Review of ecological impacts of forest fires and harvesting on lakes of the boreal ecozone in Québec
This paper presents a comparative review on ecological impacts of wildfires and harvesting on water quality and aquatic community attributes of boreal lakes in eastern Canada. This project was carried out from 1996 to 1998, as part of the research program of the Sustainable Forest Management Network Centre of Excellence (NCE-SFM), on 38 lakes of the Boreal Canadian Shield (Québec, Canada). Changes in water quality, and in diversity, biomass and community structure of pelagic plankton and fish populations were monitored for 3 years in 20 reference lakes, 9 cut lakes and 9 burnt lakes. Our study shows that wildfires and logging have significant impacts on water quality as well as primary and secondary producers in boreal lakes. However, fire and logging does not have similar impacts. In general, cut and burnt lakes showed higher concentrations of total phosphorus (TP: 2-3 folds) than reference lakes. However nitrates concentrations (NO3-) were higher in burnt lakes than in reference and cut lakes, whereas dissolved organic carbon (DOC), light extinction coefficient (e PAR) and water colour were higher in cut lakes than in reference lakes, burnt lakes showing intermediate values. Wildfire and logging also affect ionic composition of lake water. Both burnt and cut lakes had higher concentrations of potassium (K+), chloride (Cl-) and calcium (Ca2+) than reference lakes, while sulphates (SO42-) and magnesium (Mg2+) increased only in burnt lakes. The observed increases in organic and mineral elements reflect their export from the watershed and were related to the drainage ratio. As mobile ions are rapidly flushed from the perturbed watersheds, ionic water composition returned to normal levels within three years after the perturbation. In contrast, some changes in water quality (P and N enrichment in perturbed lakes; DOC increase in cut lakes) seems to be on longer term and might have a more important effect on limnological features of lakes such as thermal stratification, mixed and euphotic depth, plankton biomass and bioaccumulation of mercury in zooplankton and fish. Wildfires and logging do not have significant impacts on species richness of planktonic communities, except for lakes with large drainage basins and those where the perturbation was higher than 40% of the watershed area. However, wildfires and logging affect biomass of plankton communities. In burnt lakes, the nutrient enrichment induced important increases in Chl. a concentration and phytoplankton biomass, and a small increase in limnoplankton biomass (seston > 53 μm). Diatoms, rotifers and large crustaceans showed significant increases. In burnt lakes, increases in phytoplankton were observed during the three years of the survey and were highest in the first 2 years after fires. Limnoplankton increases lasted only 2 years and were highest the second year after fires. In cut lakes, nutrient enrichment was not reflected in a proportional increase in plankton production due to increase in DOC concentrations and lake water colour that reduced water transparency. Phytoplankton slightly increased only the first year after logging and no increase was observed for limnoplankton and zooplankton. Logging negatively affect calanoids, a zooplankton group characteristic of clear and oligotrophic lakes. No significant differences were observed in the occurrence, abundance and growth of fish populations among reference and perturbed lakes. However, the proportion of small yellow perch and white sucker were significantly lower in populations of impacted lakes (burned and logged lakes pooled). In most cases, impacts on water quality and plankton communities were related to the intensity of fire or logging, when expressed as the percent area of watershed harvested or burnt divided by the lake surface or volume. Simple models may then be developed and help to predict the effect of harvesting practices on lake ecosystems. However, environmental variation in watershed and lakes characteristics, especially drainage ratio and lake water residence time, are important confounding factors in the responses of lakes to watershed perturbations by fire or logging. Lakes with drainage ratio > 4 and with more of 30% of their watershed perturbed are the most sensitive to fire and logging. These factors should be considered when developing lake management models in the boreal forest impacted by fire and logging.