Effects of shoreline permafrost thaw on nutrient dynamics and diatom ecology in a subarctic tundra pond

In permafrost landscapes, ponds and lakes are ubiquitous and important features that support long-term carbon storage and diverse diatom communities. With climate warming, thermokarst lake expansion may modify the ability of these aquatic systems to maintain these functions, as a consequence of changes to nutrient inputs and cycling. A recent (2006–2008) thermokarst shoreline expansion event adjacent to small Frisbee Pond, in the Hudson Bay Lowlands, Canada, presented an opportunity to evaluate the effects of thaw-driven disturbance on pond nutrient dynamics. A sediment core was collected in 2015 and analysed for diatom assemblage composition, total nutrient (carbon, nitrogen and phosphorus) accumulation rates, and stable isotopes of organic carbon (δ13C) and nitrogen (δ15N). Results indicate that recent shoreline expansion in this subarctic peatland pond caused an increase in total diatom accumulation, although shifts in diatom community composition were initiated prior to the shoreline slump, particularly an increase in the relative abundance of Denticula kuetzingii. Nutrient accumulation rates in the sediment increased dramatically both prior to and following shoreline slumping, with isotope values indicating an increased supply of allochthonous carbon and atmospheric nitrogen. Investigations of hydroclimate conditions prior to the shoreline slump indicate warming of summer air temperatures and an increase in precipitation over the long-term mean, consistent with changing climate conditions that caused thermokarst disturbance in other permafrost environments. This study has important implications for understanding hydro-ecological responses of small surface water bodies to general climate forcing, in combination with rapid thermokarst shoreline expansion, under a changing climate.