Wildfire effects on stream food webs and nutrient dynamics in Glacier National Park, USA

Abstract

We documented immediate and mid-term (5 years) impacts on streams from a large (15,500 ha) wildfire in northwestern Montana. Fire-related impacts were ecosystem-wide, extending from water chemistry to fish. During the initial firestorm, phosphorus and nitrogen levels increased 5-to 60-fold above background levels resulting from aerial deposition from smoke and ash. Nutrients returned to background concentrations within several weeks after the fire. During subsequent years, nutrient concentrations periodically increased in fire-impacted sites compared to reference sites, especially during spring run-off.Evidence of post-fire changes was also documented in the aquatic food web via stable isotope analyses. Macroinvertebrates and fish from fire-impacted sites were significantly more enriched in 15 N and depleted in 13 C than consumers from forested reference sites (P < 0:001). The post-fire isotopic shift in consumers was consistent with increased utilization of algae and/or other autochthonous food sources together with decreased reliance on terrestrial leaf litter and other allochthonous food sources. Such a post-fire shift from a detritus based on a periphyton-based food web fits predictions of the river continuum concept following canopy removal and nutrient enrichment.Following decades of active fire suppression, forest managers are now contemplating aggressive efforts to reduce the fuel build-up noted in forests throughout the western US. Such efforts could involve increased use of fire and mechanical thinning and harvest. Results from our work and others suggest that expanded fire activity could mobilize substantial quantities of highly available nutrients to lakes and streams. With significant nutrient delivery mechanisms involving water, as well as airborne transport via smoke and ash, the potential for increased nutrient loadings to surface waters could extend well beyond the catchment of any particular fire. As natural resource managers contemplate expanding the use of fire as a forest restoration tool, they face the dilemma that such efforts could run counter to a decades-long effort to reduce nutrient loadings to lakes and other surface waters threatened by eutrophication. #

Forest fire increases mercury accumulation by fishes via food web restructuring and increased mercury inputs

“…In North America, forest fire has been suppressed for Ͼ100 years; however, prescribed burning has recently become a popular counteractive management option (23). Prescribed burning (17,23) and climate change are expected to increase forest fire occurrence (24,25). Our study demonstrates that forest fire and climate change are linked in previously unanticipated ways to factors that control MeHg concentrations in fishes.…”

“…Nitrate ϩ nitrite increased 9-fold from Յ2.0 g/liter, total dissolved nitrogen increased 2-fold from 89 g/liter, total phosphorus increased 4-fold from 5.0 g/liter, and total dissolved phosphorus increased 5-fold from 1.0 g/liter. All nutrient concentrations continued to increase in 2001 and may persist for several years (16,17).…”

“…For instance, forest fire alters soil and vegetation within burned catchments, which can affect nutrient transport (15), and postfire impacts on water chemistry (i.e., nutrients, DOC, SO 4 2Ϫ , and pH) and water temperature have been documented (15). Fire-mobilized nutrients (usually N and sometimes P) (16) can enhance lake productivity (17). Forest fire also has been shown to cause small increases in ␦ 15 N of aquatic organisms (17)(18)(19), which some attributed to increased inputs of inorganic nitrogen from the catchment (18,19), not food web alterations.…”

“…Fire-mobilized nutrients (usually N and sometimes P) (16) can enhance lake productivity (17). Forest fire also has been shown to cause small increases in ␦ 15 N of aquatic organisms (17)(18)(19), which some attributed to increased inputs of inorganic nitrogen from the catchment (18,19), not food web alterations. Few have studied the effects of forest fire on the biogeochemical cycling of Hg.…”

Present Effects of Past Wildfires on the Diets of Stream Fish

“…The immediate effects of wildfire on stream biotic structure and function have been well documented (Minshall et al 1989, Minshall 432 WESTERN NORTH AMERICAN NATURALIST [Volume 67 and Brock 1991, Rinne 1996, Rieman and Clayton 1997, Gresswell 1999, Spencer et al 2003. Less clear are the mid-to long-term patterns in biotic response (but see Robinson et al 2005 andDunham et al 2007 for physical data).…”