Magnesium and zinc stable isotopes as a new tool to understand Mg and Zn sources in stream food webs

Abstract

Non‐traditional stable isotopes of metals were recently shown as new dietary tracers in terrestrial and marine mammals. Whether these metal stable isotopes can be used to understand feeding habits in stream food webs is not known yet. In this study, we explored the potential of stable isotopes of essential Mg (δ26Mg) and Zn (δ66Zn) as a new tool in stream ecology. For this purpose, we determined δ26Mg and δ66Zn values of stream organisms and their potential metal sources in upper and lower reaches of two streams in the Lake Biwa catchment, Central Japan. Our goals were (1) to explore variations in δ26Mg and δ66Zn across organisms of different feeding habits and (2) to understand Mg and Zn sources to stream organisms. Overall, δ26Mg and δ66Zn values of organisms were neither related to each other, nor to δ13C and δ15N values, indicating different elemental sources and factors controlling isotopic fractionation depending on element and taxa. Low δ26Mg values in filter‐feeding caddisfly larvae and small gobies indicated aqueous Mg uptake. Higher δ26Mg values in leaf‐shredding crane fly and grazing mayfly larvae suggested Mg isotopic fractionation during Mg uptake from the diet. While the δ26Mg values of stonefly nymphs reflected those of caddisfly larvae as a potential prey, the highest δ26Mg values found in dobsonfly nymphs can be explained by 26Mg enrichment during maturing. δ66Zn values of caddisfly and mayfly larvae indicated Zn was a mixture of aqueous and dietary available Zn, while higher δ66Zn values in crane fly larvae pointed to Zn isotopic fractionation during Zn uptake from plant litter. δ66Zn values in stonefly and dobsonfly nymphs were often in the range of those of caddisfly larvae as their prey, while dragonfly nymphs and small goby were depleted in 66Zn relative to their dietary Zn sources. We conclude that δ26Mg is a promising indicator to assess Mg sources in stream ecology depending on taxa, while the use of δ66Zn is limited due to the complexity in Zn sources.

Accumulation of trace metals in freshwater macroinvertebrates across metal contamination gradients

“…The predator Rhyacophila accumulated the lowest median trace metal body burdens of the insect taxa (Cain et al, 2004;Kiffney and Clements, 1993;Liess et al, 2017;Rodriguez et al, 2018;Smock, 1983a) whilst the detritivores Leuctra and Hydropsyche generally accumulated the highest median body burdens. We found median body burdens of Cd and Zn were highest in Baetis, a result consistent with the findings of others (Burrows and Whitton, 1983;Kiffney and Clements, 1993;Nitzsche et al, 2020;Timmermans et al, 1989). Our data suggest the accumulation of trace metals by a predator from prey (Rhyacophila) were less than accumulations of metals by biota predominantly consuming basal resources (Baetis, Leuctra and Hydropsyche).…”

A Neandertal dietary conundrum: Insights provided by tooth enamel Zn isotopes from Gabasa, Spain

“… Cannibalism or hypercarnivorism: Cannibalism or carnivore meat consumption could be associated with low Zn isotope ratios, but the absence of cut marks on hominin and carnivore bones rules out this hypothesis. Food with unusually low δ 66 Zn values: Archeological evidence for some isotopically light foods that Neandertals might have specifically targeted may be missing [e.g., animal livers ( 53 , 77 , 78 ), birds, mushrooms, fruits, and leaves ( 79 – 81 ), insects ( 82 , 83 ), or aquatic resources ( 43 )]. However, δ 66 Zn enamel of the consumers of those foods has been measured and is not associated with lower values than that of sympatric carnivores, both in Gabasa (e.g., lynx are known to consume liver, deer consume leaves, while foxes, bears, and rabbits have been known to consume mushrooms) and other food webs [apes and tapirs for insects and fruits ( 36 )].…”

Metals and metal isotopes in insect wings: Implications for diet, geolocation and pollution exposure

“…2019), illustrating the large knowledge gap that the newly-developing field of metallomics is seeking to fill. Metal isotopes that fractionate with physiological processes can be particularly useful to advance understanding but have been under-utilized in insects thus far (but see Nitzsche et al . 2020, 2022).…”