Abstract
The measurement of stable carbon (δ13C) and nitrogen (δ15N) isotopes in tissues of organisms has formed the foundation of isotopic food web reconstructions, as these values directly reflect assimilated diet. In contrast, stable hydrogen (δ2H) and oxygen (δ18O) isotope measurements have typically been reserved for studies of migratory origin and paleoclimate reconstruction based on systematic relationships between organismal tissue and local environmental water. Recently, innovative applications using δ2H and, to a lesser extent, δ18O values have demonstrated potential for these elements to provide novel insights in modern food web studies. We explore the advantages and challenges associated with three applications of δ2H and δ18O values in food web studies. First, large δ2H differences between aquatic and terrestrial ecosystem end members can permit the quantification of energy inputs and nutrient fluxes between these two sources, with potential applications for determining allochthonous vs. autochthonous nutrient sources in freshwater systems and relative aquatic habitat utilization by terrestrial organisms. Next, some studies have identified a relationship between δ2H values and trophic position, which suggests that this marker may serve as a trophic indicator, in addition to the more commonly used δ15N values. Finally, coupled measurements of δ2H and δ18O values are increasing as a result of reduced analytical challenges to measure both simultaneously and may provide additional ecological information over single element measurements. In some organisms, the isotopic ratios of these two elements are tightly coupled, whereas the isotopic disequilibrium in other organisms may offer insight into the diet and physiology of individuals. Although a coherent framework for interpreting δ2H and δ18O data in the context of food web studies is emerging, many fundamental uncertainties remain. We highlight directions for targeted research that will increase our understanding of how these markers move through food webs and reflect ecological processes.
Highlights
The application of stable isotope measurements of both the organic and inorganic components of food webs has had a long and impressive history, and such measurements are an indispensable tool for elucidating food web structure, nutrient and contaminant flows, and the foraging ecology of individuals and populations (Peterson and Fry, 1987; Rundel et al, 1989; Lajtha and Michener, 1994; Fry, 2006; Michener and Lajtha, 2007; Martínez del Rio et al, 2009; Boecklen et al, 2011)
With massive environmental perturbations that characterize the current (Anthropocene) era, there will be a commensurate increase in the use and need for refinement of isotopic tools to investigate pressing environmental issues (Dawson and Seigwolf, 2007). This field has relied almost exclusively on the stable isotopes of relatively few elements, with the overwhelming contributions coming from the use of δ13C and δ15N measurements and to a lesser extent those of δ34S (Krouse and Grineko, 1971). This focus on C, N, and S derives from the fact that these elements are major components of animal and plant tissues that are sourced nearly exclusively from diet, their stable isotopes are ideal indicators of sources of primary production (e.g., δ13C, δ34S) or trophic position (δ15N)
As research using δ2H and δ18O values in food web ecology continues to expand, we expect that controlled studies to explore the mechanisms and physiological processes affecting the isotopic values of H and O during the integration and synthesis of tissues will contribute to advances in interpreting isotopic data (Figure 1)
Summary
The application of stable isotope measurements of both the organic and inorganic components of food webs has had a long and impressive history, and such measurements are an indispensable tool for elucidating food web structure, nutrient and contaminant flows, and the foraging ecology of individuals and populations (Peterson and Fry, 1987; Rundel et al, 1989; Lajtha and Michener, 1994; Fry, 2006; Michener and Lajtha, 2007; Martínez del Rio et al, 2009; Boecklen et al, 2011). A large fraction of H atoms are bonded directly to the Cskeleton of most biomolecules, whereas most O is found within functional groups, and theoretical assessments and experimental data suggest that atomic routing from diet to consumer tissues is in general much stronger for H than for O (Ehleringer et al, 2008; Bowen et al, 2009; Wang et al, 2009; Nielson and Bowen, 2010; Wolf et al, 2011; Soto et al, 2013c) This means that in most cases the expectation will be that H isotopes will carry a “purer” dietary signature and O will bear a stronger reflection of consumer physiological water balance.
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