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Cet article vise à évaluer les effets à moyen terme des feux et des coupes de forêtsur la biomasse, la richesse spécifique et la composition du zooplancton des lacs de laforêt boréale au Québec. L'étude a été réalisée durant trois ans (1996-1998) après lesperturbations sur 3 sous-ensembles de lacs similaires au niveau de leur morphométrie,incluant 7 lacs naturels non perturbés sur leur bassin versant, 7 lacs affectés par descoupes sur plus de 43% de la surface du bassin versant et 7 lacs affectés par des feux surplus de 50% de la surface du bassin versant. Notre étude indique que le zooplancton a uneforte résilience aux perturbations des lacs de la forêt boréale par les feux et les coupes.Les assemblages d'espèces sont très stables et varient peu entre les lacs naturels et leslacs perturbés. Les Rotifères sont le seul groupe affecté par les perturbations; leurrichesse spécifique et leur biomasse sont plus fortes dans les lacs perturbés que dans leslacs naturels. Les feux ont un effet d'eutrophisation plus marqué que les coupes. Dans leslacs affectés par les feux, les apports accrus de nutriments favorisent la croissance duphytoplancton puis des rotifères. Toutefois, dans les lacs affectés par les coupes, la fortecouleur de l'eau due aux apports accrus en carbone organique dissous est un facteurlimitant, et la hausse du phytoplancton et des rotifères est moins accentuée. La richessespécifique et la biomasse des Crustacés (Cladocères, Copépodes Calanoïdes et Cyclopoïdes) nevarient pas avec les perturbations. Sur l'ensemble du suivi, le niveau de perturbation n'estpas fortement relié aux changements dans la biomasse des groupes taxinomiques quis'expliquent plutôt par les variations annuelles de la température de l'eau et desconcentrations en chlorophylle a.This paper aims to evaluate mid-term impacts of forest wildfires and harvesting on thebiomass, species richness and assemblages of zooplankton communities in boreal lakes inQuébec. The research was carried out during a post-perturbation 3-year survey (1996-1998) onthree subsets of lakes (natural, cut, burnt lakes). It complements previous short-termstudies conducted 1 year after perturbations in the same region by PATOINE et al. (2000,2002a) on different subsets of lakes, which failed to detect significant effects. Thiscomparative study attempts to minimize the spatial variability among natural and perturbedlakes by selecting lakes with similar morphometric conditions. For this selection, weapplied a multivariate approach developed by DUHAIME and PINEL-ALLOUL (2005). Making surethat natural and perturbed lakes have similar morphometric features allows minimization ofthe confounding effects of lake and watershed conditions on organic carbon and nutrientexports from perturbed watersheds, and subsequently on lake water quality andbiota.The initial 3-year survey was conducted on 31 lakes including 15 natural lakes withoutany perturbation in their watershed over the past 70 years, and 9 burnt lakes with more than50% of their watershed intensively burnt, and 7 logged lakes with more than 43% of theirwatershed clear-cut. Lakes were visited three times per summer (May-June, July,August-September) between 6:00 to 15:00h, during the 3 years (1996-98) following the firesand harvesting of 1995. Watershed and lake morphometry, and water quality, and algalcommunities were monitored and studied by CARIGNAN et al. (2000) and PLANAS et al. (2000).Fish communities were sampled and studied by SAINT-ONGE and MAGNAN (2000). Zooplankton wassampled using a cantilever net (53 µm mesh size) on vertical hauls from 1 m above thesediments to the lake surface. Sampling methods and analysis of zooplankton were previouslydescribed by PATOINE et al. (2000, 2002a). We estimated species richness and assemblages, aswell as the biomass of each zooplankton group (Rotifera, Cladocera, Copepoda Cyclopoida andCalanoida) and of the total community in each year and each lake. Simple ANOVA were used toevaluate global changes over the mid-term survey in zooplankton attributes among the threesubsets of lakes (natural, cut, burnt). RMANOVA (analysis of variance with repeatedmeasures) were used to test the effects of watershed conditions related to perturbations(natural, cut, burnt lakes), the effects of annual variations during the survey and theirinteraction. Redundancy analysis (RDA) served to evaluate the relationships between changesin the biomass of zooplankton groups during the survey in the three subsets of lakes and thechanges in environmental factors and perturbation intensity. Environmental factors retainedfor the analysis were the drainage ratio (ratio of watershed area to lake area), lake watertransparency, concentrations of total phosphorus, total nitrogen and dissolved organiccarbon, water temperature in the euphotic zone, chlorophyll a concentrations, and biomassclasses of dominant fish (yellow perch, white sucker). The perturbation intensity, expressedas the percentage of watershed area clear-cut or burnt, was also included in themodel.Based on their similarity in morphometric variables (drainage ratio, lake volume, areaand maximum depth, percentage of wetlands in the watershed), we retained for the study 7 cutlakes (C2, C9, C12, C23, C24, C29, C48), 7 burnt lakes (FBP9, FBP10, FP2, FP24, FP30, FP31,FP32), and 7 natural lakes (N5, N16, N56, N70, N82, N88, N109). Variations in morphometriccharacteristics were found within each subset of lakes (reference, burnt, logged), and wedid not detect significant variation among groups. However, we did find significantdifferences in water quality variables among subsets of lakes. Thus, in our study, we haveminimized the effects of variable morphometry among watersheds and lakes that could confoundthe responses of lakes to perturbations, while maintaining the same effects of fires andharvesting on water quality, as previously observed (CARIGNAN et al. 2000). This allowed usto test scaling-up effects of these changes on zooplankton communities.Our study shows that zooplankton communities in boreal lakes have strong resilience toenvironmental perturbations by wildfires and clear-cut logging. Species assemblages werevery stable and varied little between natural and perturbed lakes. Some rotifer species suchas Keratella taurocephala, known as a species adapted to acidic and humic lakes, had higherabundance in the cut lakes most enriched in dissolved organic carbon. Another rotiferspecies, Polyarthra vulgaris, adapted to eutrophic lake and reservoir ecosystems, was moreabundant in the burnt lakes that were the most enriched in nutrients. Only short life-spanand r-strategic species such as rotifers showed increases in species richness in theperturbed lakes compared to natural lakes. Compared to previous studies, our study showedthat this increase in the number of rotifer species lasted for the 3-year survey. Rotifersalso showed higher biomass in perturbed lakes (especially burnt lakes), but only during thefirst two years of the survey. In general, fires had greater eutrophication effects than didclear-cut harvest. In lakes impacted by fires, increases in nutrient inputs (totalphosphorus, total nitrogen) favoured phytoplankton and rotifer growth, while in lakesimpacted by logging, increases in dissolved organic carbon and water color limited theincrease of phytoplankton and rotifers. For the crustaceans (Cladocera, Calanoida,Cyclopoida), neither species richness nor biomass was related to watershed perturbations. Ingeneral, our study indicates that natural sources of temporal variation between years hadgreater effects on the biomass of some zooplankton groups than did the perturbations.Overall, increases in Rotifera biomass were primarily related to higher mean summer watertemperature whereas increases in Cyclopoida biomass were related to higher chlorophyll aconcentrations. Perturbation intensity, nutrient concentrations, and drainage ratio alsotended to increase the responses of these zooplankton groups, but their influence over themid-term survey was not significant. A higher biomass of Calanoida was observed in lakes(mainly natural ones) with high water transparency and low nutrients. Variation in Cladocerabiomass was not related to lake enrichment, higher temperature or perturbation intensity.Our mid-term study generally confirms the effects of wildfires and logging, as previouslysuggested, but not significantly detected by other studies (PATOINE et al. 2000, 2002a)conducted only during the first year after the perturbations. It shows the usefulness ofmid-term monitoring to better assess the ecological impacts of natural and anthropogenicperturbations on aquatic ecosystems

Suivi à moyen terme des impacts écologiques des feux et des coupes forestières sur la communauté zooplanctonique des lacs de l'écozone boréale