Pelagic and benthic algal responses in eastern Canadian Boreal Shield lakes following harvesting and wildfires

Pelagic and benthic algal biomass and pelagic algal community structure wer e measured in Boreal Shield lakes impacted by forest harvesting and wildfir es (Haute-Mauricie, Quebec). Sixteen reference lakes in which the watershed has been unperturbed for at least 40 years, seven harvested lake watershed s (logged in 1995), and nine lake watersheds burnt in 1995 were sampled for 3 years following harvesting or wildfires. From 1996 to 1998, repeated- me asures ANOVA showed significant effects between treatment and sampling year s for pelagic chlorophyll a (Chl a) and biomass, but for 1997-1998 benthic Chi a, repeated-measures ANOVA showed only significant treatment effects. C hi a concentrations increased 1.4- to 3-fold in perturbed lakes as compared with reference lakes. Areal pelagic Chi a (milligrams per square metre) wa s lower than estimated littoral Chl a in perturbed lakes. The pelagic algal community was dominated by mixotrophic nanoflagellates in reference lakes. Watershed perturbation induced differential changes in pelagic algal commu nities: mixotrophic nanoflagellates increased in harvested lakes and photoa utotrophic diatoms in burnt lakes. Considering only perturbed lakes, algal biomass was proportional to the fraction of the catchment area perturbed di vided by the surface area of lakes in the catchment.