Benthic macroinvertebrate biomass and wildfires: evidence for enrichment of boreal subarctic lakes

1 We quantified the effects of forest fire on littoral benthic macroinverte brate biomass on a boreal subarctic plateau in Alberta, Canada. Water chemi stry and benthos were collected from six lakes, 1 and 2 years following a 1 995 fire which burned about 91% of their catchments (i.e. recently burnt la kes), and from four other lakes whose catchments burned between 1961 and 19 85 (i.e. previously burnt lakes). Seven lakes whose catchments had not burn ed since at least 1957 served as reference systems. 2 Total benthic macroinvertebrate biomass and biomass of Chironomidae were 1.5- and 2-fold (P < 0.05) greater within recently burnt lakes than in refe rence systems, whereas the biomass of Oligochaeta (P = 0.06) and Amphipoda (P = 0.07) were marginally higher in burnt than reference lakes. Burnt lake s had greater colour (P < 0.05) and marginally (P = 0.06) higher concentrat ions csf soluble reactive phosphorus than reference lakes. 3 Nutrient diffusing substrata deployed in a previously and a recently burn t lake indicated that littoral epilithic communities were co-limited by the availability of phosphorus (P) and nitrogen (N), although co-limitation wa s stronger in the previously burnt than the recently burnt lake. Epilithic chlorophyll a on natural stone surfaces in the recently burnt lake was also 3.5 times higher (P < 0.05) than that from the previously burnt lake. 4 Among all 17 lakes, total benthic biomass and biomass of Chironomidae, Am phipoda and Nematoda, were significantly (P < 0.05) or marginally (P = 0.06 ) related to soluble reactive phosphorus (SRP) but not dissolved inorganic nitrogen or colour. These regressions explained between 11% and 64% of vari ation in benthic biomasses. 5 Linear regressions and second-order polynomials explained 18% and 24% of the variation in concentrations of SRP and water colour with time since fir e, respectively, and between 22% and 70% of variation in total biomass and biomass of the five dominant invertebrate groups. These analyses suggest th at benthic biomasses continue to be elevated for about 15-20 years followin g fire before declining to pre-disturbance levels.