Using hydrogen isotopes of freshwater fish tissue as a tracer of provenance

Abstract

Hydrogen isotope (δ2H) measurements of consumer tissues in aquatic food webs are useful tracers of diet and provenance and may be combined with δ13C and δ15N analyses to evaluate complex trophic relationships in aquatic systems. However, δ2H measurements of organic tissues are complicated by analytical issues (e.g., H exchangeability, lack of matrix‐equivalent calibration standards, and lipid effects) and physiological mechanisms, such as H isotopic exchange with ambient water during protein synthesis and the influence of metabolic water. In this study, δ2H (and δ15N) values were obtained from fish muscle samples from Lake Winnipeg, Canada, 2007–2010, and were assessed for the effects of species, feeding habits, and ambient water δ2H values. After lipid removal, we used comparative equilibration to calibrate muscle δ2H values to nonexchangeable δ2H equivalents and controlled for H isotopic exchange between sample and laboratory ambient water vapor. We then examined the data for evidence of trophic δ2H enrichment by comparing δ2H values with δ15N values. Our results showed a significant logarithmic correlation between fork length and δ2H values, and no strong relationships between δ15N and δ2H. This suggests the so‐called apparent trophic compounding effect and the influence of metabolic water into tissue H were the potential mechanisms for δ2H enrichment. We evaluated the importance of water in controlling δ2H values of fish tissues and, consequently, the potential of H isotopes as a tracer of provenance by taking account of confounding variables such as body size and trophic effects. The δ2H values of fish appear to be a good tracer for tracking provenance, and we present a protocol for the use of H isotopes in aquatic ecosystems, which should be applicable to a broad range of marine and freshwater fish species. We advise assessing size effects or working with fish of relatively similar mass when inferring fish movements using δ2H measurements.

Terrestrial contributions to Afrotropical aquatic food webs: The Congo River case

“…The range of δ 2 H values for consumers was also large with, for instance, fish having hydrogen isotope values mainly from −80‰ to −20‰ (one value around −90‰). No correlation was found between fish size and δ 2 H values because the standard length of most sampled fish in the mainstream Congo River in 2012 was within a relatively small range between 40 and 150 mm (Figure ), which avoided any potential effects of size when using δ 2 H (Soto, Hobson, & Wassenaar, ). Nonetheless, dietary habits and trophic level had a significant effect on δ 2 H and δ 15 N values (ANOVA, p < .01) for fish in the 2012 mainstream sampling (Figure ).…”

“…The higher presence of algal contribution to herbivorous species could be associated to the ingestion of periphyton attached to plants. Alternative causes to this fish δ 2 H variation could be the substantial contribution of aquatic plants into consumer diet (Syväranta, Scharnweber, Brauns, Hilt, & Mehner, ) and/or previously described size effects (Soto et al, ; Soto, Wassenaar, Hobson, & Catalan, ). Both explanations were discarded because firstly the abundance of aquatic plants was very low in the system and secondly the range of fish size classes sampled was also small.…”

“…For terrestrial invertebrates, isotope values ranged from −32.0‰ to −11.4‰ for δ 13 C and 3.7‰ to 15.2‰ for F I G U R E 2 Biplots of the stable isotopic composition of C, N, and H of primary producers and main consumers (invertebrates and fish) from the mainstream Congo River in 2012. All isotope values are raw data without removing the effect of water contribution (H) or without applying any C and N trophic discrimination F I G U R E 3 Boxplots of the stable isotopic composition of C, N, and H of fish specimens grouped by dietary habits and trophic level (see Table S1 for details on the fish species involved in each group) and 150 mm ( Figure S3), which avoided any potential effects of size when using δ 2 H (Soto, Hobson, & Wassenaar, 2016). Nonetheless, dietary habits and trophic level had a significant effect on δ 2 H and δ 15 N values (ANOVA, p < .01) for fish in the 2012 mainstream sampling ( Figure 3).…”

An assessment of assumptions and uncertainty in deuterium‐based estimates of terrestrial subsidies to aquatic consumers

“…, Soto et al. ). However, studies vary considerably in how they characterize the trophic levels of different consumers.…”

“…, Soto et al. ), and the outcome of hydrogen isotope‐based analyses can therefore be strongly affected by how consumer trophic levels are assigned. The equation above also assumes that ω is constant across all trophic levels, whereas the data compiled in Table suggests ω could be quite variable from one consumer or system to another.…”