Deuterium as a food source tracer: Sensitivity to environmental water, lipid content, and hydrogen exchange

Hydrogen stable isotopes (dH) are used for quantifying resources supporting food webs. However, application of dH in mixing models requires; (1) correction for environmental water (x) in consumer tissues, (2) consideration of differential fractionation among biochemical constituents, and (3) consideration of differential H-exchange among samples and standards. We present data and sensitivity analyses addressing each of these issues and provide recommendations for future isotope food web studies. First, we determined from field data that maximum x for aquatic consumers averaged 0.23 6 0.03, similar to the median x from a survey of published values (0.22 6 0.02). Resource use estimates based solely on dH data were sensitive to the selected x value. Second, to quantify the potential bias in bulk tissue analysis from differential tissue fractionation, we calculated the change in whole organism dH before and after lipid extraction for 61 aquatic samples. The average change in consumers’ dH after lipid extraction was a positive shift of 11.8& relative to the pre-extraction value. This shift resulted in a minor change in resource use estimates when correcting for lipids. Finally, we evaluated the impact of correcting for H-exchange in samples using standards with dissimilar H-exchange portions. The impact of the correction factor for H-exchange on resource use estimates could be large if suitable standards are not used for comparison. From these analyses we conclude that despite these complicating factors, analysis of resource use is possible using whole organisms’ dH, especially in combination with cautionary sensitivity analysis. Recently, hydrogen isotope ratios (dH) have been used to quantify trophic resources in marine and freshwater consumers (e.g., Bortolotti et al. 2014; Berggren et al. 2014; Hondula et al. 2014). One of the key applications of hydrogen (H) isotopes in aquatic ecosystems has been to assess the degree to which aquatic consumers use organic matter from the surrounding watershed, known as allochthony (e.g., Cole et al. 2011; Karlsson et al. 2012; Kelly et al. 2014). There is a large separation in dH values between allochthonous and autochthonous organic matter (material originating within the system) as well as among some aquatic primary producers such as different species of macroalgae and macrophytes (Hondula et al. 2014). This large separation in end members (organic matter sources) allows for more robust estimates of consumer resource use. However, applying H isotopes to estimate consumer resources requires: (1) correcting for environmental water dH in consumer tissues, (2) evaluating the influence of differential fractionation among biochemical constituents used for sources and end-members due largely to varying lipid content, and (3) properly accounting for H-exchange during laboratory analysis. Environmental water, sometimes referred to as “dietary water,” is the H in a consumer’s tissue that is derived directly from water in the surrounding aquatic environment and not from food resources (Hobson et al. 1999; Solomon et al. 2009). Incorporation of environmental water into consumer tissues is useful for applications such as reconstructing animal and human migrations using dH precipitation gradients (Hobson et al. 2004; O’Grady et al. 2012). For studies of aquatic consumer resource use dietary water is problematic. The environmental water correction (x), formalized by Solomon et al. (2009), varies by consumer and increases with increasing trophic level (Birchall et al. 2005). The influence of environmental water on consumer dH is calculated as a mixing model dHConsumer5ðxtot3dH2OÞ1 ! ð12xtotÞ3dHResources " (1) where xtot is the trophically compounded value of x (x 5 xtot for primary consumers), the environmental water correction parameter (bounded between 0 and 1), and dHConsumer, dH2O, and d HResources are the H isotope ratio values of the consumer, water, and end members, respectively (Solomon *Correspondence: [email protected] 213 LIMNOLOGY and OCEANOGRAPHY: METHODS Limnol. Oceanogr.: Methods 13, 2015, 213–223 VC 2015 Association for the Sciences of Limnology and Oceanography doi: 10.1002/lom3.10019