Influence of Wildfire on Downstream Transport of Dissolved Carbon, Nutrients, and Mercury in the Permafrost Zone of Boreal Western Canada

Northern regions are undergoing rapid change with wildfires increasing in frequency and severity alongside thawing permafrost and altered water balance. These disturbances could cause significant change in the export of carbon, nutrients, and metals to aquatic systems, with implications for food webs and ecosystem processes. Here, we examine chemical data from a series of 52 streams and rivers that were sampled across a 250,000 km2 expanse of the Taiga Plains and Taiga Shield ecozones of the Northwest Territories, Canada. Samples were collected immediately after and for 3 years following a "megafire" that occurred in this region in 2014, and included wildfire-affected and non-affected catchments. While wildfire has been observed to cause significant impacts on water quality in other regions, we here report weak relationships with percent watershed burn with minor to moderate effect sizes, the greatest being a reduction in dissolved organic carbon (-32% concentration). Watershed-specific properties were a strong driver of large spatial variability in stream water chemistry, which may overwhelm or obscure lesser wildfire effects. The watershed chemical yield-specific response to wildfire was weaker than the response for concentrations, due to substantial variation and uncertainty in runoff among sites and years. This suggests that watershed chemical yields in this region are more sensitive to changes in water balance due to climate than to altered wildfire regimes. Our study examines how wildfires affect the chemistry of water in northern regions. We collected data from 52 streams and rivers in the Taiga Plains and Taiga Shield ecozones of Canada. The results show that the effects of wildfires on water quality are relatively small. Other factors, such as climate and the presence of wetlands, have a much stronger influence on water chemistry in each ecozone. Understanding these changes in water chemistry is crucial for managing aquatic ecosystems. However, we found that the impact of wildfires on water quality in this region differs from that in other locations. The variability in runoff among different sites and years also adds uncertainty to our findings. Looking ahead, climate change and altered hydrology may have more substantial impacts on fluvial networks compared to wildfires in this rapidly changing region. Our research provides valuable insights into the relative importance of various landscape factors in regulating water quality, including the effects of wildfires. By examining the chemical changes occurring due to wildfires and considering the broader environmental context, we contribute to a better understanding of how these factors influence water quality in northern regions. Wildfires in the Taiga regions of Canada had modest effects on water chemistry, with the largest impact being a reduction in DOC (-32%)Variability in stream water chemistry was driven by watershed-specific properties overshadowing the influence of wildfiresFuture impacts on streams are likely more substantially driven by climate and altered hydrology than increasing wildfire frequency