Understanding river food webs requires distinguishing energy derived from primary production in the river itself (autochthonous) from that produced externally (allochthonous), yet there are no universally applicable and reliable techniques for doing so. We compared the natural abundance stable isotope ratios of hydrogen (deltaD) of allochthonous and autochthonous energy sources in four different aquatic ecosystems. We found that autochthonous organic matter is uniformly far more depleted in deuterium (lower deltaD values) than allochthonous: an average difference of approximately 100% per hundred. We also found that organisms at higher trophic levels, including both aquatic invertebrates and fish, have deltaD values intermediate between aquatic algae and terrestrial plants. The consistent differences between leaves and algae in deltaD among these four watersheds, along with the intermediate values in higher trophic levels, indicate that natural abundance hydrogen isotope signatures are a powerful tool for partitioning energy flow in aquatic ecosystems.
Aquatic food webs are subsidized by allochthonous resources but the utilization of these resources by consumers can be difficult to quantify. Stable isotope ratios of hydrogen (deuterium:hydrogen; deltaD) potentially distinguish allochthonous inputs because deltaD differs between terrestrial and aquatic primary producers. However, application of this tracer is limited by uncertainties regarding the trophic fractionation of deltaD and the contributions of H from environmental water (often called "dietary water") to consumer tissue H. We addressed these uncertainties using laboratory experiments, field observations, modeling, and a literature synthesis. Laboratory experiments that manipulated the deltaD of water and food for insects, cladoceran zooplankton, and fishes provided strong evidence that trophic fractionation of deltaD was negligible. The proportion of tissue H derived from environmental water was substantial yet variable among studies; estimates of this proportion, inclusive of lab, field, and literature data, ranged from 0 to 0.39 (mean 0.17 +/- 0.12 SD). There is a clear need for additional studies of environmental water. Accounting for environmental water in mixing models changes estimates of resource use, although simulations suggest that uncertainty about the environmental water contribution does not substantially increase the uncertainty in estimates of resource use. As long as this uncertainty is accounted for, deltaD may be a powerful tool for estimating resource use in food webs.
The study reveals that for future research on N2O isotopocules, standardisation against N2O reference material is essential to improve interlaboratory compatibility. For atmospheric monitoring activities, we suggest N2O in whole air as a unifying scale anchor.
Multiple‐stable‐isotope analysis was used to infer anadromous and nonanadromous origins of adult brook trout Salvelinus fontinalis and maternal migration history of age‐0 progeny in the Tabusintac River, New Brunswick. Forty‐seven adults collected above head of tide displayed deviations (δ) from standard ratios of 13C/12C, 15N/14N, and 34S/32S of −30.3‰ to −16.0‰, 7.4‰ to 16.8‰, and 1.5‰ to 14.1‰, respectively; higher values (positive or less negative) denote relatively greater enrichment in the heavier isotope. Isotopically enriched brook trout exhibited isotope profiles typical of fish from marine environments, and those that were isotopically depleted were considered to be of freshwater origin. Age‐0 brook trout from Home Camp Pool, the most downstream freshwater rearing site sampled, were more enriched (δ13C = −24.4 ± 2.7‰ (mean ± SD), δ15N = 12.5 ± 3.1‰, and δ34S = 7.4 ± 1.8‰) than those at two other sites, and were believed to be progeny of anadromous females. Age‐0 brook trout from the Bathurst Highway site (δ13C = −27.9 ± 0.6‰, δ15N = 8.9 ± 0.8‰, and δ34S = 5.8 ± 0.8‰) and the Little Eskedelloc River (δ13C =−28.6 ± 0.5‰, δ15N = 8.1 ± 0.4‰, and δ34S = 2.9 ± 0.8‰), with less isotopic enrichment, were most likely from nonanadromous parents. Stable‐isotope ratios varied with fork length; at Home Camp Pool, this relationship was thought to represent an “isotope dilution factor” as recently emerged juveniles assimilated new food from freshwater, grew, and masked the marine signatures of their maternal parents. This study suggests that stable‐isotope ratios may be used to distinguish between sympatric anadromous and nonanadromous adult brook trout and their progeny as long as brook trout are collected before they dilute their maternal isotope signatures.
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