Two decades after the fire: Impacts on benthic macroinvertebrate communities evident 21 years after wildfire in the south-west Rocky Mountains of Alberta, Canada

Wildfire is a key feature of the disturbance regime in Canada’s Rocky Mountain headwaters, and can produce lasting impacts on water quality and stream ecology. Wildfire disturbance can impact aquatic habitats in a variety of ways, including post-fire shifts in the nutrient balance of aquatic systems, which can drive cascading impacts on plant, macroinvertebrate, and fish communities. These long-term impacts are driven by persistent post-disturbance delivery of fine-grained sediments and associated limiting nutrients, such as phosphorus (P), that regulate the ecology of oligotrophic streams in this region. The post-fire recovery of aquatic ecosystems is widely studied, but observations of impacts on macroinvertebrate communities lasting greater than a decade are uncommon.

Here we describe the lasting impact of the severe 2003 Lost Creek wildfire on benthic macroinvertebrate communities in 3 burned and 2 unburned (reference) catchments. Fire impacts to macroinvertebrate communities were assessed by comparing the abundance of key disturbance sensitive taxa (EPT; Ephemeroptera, Plecoptera, Trichoptera) relative to disturbance tolerant taxa (Diptera) as an index (EPT:D). Benthic macroinvertebrate samples were collected from burned and reference catchments in fall 2024 (21 years after the wildfire) and compared to similar sampling conducted in the same catchments in 2007 (4 years after the wildfire), and 2011 (8 years after the wildfire).

Wildfire impacts to mean EPT:D ratios were greatest 4 years after the fire (10.5 and 111.4 in burned and reference catchments, respectively; p=0.046), and remained significant 8 years later (3.1 and 6.6 in burned and reference catchments, respectively; p=0.019). Most notably, wildfire impacts to mean EPT:D ratios were still clearly evident 21 years after the wildfire (0.4 and 1.8 in burned and reference catchments, respectively; p=0.010).

Mean EPT:D ratios in severely burned catchments remained significantly lower than in unburned reference watersheds, showing that wildfires can have ecologically significant impacts on stream macroinvertebrate community composition lasting beyond two decades. These findings show notably longer lasting impacts of wildfire on aquatic ecosystems than previous studies; this is likely due to the glacial history of the area, which provides a high availability of fine-grained sediment capable of storing/transporting sediment-associated nutrients (such as P), in comparison to other mountainous regions. With the severity and frequency of fires increasing with climate change, evaluating both the magnitude and longevity of the impacts from wildfire on aquatic ecosystems across fire-prone regions is important in assessing the broader regional threats of climate driven changes in wildfire regimes.