Thaw-induced impacts on land and water in discontinuous permafrost: A review of the Taiga Plains and Taiga Shield, northwestern Canada

Rising air temperatures, intensifying wildfire activity, and human disturbance are driving rapid permafrost thaw across the subarctic, particularly for thaw-sensitive discontinuous permafrost. The Taiga Plains and Taiga Shield ecozones of northwestern Canada have experienced rapid and widespread permafrost thaw over recent decades, creating significant community concerns and knowledge gaps. In direct response, this review: (1) outlines the observed thaw-induced changes in landcover, hydrology, and water quality; (2) discusses the underlying drivers and mechanisms of these changes; and (3) identifies knowledge gaps to guide future research in the discontinuous permafrost zone of the Taiga Plains and Shield (study region). In the Taiga Plains, permafrost is mainly associated with peatlands where its thaw increases the extent of thermokarst wetlands at the expense of treed peatlands underlain by permafrost. This thaw-induced landcover change enhances the hydrologic connectivity of the landscape, which increases basin-scale runoff and annual streamflow, and enables wetland drainage such that permafrost-free treed wetlands develop. Thaw-induced landcover changes in the lake- and bedrock-dominated Taiga Shield are not well known but are expected to occur as limited or minor thermokarst pond development and changing lake extent due to the low (<5%) peatland coverage of this ecozone. Permafrost thaw also increases the connectivity between surface water and groundwater, leading to increasing winter baseflows and possibly icing (aufeis) development. Such increases in hydrologic connectivity can enhance the mobilization of parameters of concern for water quality, both in the Taiga Plains and Shield. The thawing of peatlands will likely increase the transport and concentrations of dissolved organic carbon and metals bound to organic compounds, including methylmercury. Further work is needed to fully understand the biogeochemical processes operating in these systems and the degree to which thawing peatlands will impact water quality and quantity at the larger basin scale. The greatest knowledge gaps across the study region surround the evolution of thaw-activated groundwater flow systems and the consequences for wetland biogeochemistry, the rates and patterns of permafrost thaw, contaminant transport, and streamflow of larger river systems. This synthesis not only informs future research directions in the study region but extends to similar subarctic peatland and Shield environments common throughout the circumpolar north.