Challenges for interpreting stable isotope fractionation of carbon and nitrogen in tropical aquatic ecosystems

Summary Stable isotopes are useful for elucidating food webs, and oneessentialaspectofinterpretationisaccuratelydeterminingtheenrichment between trophic levels, especially when used inmixing models. The fractionation of the stable isotopes ofnitrogen(Δδ 15 N)andcarbon(Δδ 3 C)betweentrophiclevelsintropicalaquaticecosystemsseemstodiffercomparedtotypicalvalues found in temperate aquatic ecosystems of about 3.4‰forδ 15 Nand0.5‰forδ 13 C.Inrecentstudiesofuplandstreamecosystems in Panama, with and without tadpoles, we foundlower fractionation of δ 15 N, typically 1.0–1.7‰, and muchhigherfractionationofδ 13 C,typically1–1.6‰.Similarvalueshavebeenobservedinothertropicalsystems,includingPuertoRicoandCostaRicastreamsandLakeTanganyika.Theselargedifferencesintrophicfractionationareenigmatic.Weexploresources of variation in fractionation such as tissue turnoverrate,streamflow,taxonomicdifferences,functionalgroupdif-ferences, dietary balance, growth rate, and microbial activityto assess possible contributing factors. The ecological infor-mation embedded in this variation in trophic fractionationshould be embraced and exploited.