). For each sampled invertebrate community, the six Laymans IFIs were then calculated in the δ -space (δ13C vs. δ15N).ResultsAs expected, isotopic functional richness indices, measuring the overall extent of community trophic space, were strongly sensitive to changes in isotopic baselines unlike other IFIs. Indeed, other IFIs were calculated based on the distribution of species within δ-space and were not strongly impacted by changes in the vertical or horizontal distribution of specimens in the δ-space. Our results highlighted that IFIs exhibited different sensitivities to changes in isotopic baselines, leading to potential misinterpretations of IFIs in river studies where isotopic baselines generally show high temporal and spatial variabilities. The identification of isotopic baselines and their associated variability, and the use of independent trophic tracers to identify the actual energy pathways through food webs must be a prerequisite to IFIs-based studies to strengthen the reliability of ecological inferences of food web structural properties.

Highlights

Stable isotopes analysis, mainly those of carbon and nitrogen, of aquatic consumers is a common technique to provide quantitative and qualitative measurements of energy flows in food webs (Cabana & Rasmussen, 1996; Post, 2002; Vander Zanden et al, 2016)
Waterfalls induce CO2-outgassing and associated shifts in dissolved inorganic carbon (DIC)- δ13C values in acidic running waters where carbonate dissolution cannot compensate for the loss of CO2 (Palmer et al, 2001; Doctor et al, 2008)
Waterfalls induced consistent increase in DIC-δ13C values induced by rapid CO2-outgassing (Fig. 1)

Summary

Introduction

Mainly those of carbon and nitrogen, of aquatic consumers is a common technique to provide quantitative and qualitative measurements of energy flows in food webs (Cabana & Rasmussen, 1996; Post, 2002; Vander Zanden et al, 2016). This study aimed to assess whether ecological inferences from isotopic functional indices (IFIs) are impacted by changes in isotopic baselines in aquatic food webs. Isotopic functional richness indices, measuring the overall extent of community trophic space, were strongly sensitive to changes in isotopic baselines unlike other IFIs. other IFIs were calculated based on the distribution of species within δ-space and were not strongly impacted by changes in the vertical or horizontal distribution of specimens in the δ-space. The identification of isotopic baselines and their associated variability, and the use of independent trophic tracers to identify the actual energy pathways through food webs must be a prerequisite to IFIs-based studies to strengthen the reliability of ecological inferences of food web structural properties

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