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River aufeis (ow 0 fīse) are widespread features of the arctic cryosphere. They form when river channels become locally restricted by ice, resulting in cycles of water overflow and freezing and the accumulation of ice, with some aufeis attaining areas of $ 25 + km 2 and thicknesses of 6+ m. During winter, unfrozen sediments beneath the insulating ice layer provide perennial groundwater-habitat that is otherwise restricted in regions of continuous permafrost. Our goal was to assess whether aufeis facilitate the occurrence of groundwater invertebrate communities in the Arctic. We focused on a single aufeis ecosystem ($ 5 km 2 by late winter) along the Kuparuk River in arctic Alaska. Subsurface invertebrates were sampled during June and August 2017 from 50 3.5-cm diameter PVC wells arranged in a 5 × 10 array covering $ 40 ha. Surface invertebrates were sampled using a quadrat approach. We documented a rich assemblage of groundwater invertebrates (49 [43-54] taxa, X [95% confidence limits]) that was distributed below the sediment surface to a mean depth of $ 69 ± 2 cm (X ± 1 SE) throughout the entire well array. Although community structure differed significantly between groundwater and surface habitats, the taxa richness from wells and surface sediments (43 [35-48] taxa) did not differ significantly, which was surprising given lower richness in subsurface habitats of large, riverine gravel-aquifer systems shown elsewhere. This is the first demonstration of a rich and spatially extensive groundwater fauna in a region of continuous permafrost. Given the geographic extent of aufeis fields, localized groundwater-dependent ecosystems may be widespread in the Arctic.

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