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Plusieurs approches tentent de mesurer l'impact des perturbations anthropiques sur lesécosystèmes. L'approche BACI (Before-After Control-Impact) consiste à suivre deux groupes desites (contrôle et impact), avant et après une perturbation, afin de mesurer l'effet decette dernière sur les écosystèmes. Les études BACI permettent de contrôler la variabiliténaturelle entre les groupes de sites, par le suivi des mêmes sites d'impact avant et aprèsla perturbation, tout en minimisant la variabilité naturelle entre les années grâce au suivide sites de contrôle échantillonnés également avant et après la perturbation. Puisque lavariation naturelle entre les années dans les sites d'impact est estimée à partir de celledes sites de contrôle, il est nécessaire de sélectionner des sites de contrôle dont lescaractéristiques limnologiques sont semblables à celles des sites d'impact. Ceci estessentiel pour une bonne application de l'approche BACI, afin de s'assurer que les sitesnaturels et perturbés répondent de la même façon aux variations naturelles interannuellesdans l'environnement et que les différences observées dans les sites d'impact avant et aprèsla perturbation soient attribuables à celle-ci.Cet article propose une méthode de sélection des sites de contrôle dans le cadre d'uneétude BACI portant sur l'impact des coupes forestières sur le zooplancton des lacs de laforêt boréale au Québec. Le zooplancton de 16 lacs de la forêt boréale a été échantillonnéun an avant (2000) et deux ans après (2001-2002) des coupes forestières sur le bassinversant de certains lacs. Six lacs ont subi des coupes importantes sur 44 à 77 % du bassinversant (lacs de coupe: DA2, DF2, DF7, DF9, K4, K8) et 10 lacs sont restés à l'état naturelou n'ont subi que des coupes négligeables sur moins de 2 % du bassin versant (lacs deréférence: K2, AB34, AB35, AB40, CSL5, DA4, DF4, N35, N89, N43). Parmi ces dix lacs deréférence, nous avons sélectionné les six lacs les plus semblables aux lacs de coupe, àl'aide d'analyses en composantes principales (ACP) basées sur la similarité des variablesmorphométriques, de la qualité de l'eau et du zooplancton avant la coupe (2000). De plus,les variables ayant la plus grande contribution à la variation totale au niveau de ces troisgroupes de variables ont été déterminées. Quatre lacs ont été exclus (K2, N89, AB35, AB40)et six lakes (AB34, CSL5, DA4, DF4, N35, N43) ont été sélectionnés comme référence.Finalement, la validité du choix des six lacs de référence a été testée par des analyses deredondance (RDA) avec une variable binaire qui permettait de distinguer les lacs de coupedes lacs de référence sélectionnés. Les analyses de redondance ont montré que les variablesde la morphométrie, de la qualité de l'eau et du zooplancton des lacs de référencesélectionnés ne différaient pas significativement de celles des lacs de coupe avant laperturbation. En conséquence, les différences observées après la perturbation dans les sixlacs de coupe, relativement aux variations naturelles dans les six lacs de référencesélectionnés, devraient être attribuables à l'effet de la coupe forestière. La méthode desélection développée dans le cadre de cette étude peut être utilisée pour évaluer à l'aided'une approche BACI les effets de toute perturbation anthropique sur lesécosystèmesSeveral designs can be used to assess the effects of human perturbations onecosystems. However, the main difficulty is to isolate natural sources of variation from thevariation induced by the perturbation. Several studies have shown that the naturaldifferences between the reference and the impacted sites may influence their responses tothe perturbation. In a comparative design comparing the conditions at the reference sitesand the impacted sites after the perturbation, it is not possible to control for the naturalsources of variation between these two groups of sites that occurred before theperturbation. This natural variation among sites is taken into account in a Before-Afterdesign in which the same sites are monitored before and after perturbation, but the naturalsources of variation among years cannot be separated from the variation induced by theperturbation. In this study, we used a BACI (Before-After Control-Impact) design to measurethe effect of a perturbation on an ecosystem by following two groups of sites (control andimpacted) before and after the perturbation. A BACI design is the only one that allowscontrolling for the natural variability among sites by following the same impacted sitesbefore and after the perturbation. This approach also takes into account the naturalvariability among years by monitoring concomitantly the control sites before and after theperturbation. Since the natural variability among years at the impacted sites is estimatedby the variability measured at the control sites, it is essential to select control sitesthat are the most similar to the impacted sites with respect to their limnologicalcharacteristics. This is a requirement for the good use of the BACI design, to make surethat control and impacted sites have the same responses towards year-to-year naturalvariations in their environment and to ensure that the differences observed at the impactedsites before-after the perturbation are caused by it.This paper proposes a method for the selection of control sites in a BACI design tostudy the impact of forest harvesting on zooplankton communities in lakes of the borealforest in Québec. Zooplankton in 16 lakes was sampled one year before (2000) and two yearsafter (2001-2002) forest harvesting in the watersheds of some lakes. Six lakes were impactedby important harvesting on 44 to 77% of their watershed (cut lakes: DA2, DF2, DF7, DF9, K4,K8) and ten lakes were considered as natural lakes with forest harvesting on less than 2% oftheir watershed (reference lakes: K2, AB34, AB35, AB40, CSL5, DA4, DF4, N35, N89, N43).Among these ten natural lakes, we selected the six reference lakes that had limnologicalfeatures most similar to the cut lakes, based on the morphometry, water quality andzooplankton variables before forest harvesting (2000). We used principal component analyses(PCA) to compare the lakes (10 natural lakes and six lakes to be impacted by watershedharvesting) using ordination biplots. A PCA was done for each group of variables. Lakevolume, maximal depth, and area and the slope of the watershed were the variables having themost important contributions to the total variation in morphometry. Positive correlationswere found between the slope of the watershed and the maximal depth of the lake and betweenthe lake area and the lake volume. This PCA allowed us to eliminate two reference lakes thathad higher values in lake volume and area for one lake (K2) and in lake volume and maximaldepth for the other one (AB35), compared to the cut lakes. When comparing water qualityvariables, total phosphorus (TP), total nitrogen (TN), dissolved organic carbon (DOC),Secchi depth, Ca2+, Mg2+, alkalinity (ALK) and pHcontributed more to the total variation than other variables. Among these variables, DOC, TPand TN were positively correlated together whereas they were negatively correlated to watertransparency (Secchi). Two reference lakes (AB35, N89) had higher transparency and lowervalues in DOC, TP and TN and one reference lake (AB40) had lower values in pH, ALK, Ca2+ andMg2+ compared to cut lakes. Since one of these three lakes had previously been eliminated onthe basis of its morphometric variables, we had a total of four reference lakes excludedfrom the study. The PCA on zooplankton variables revealed that only the largest class ofzooplankton (>500 µm) contributed significantly to zooplankton total variation. Ingeneral, reference lakes and cut lakes had a similar distribution on the ordination biplotso no lake was eliminated with respect to zooplankton variables. Finally, four referencelakes were excluded (K2, N89, AB35, AB40) and six reference lakes were selected (AB34, CSL5,DA4, DF4, N35, N43).The validation of the selection of the six reference lakes was made using redundancyanalyses (RDA) with a binary variable discriminating between the six cut lakes and the sixreference lakes selected in the PCAs. Multivariate methods such as RDA have the advantage oftesting if the two groups of lakes are different with respect to many variables and not justone, as would be the case when using a univariate method. Redundancy analyses showed thatthe morphometric, water quality and zooplankton variables in the six selected referencelakes were not significantly different from those in the six cut lakes before harvesting.The RDA results supported the selection of the six reference lakes based on the ordinationbiplots from the three PCA made on the three groups of variables. We could then presume thatany difference observed after the perturbation in the cut lakes, relative to the naturalvariation among years in the six reference lakes, could be attributed to the effect offorest harvesting. Finally, this method of selection of control sites could be used for anyBACI study testing the effects of human perturbation on ecosystems

revue des sciences de l'eau

Méthode de sélection de lacs de référence dans le cadre d'une étude Before-After Control-Impact (BACI) évaluant les effets des coupes forestières sur le zooplancton des lacs de la forêt boréale