Wildfires can be either a threat or a boon to freshwater biodiversity and ecosystem function, and understanding the complex drivers leading to these contradictory outcomes is needed to support more resilient ecosystems. One prominent way fire affects freshwater ecosystems is the “fire pulse,” or a post-fire increase in bottom-up productivity driven by increased nutrients and light availability. Decades of research show that the fire pulse can be spatially and temporally variable and inconsistently propagate through food webs (e.g., periphyton-invertebrates-fish or phytoplankton-zooplankton). This has been linked to variation in 1) fire severity; 2) changes in light following the loss and regeneration of riparian vegetation; 3) nutrient and light-limiting ash inputs; and 4) community traits and interactions that direct energy flow. Spatial and temporal variation in the fire pulse raises the possibility that contradictory responses among patches may contribute to a dynamic portfolio of ecological responses across the riverscape. Theory suggests that such a portfolio may increase the resiliency of affected ecosystems.
We evaluated this possibility by leveraging three studies with 70 years of monitoring from the Bitterroot River watershed (MT; 15 years post-fire), Big Creek watershed (ID; 37 years encompassing multiple fires), and Three Lakes reservoir-complex (CO; 18 years with 5 post-fire). The stream studies evaluated changes in habitat, aquatic invertebrates, fish-centric food webs, and aquatic-terrestrial linkages, and the lake study considered nutrient inputs and responses by plankton.
Collectively, the fire pulse was spatially and temporally variable, and the affected foodweb pathways and ecosystem functions exhibited diverse responses. In the Bitterroot River watershed, fire was associated with elevated invertebrate drift and caloric content of fish gut contents for most sites, but experienced limited increases in aquatic insect emergence. Across the Big Creek watershed, chlorophyl-a and benthic invertebrate biomass were elevated post-fire, but the length of this pulse ranged from a few years to over a decade. The Three Lakes reservoir-complex experienced nutrient fluxes, but the pulse inconsistently propagated to phytoplankton and zooplankton. Spatial and temporal dynamics in the fire pulse contributed to a portfolio of post-fire responses, which was associated with the maintenance of biodiversity and ecosystem function across the riverscapes.