Permafrost ecohydrology of beaver ponds in Arctic Alaska

Beavers (Castor canadensis) are rapidly colonizing the North American Arctic, transforming aquatic and riparian tundra ecosystems. Arctic tundra may respond differently than temperate regions to beaver engineering due to the presence of permafrost and the paucity of unfrozen water during winter. Here, we provide a detailed investigation of 11 beaver pond complexes across a climatic gradient in Arctic Alaska, addressing questions about the permafrost setting surrounding ponds, the influence of groundwater inputs on beaver colonization and resulting ponds, and the change in surface water and aquatic overwintering habitat. Using field measurements, in-situ dataloggers, and remote sensing, we evaluate permafrost, water quality, pond ice phenology, and physical characteristics of impoundments, and place our findings in the context of pond age, local climate, permafrost setting, and the presence of perennial groundwater inputs. We show beavers are accelerating the effects of climate change by thawing permafrost adjacent to ponds and increasing liquid water during winter. Beavers often exploited groundwater upwellings in discontinuous permafrost, and summertime water temperatures at groundwater-fed (GW) beaver ponds were roughly 5°C lower than sites lacking perennial groundwater inputs (NGW). Late winter liquid water was present at all but a recently abandoned pond complex, although liquid water below seasonal ice cover was shallow (0–82 cm at GW ponds; 0–15 cm at NGW ponds) and ice was thick (median: 83 cm at GW ponds; 120 cm at NGW ponds). Water was less acidic at GW than NGW sites and had higher specific electrical conductivity and more dissolved oxygen. We estimated 3.2 dams/km of stream at sites on the recently-colonized (last ~10 years) Baldwin Peninsula and 8.0 dams/km on the Seward Peninsula, where beavers have been present longer (~20+ years) and groundwater-surface water connectivity is more common. Our study highlights the importance of climatic and physiographic context, especially permafrost presence and groundwater inputs, in determining the characteristics of the Arctic beaver pond environment. As beavers continue their expansion into tundra regions, these characteristics will describe the future of aquatic and riparian Arctic ecosystems.