Abstract
Pelagic and benthic algal biomass and pelagic algal community structure were measured in Boreal Shield lakes impacted by forest harvesting and wildfires (Haute-Mauricie, Quebec). Sixteen reference lakes in which the wa- tershed has been unperturbed for at least 40 years, seven harvested lake watersheds (logged in 1995), and nine lake watersheds burnt in 1995 were sampled for 3 years following harvesting or wildfires. From 1996 to 1998, repeated- measures ANOVA showed significant effects between treatment and sampling years for pelagic chlorophyll a (Chl a) and biomass, but for 1997-1998 benthic Chl a, repeated-measures ANOVA showed only significant treatment effects. Chl a concentrations increased 1.4- to 3-fold in perturbed lakes as compared with reference lakes. Areal pelagic Chl a (milligrams per square metre) was 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 communities: mixotrophic nanoflagellates increased in harvested lakes and photoautotrophic diatoms in burnt lakes. Considering only perturbed lakes, algal biomass was proportional to the fraction of the catchment area per - turbed divided by the surface area of lakes in the catchment.
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