Michael P. Carey Joshua C. Koch Xiaomei Xu Brett Poulin Jennifer Walker Christian E. Zimmerman Jonathon A O'Donnell 2020 <p><span>Permafrost thaw in the Arctic is mobilizing old carbon (C) from soils to aquatic ecosystems and the atmosphere. Little is known, however, about the assimilation of old C by aquatic food webs in Arctic watersheds. Here, we used C isotopes (δ</span>¹³<span>C, Δ</span>¹⁴<span>C) to quantify C assimilation by biota across 12 streams in arctic Alaska. Streams spanned watersheds with varying permafrost hydrology, from ice-poor bedrock to ice-rich loess (that is, yedoma). We measured isotopic content of (1) C sources including dissolved organic C (DOC), dissolved inorganic C (DIC), and soil C, and (2) stream biota, including benthic biofilm and macroinvertebrates, and resident fish species (Arctic Grayling (</span><i>Thymallus arcticus</i><span>) and Dolly Varden (</span><i>Salvelinus malma</i><span>)). Findings document the assimilation of old C by stream biota, with depleted Δ</span>¹⁴<span>C values observed at multiple trophic levels, including benthic biofilm (</span>¹⁴<span>C ages = 5255 to 265&nbsp;years before present (y BP)), macroinvertebrates (4490 y BP to modern), and fish (3195 y BP to modern). Mixing model results indicate that DOC and DIC contribute to benthic biofilm composition, with relative contributions differing across streams draining ice-poor and ice-rich terrain. DOC originates primarily from old terrestrial C sources, including deep peat horizons (39–47%; 530 y BP) and near-surface permafrost (12–19%; 5490 y BP). DOC also accounts for approximately half of fish isotopic composition. Analyses suggest that as the contribution of old C to fish increases, fish growth and nutritional status decline. We anticipate increases in old DOC delivery to streams under projected warming, which may further alter food web function in Arctic watersheds.</span></p> application/pdf 10.1007/s10021-019-00413-6 en Springer Permafrost hydrology drives the assimilation of old carbon by stream food webs in the Arctic article