A group model for stable multi-subject ICA on fMRI datasets

Spatial Independent Component Analysis (ICA) is an increasingly used data-driven method to analyze functional Magnetic Resonance Imaging (fMRI) data. To date, it has been used to extract sets of mutually correlated brain regions without prior information on the time course of these regions. Some of these sets of regions, interpreted as functional networks, have recently been used to provide markers of brain diseases and open the road to paradigm-free population comparisons. Such group studies raise the question of modeling subject variability within ICA: how can the patterns representative of a group be modeled and estimated via ICA for reliable inter-group comparisons? In this paper, we propose a hierarchical model for patterns in multi-subject fMRI datasets, akin to mixed-effect group models used in linear-model-based analysis. We introduce an estimation procedure, CanICA (Canonical ICA), based on i) probabilistic dimension reduction of the individual data, ii) canonical correlation analysis to identify a data subspace common to the group iii) ICA-based pattern extraction. In addition, we introduce a procedure based on cross-validation to quantify the stability of ICA patterns at the level of the group. We compare our method with state-of-the-art multi-subject fMRI ICA methods and show that the features extracted using our procedure are more reproducible at the group level on two datasets of 12 healthy controls: a resting-state and a functional localizer study

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

Plusieurs approches tentent de mesurer l'impact des perturbations anthropiques sur les écosystèmes. L'approche BACI (Before-After Control-Impact) consiste à suivre deux groupes de sites (contrôle et impact), avant et après une perturbation, afin de mesurer l'effet de cette dernière sur les écosystèmes. Les études BACI permettent de contrôler la variabilité naturelle entre les groupes de sites, par le suivi des mêmes sites d'impact avant et après la perturbation, tout en minimisant la variabilité naturelle entre les années grâce au suivi de sites de contrôle échantillonnés également avant et après la perturbation. Puisque la variation naturelle entre les années dans les sites d'impact est estimée à partir de celle des sites de contrôle, il est nécessaire de sélectionner des sites de contrôle dont les caractéristiques limnologiques sont semblables à celles des sites d'impact. Ceci est essentiel pour une bonne application de l'approche BACI, afin de s'assurer que les sites naturels et perturbés répondent de la même façon aux variations naturelles interannuelles dans l'environnement et que les différences observées dans les sites d'impact avant et après la perturbation soient attribuables à celle-ci.Cet article propose une méthode de sélection des sites de contrôle dans le cadre d'une étude BACI portant sur l'impact des coupes forestières sur le zooplancton des lacs de la forêt boréale au Québec. Le zooplancton de 16 lacs de la forêt boréale a été échantillonné un an avant (2000) et deux ans après (2001-2002) des coupes forestières sur le bassin versant de certains lacs. Six lacs ont subi des coupes importantes sur 44 à 77 % du bassin versant (lacs de coupe: DA2, DF2, DF7, DF9, K4, K8) et 10 lacs sont restés à l'état naturel ou n'ont subi que des coupes négligeables sur moins de 2 % du bassin versant (lacs de référence: K2, AB34, AB35, AB40, CSL5, DA4, DF4, N35, N89, N43). Parmi ces dix lacs de référence, nous avons sélectionné les six lacs les plus semblables aux lacs de coupe, à l'aide d'analyses en composantes principales (ACP) basées sur la similarité des variables morphométriques, 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 trois groupes 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 de redondance (RDA) avec une variable binaire qui permettait de distinguer les lacs de coupe des lacs de référence sélectionnés. Les analyses de redondance ont montré que les variables de la morphométrie, de la qualité de l'eau et du zooplancton des lacs de référence sélectionnés ne différaient pas significativement de celles des lacs de coupe avant la perturbation. En conséquence, les différences observées après la perturbation dans les six lacs de coupe, relativement aux variations naturelles dans les six lacs de référence sélectionnés, devraient être attribuables à l'effet de la coupe forestière. La méthode de sélection développée dans le cadre de cette étude peut être utilisée pour évaluer à l'aide d'une approche BACI les effets de toute perturbation anthropique sur les écosystèmesSeveral designs can be used to assess the effects of human perturbations on ecosystems. However, the main difficulty is to isolate natural sources of variation from the variation induced by the perturbation. Several studies have shown that the natural differences between the reference and the impacted sites may influence their responses to the perturbation. In a comparative design comparing the conditions at the reference sites and the impacted sites after the perturbation, it is not possible to control for the natural sources of variation between these two groups of sites that occurred before the perturbation. This natural variation among sites is taken into account in a Before-After design in which the same sites are monitored before and after perturbation, but the natural sources of variation among years cannot be separated from the variation induced by the perturbation. In this study, we used a BACI (Before-After Control-Impact) design to measure the effect of a perturbation on an ecosystem by following two groups of sites (control and impacted) before and after the perturbation. A BACI design is the only one that allows controlling for the natural variability among sites by following the same impacted sites before and after the perturbation. This approach also takes into account the natural variability among years by monitoring concomitantly the control sites before and after the perturbation. Since the natural variability among years at the impacted sites is estimated by the variability measured at the control sites, it is essential to select control sites that are the most similar to the impacted sites with respect to their limnological characteristics. This is a requirement for the good use of the BACI design, to make sure that control and impacted sites have the same responses towards year-to-year natural variations in their environment and to ensure that the differences observed at the impacted sites before-after the perturbation are caused by it.This paper proposes a method for the selection of control sites in a BACI design to study the impact of forest harvesting on zooplankton communities in lakes of the boreal forest in Québec. Zooplankton in 16 lakes was sampled one year before (2000) and two years after (2001-2002) forest harvesting in the watersheds of some lakes. Six lakes were impacted by 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% of their 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 limnological features most similar to the cut lakes, based on the morphometry, water quality and zooplankton 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 watershed harvesting) using ordination biplots. A PCA was done for each group of variables. Lake volume, maximal depth, and area and the slope of the watershed were the variables having the most important contributions to the total variation in morphometry. Positive correlations were found between the slope of the watershed and the maximal depth of the lake and between the lake area and the lake volume. This PCA allowed us to eliminate two reference lakes that had higher values in lake volume and area for one lake (K2) and in lake volume and maximal depth for the other one (AB35), compared to the cut lakes. When comparing water quality variables, total phosphorus (TP), total nitrogen (TN), dissolved organic carbon (DOC), Secchi depth, Ca2+, Mg2+, alkalinity (ALK) and pH contributed more to the total variation than other variables. Among these variables, DOC, TP and TN were positively correlated together whereas they were negatively correlated to water transparency (Secchi). Two reference lakes (AB35, N89) had higher transparency and lower values in DOC, TP and TN and one reference lake (AB40) had lower values in pH, ALK, Ca2+ and Mg2+ compared to cut lakes. Since one of these three lakes had previously been eliminated on the basis of its morphometric variables, we had a total of four reference lakes excluded from the study. The PCA on zooplankton variables revealed that only the largest class of zooplankton (>500 µm) contributed significantly to zooplankton total variation. In general, reference lakes and cut lakes had a similar distribution on the ordination biplot so no lake was eliminated with respect to zooplankton variables. Finally, four reference lakes 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 redundancy analyses (RDA) with a binary variable discriminating between the six cut lakes and the six reference lakes selected in the PCAs. Multivariate methods such as RDA have the advantage of testing if the two groups of lakes are different with respect to many variables and not just one, as would be the case when using a univariate method. Redundancy analyses showed that the morphometric, water quality and zooplankton variables in the six selected reference lakes 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 ordination biplots from the three PCA made on the three groups of variables. We could then presume that any difference observed after the perturbation in the cut lakes, relative to the natural variation among years in the six reference lakes, could be attributed to the effect of forest harvesting. Finally, this method of selection of control sites could be used for any BACI study testing the effects of human perturbation on ecosystems

Variations spatiotemporelles des compartiments autotrophes et hétérotrophes de la boucle microbienne dans les lacs du sud du Québec

Les variations de la biomasse des compartiments autotrophes et hétérotrophes de la boucle microbienne ont été examinées dans neuf lacs de deux régions du sud du Québec. Six lacs étaient situés dans les Laurentides et trois lacs dans les Cantons de l'Est. Ils se différenciaient en fonction de leur statut trophique, de la géologie du bassin versant, et de la physicochimie des eaux. Dans chaque lac, l'échantillonnage a été réalisé à trois profondeurs correspondant aux strates épi-, méta- et hypolimnétiques et à cinq dates au cours de l'été (1990-1992). Les biomasses moyennes de picoplancton autotrophe (PPA : 16-80 µg·C·L-1), de picoplancton hétérotrophe (PPH : 97-647 µg·C·L-1), de nanoplancton autotrophe (NPA : 7-37 µg·C·L-1) et de nanoplancton hétérotrophe (NPH : 9-29 µg·C·L-1) notées dans les lacs du sud du Québec le long d'un gradient trophique de PT variant de 7 à 73 µg·L-1 étaient du même ordre de grandeur que celles rapportées pour d'autres écosystèmes d'eau douce au Canada.La majeure partie de la variation dans les biomasses des compartiments microbiens étaient reliée aux variations inter-lacs mais il existaient aussi des sources de variation significatives au sein des lacs, soit au niveau spatial entre les strates limnétiques ou au niveau temporel entre les dates d'échantillonnage durant l'été. Toutefois, les patrons de variation spatiotemporelle intra-lac variaient d'un lac à l'autre. Les variations inter-lacs de la biomasse des quatre compartiments microbiens ont été mises en relation avec les changements dans les concentrations de phosphore total (PT) et dans les densités de macrozooplancton. Le niveau trophique des lacs, exprimés en PT, et l'abondance du macrozooplancton avaient un effet sur les biomasses des compartiments picoplanctoniques. La biomasse de PPA et PPH décroissait dans les lacs eutrophes des Cantons de l'Est ayant des concentrations de PT supérieures à 20 µg·L-1 et dans les lacs ayant de fortes densités de macrozooplancton ou un faible rapport micro-macrozooplancton. Notre étude n'a pas mis clairement en évidence les effets du niveau trophique ou du macrozoopancton sur les compartiments nanoplanctoniques (NPA, NPH).The biomass of autotrophic and heterotrophic microbial compartments were measured in nine lakes in two regions of southern Quebec. Six lakes were located in the Laurentides while three lakes were situated in the Eastern Townships. They varied in trophic status, watershed geology, and water chemistry. Each lake was sampled at three depths, corresponding to the epi-, meta-, and hypolimnion strata, and at five dates over the summer season (1991-1992). The mean biomass of autotrophic picoplankton (PPA: 16-80 µg·C·L-1), heterotrophic picoplankton (PPH: 97-647 µg·C·L-1), autotrophic nanoplankton (NPA: 7-37 µg·C·L-1), and heterotrophic nanoplankton (NPH: 9-29 µg·C·L-1) found in Quebec lakes over a PT range of trophy varying from 7 to 73 µg·L-1 were of the same order as those reported in other freshwater environments in Canada.Most of the variation in the biomass of the four microbial compartments was related to among-lake variation. Some variation was also due to within-lake spatial variation among limnetic strata or temporal variation over the summer season. However, within-lake patterns of variation patterns were different among lakes. Among-lake variations in the biomass of the four microbial compartments were examined in relation to changes in total phosphorus concentrations (PT) and macrozooplankton densities between lakes. Both lake trophy, expressed by PT, and macrozooplankton abundance influence the biomass of picoplankton compartments. The biomass of PPA and PPH decline in the most eutrophic lakes of the Eastern Townships where PT > 20 µg·L-1. PPA and PPH biomass were also reduced in lakes with the highest density of macrozooplankton or with low micro-/macrozooplankton ratio. Our study did not clearly detected the effects of lake trophy or macrozooplankton on nanoplanktonic compartments

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

Cet article vise à évaluer les effets à moyen terme des feux et des coupes de forêt sur la biomasse, la richesse spécifique et la composition du zooplancton des lacs de la forêt boréale au Québec. L'étude a été réalisée durant trois ans (1996-1998) après les perturbations 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 des coupes sur plus de 43% de la surface du bassin versant et 7 lacs affectés par des feux sur plus de 50% de la surface du bassin versant. Notre étude indique que le zooplancton a une forte 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 les lacs perturbés. Les Rotifères sont le seul groupe affecté par les perturbations; leur richesse spécifique et leur biomasse sont plus fortes dans les lacs perturbés que dans les lacs naturels. Les feux ont un effet d'eutrophisation plus marqué que les coupes. Dans les lacs affectés par les feux, les apports accrus de nutriments favorisent la croissance du phytoplancton puis des rotifères. Toutefois, dans les lacs affectés par les coupes, la forte couleur de l'eau due aux apports accrus en carbone organique dissous est un facteur limitant, et la hausse du phytoplancton et des rotifères est moins accentuée. La richesse spécifique et la biomasse des Crustacés (Cladocères, Copépodes Calanoïdes et Cyclopoïdes) ne varient pas avec les perturbations. Sur l'ensemble du suivi, le niveau de perturbation n'est pas fortement relié aux changements dans la biomasse des groupes taxinomiques qui s'expliquent plutôt par les variations annuelles de la température de l'eau et des concentrations en chlorophylle a.This paper aims to evaluate mid-term impacts of forest wildfires and harvesting on the biomass, species richness and assemblages of zooplankton communities in boreal lakes in Québec. The research was carried out during a post-perturbation 3-year survey (1996-1998) on three subsets of lakes (natural, cut, burnt lakes). It complements previous short-term studies 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. This comparative study attempts to minimize the spatial variability among natural and perturbed lakes by selecting lakes with similar morphometric conditions. For this selection, we applied a multivariate approach developed by DUHAIME and PINEL-ALLOUL (2005). Making sure that natural and perturbed lakes have similar morphometric features allows minimization of the confounding effects of lake and watershed conditions on organic carbon and nutrient exports from perturbed watersheds, and subsequently on lake water quality and biota.The initial 3-year survey was conducted on 31 lakes including 15 natural lakes without any perturbation in their watershed over the past 70 years, and 9 burnt lakes with more than 50% of their watershed intensively burnt, and 7 logged lakes with more than 43% of their watershed 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 fires and harvesting of 1995. Watershed and lake morphometry, and water quality, and algal communities 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 was sampled using a cantilever net (53 µm mesh size) on vertical hauls from 1 m above the sediments to the lake surface. Sampling methods and analysis of zooplankton were previously described by PATOINE et al. (2000, 2002a). We estimated species richness and assemblages, as well as the biomass of each zooplankton group (Rotifera, Cladocera, Copepoda Cyclopoida and Calanoida) and of the total community in each year and each lake. Simple ANOVA were used to evaluate global changes over the mid-term survey in zooplankton attributes among the three subsets of lakes (natural, cut, burnt). RMANOVA (analysis of variance with repeated measures) 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 their interaction. Redundancy analysis (RDA) served to evaluate the relationships between changes in the biomass of zooplankton groups during the survey in the three subsets of lakes and the changes in environmental factors and perturbation intensity. Environmental factors retained for the analysis were the drainage ratio (ratio of watershed area to lake area), lake water transparency, concentrations of total phosphorus, total nitrogen and dissolved organic carbon, water temperature in the euphotic zone, chlorophyll a concentrations, and biomass classes of dominant fish (yellow perch, white sucker). The perturbation intensity, expressed as the percentage of watershed area clear-cut or burnt, was also included in the model.Based on their similarity in morphometric variables (drainage ratio, lake volume, area and maximum depth, percentage of wetlands in the watershed), we retained for the study 7 cut lakes (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 morphometric characteristics were found within each subset of lakes (reference, burnt, logged), and we did not detect significant variation among groups. However, we did find significant differences in water quality variables among subsets of lakes. Thus, in our study, we have minimized the effects of variable morphometry among watersheds and lakes that could confound the responses of lakes to perturbations, while maintaining the same effects of fires and harvesting on water quality, as previously observed (CARIGNAN et al. 2000). This allowed us to test scaling-up effects of these changes on zooplankton communities.Our study shows that zooplankton communities in boreal lakes have strong resilience to environmental perturbations by wildfires and clear-cut logging. Species assemblages were very stable and varied little between natural and perturbed lakes. Some rotifer species such as Keratella taurocephala, known as a species adapted to acidic and humic lakes, had higher abundance in the cut lakes most enriched in dissolved organic carbon. Another rotifer species, Polyarthra vulgaris, adapted to eutrophic lake and reservoir ecosystems, was more abundant in the burnt lakes that were the most enriched in nutrients. Only short life-span and r-strategic species such as rotifers showed increases in species richness in the perturbed lakes compared to natural lakes. Compared to previous studies, our study showed that this increase in the number of rotifer species lasted for the 3-year survey. Rotifers also showed higher biomass in perturbed lakes (especially burnt lakes), but only during the first two years of the survey. In general, fires had greater eutrophication effects than did clear-cut harvest. In lakes impacted by fires, increases in nutrient inputs (total phosphorus, total nitrogen) favoured phytoplankton and rotifer growth, while in lakes impacted by logging, increases in dissolved organic carbon and water color limited the increase of phytoplankton and rotifers. For the crustaceans (Cladocera, Calanoida, Cyclopoida), neither species richness nor biomass was related to watershed perturbations. In general, our study indicates that natural sources of temporal variation between years had greater effects on the biomass of some zooplankton groups than did the perturbations. Overall, increases in Rotifera biomass were primarily related to higher mean summer water temperature whereas increases in Cyclopoida biomass were related to higher chlorophyll a concentrations. Perturbation intensity, nutrient concentrations, and drainage ratio also tended to increase the responses of these zooplankton groups, but their influence over the mid-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 Cladocera biomass was not related to lake enrichment, higher temperature or perturbation intensity. Our mid-term study generally confirms the effects of wildfires and logging, as previously suggested, 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 of mid-term monitoring to better assess the ecological impacts of natural and anthropogenic perturbations on aquatic ecosystems

Configurable unitary transformations and linear logic gates using quantum memories

We show that a set of optical memories can act as a configurable linear optical network operating on frequency-multiplexed optical states. Our protocol is applicable to any quantum memories that employ off-resonant Raman transitions to store optical information in atomic spins. In addition to the configurability, the protocol also offers favourable scaling with an increasing number of modes where N memories can be configured to implement an arbitrary N-mode unitary operations during storage and readout. We demonstrate the versatility of this protocol by showing an example where cascaded memories are used to implement a conditional CZ gate.Comment: 5 pages, 2 figure

Effet de la prédation et des nutriments sur les réseaux microbiens planctoniques

Les effets de l'ajout de nutriments et de poissons planctonophages sur le réseau trophique microbien (protozoaires, phytoplancton, zooplancton) ont été évalués lors d'une étude expérimentale utilisant des enclos de grande taille (8 m diam., 12 m prof.), placés dans un lac humique du Bouclier Canadien. Un plan factoriel avec 4 traitements, C (contrôle), N (ajout de nutriments), P (ajout de poissons planctonophages) et NP (ajout de nutriments et poissons) a été appliqué. L'abondance et la biomasse du zooplancton (Rotifères, Cladocères, Copépodes), des Protozoaires Ciliés et du phytoplancton (Chl. a) ont été déterminées pendant 6 semaines dans chaque strate (épi-, méta- et hypolimnion). La présence de poissons planctonophages a entraîné une baisse de la biomasse du zooplancton de grande taille (Cladocères), mais a favorisé le développement du zooplancton de petite taille (Rotifères). La diminution de biomasse des Cladocères dans les enclos avec poissons s'associait avec une hausse de l'abondance des Protozoaires Ciliés et du phytoplancton pour les enclos NP. L'ajout de nutriments a augmenté la biomasse des Cladocères, des Ciliés et du phytoplancton qui présentait une tendance monospécifique. L'étude a montré que dans un lac humique riche en carbone organique dissous, les nutriments ne sont pas le seul facteur déterminant la structure des communautés microbiennes. Celles-ci subissent également les effets en cascade de la prédation des poissons planctonophages situés au sommet de la chaîne alimentaire.An experimental study was conducted using large enclosures (8 m diam., 12 m depth.) set up in a humic lake on the Canadian Shield to determine the effects of nutrients and planktivorous fish on the microbial food web (protozoans, phytoplankton and zooplankton). A four treatments factorial design was applied as following: C (control), N (nutrients), P (planktivorous fish) and NP (nutrients and fish). Zooplankton, ciliate and phytoplankton abundance and biomass were measured for six weeks in the epi-, meta- and hypolimnion. In the fish enclosure, the decrease of cladoceran biomass produced an increase in ciliate abundance and phytoplankton biomass when both fish and nutrients were present. Nutrient loading increased the biomass of cladocerans, phytoplankton and ciliates. This study showed that, in humic lakes, the microbial community is regulated not only by nutrients, but top-down effects, via fish predation, must also be considered

Spatial mode storage in a gradient echo memory

Three-level atomic gradient echo memory (lambda-GEM) is a proposed candidate for efficient quantum storage and for linear optical quantum computation with time-bin multiplexing. In this paper we investigate the spatial multimode properties of a lambda-GEM system. Using a high-speed triggered CCD, we demonstrate the storage of complex spatial modes and images. We also present an in-principle demonstration of spatial multiplexing by showing selective recall of spatial elements of a stored spin wave. Using our measurements, we consider the effect of diffusion within the atomic vapour and investigate its role in spatial decoherence. Our measurements allow us to quantify the spatial distortion due to both diffusion and inhomogeneous control field scattering and compare these to theoretical models.Comment: 11 pages, 9 figure

Synthèse des impacts écologiques des feux et des coupes forestières sur les lacs de l'écozone boréale au Québec

Cet article présente une première synthèse comparative des impacts des feux et des coupes forestières sur la qualité des eaux et les communautés aquatiques des lacs de l'écozone boréale de l'est du Canada. La recherche a été réalisée de 1996 à 1998 dans le cadre d'un projet du Réseau de centre d'excellence sur la gestion durable des forêts (RCE-GDF) et porte sur 38 lacs du Bouclier Canadien du Québec. Les changements dans la qualité de l'eau ainsi que dans la diversité, la biomasse et la structure des communautés de plancton et de poissons ont été suivis pendant trois ans dans 20 lacs dont les bassins versants étaient non perturbés, dans 9 lacs qui avaient subit des coupes forestières et dans 9 lacs qui avaient subit des feux de forêt.Notre étude montre que les feux et les coupes ont des impacts significatifs qui diffèrent selon le type de perturbation. En général, les lacs affectés par les coupes et les feux ont des concentrations de phosphore 2 à 3 fois plus élevées que les lacs de référence. Toutefois, les lacs affectés par les feux montrent des concentrations en nitrates beaucoup plus élevées tandis que les lacs affectés par les coupes sont moins transparents et ont des eaux plus colorées, très riches en carbone organique dissous. Les feux et les coupes affectent aussi de façon différente la minéralisation des eaux et les concentrations des ions majeurs. Les feux et les coupes n'ont pas d'effet sur la biodiversité des communautés planctoniques, sauf dans les lacs ayant un grand bassin versant et plus de 40 % de perturbation. Aucune différence n'a été observée entre les lacs de référence et les lacs perturbés au niveau de l'occurrence, l'abondance et la croissance des espèces de poissons. En revanche, les feux et les coupes affectent la production et la structure des communautés aquatiques. Dans les lacs affectés par les feux, l'enrichissement en azote et phosphore provoque une hausse de la concentration en chlorophylle a, et de la biomasse du phytoplancton et du limnoplancton (seston > 53 µm), en particulier des diatomées, des rotifères et des gros crustacés, pendant deux à trois années après les feux. En revanche, dans les lacs affectés par les coupes, l'enrichissement en phosphore n'entraîne qu'une augmentation très faible et limitée à un an de la biomasse des algues et pas de changement dans les biomasses du limnoplancton, en raison de la forte couleur et de la transparence réduite des eaux, qui limite la production biologique. Les coupes ont un impact négatif sur les copépodes calanoïdes, un groupe de zooplancton caractéristique des lacs oligotrophes et transparents. Une diminution significative de la proportion des individus de petite taille a été observée chez les populations de perchaude et de meunier noir des lacs perturbés (lacs de coupe et de feux formant un seul groupe). Les impacts observés dans la qualité des eaux et la production du plancton augmentent avec l'importance relative des perturbations au niveau du bassin versant. Cependant, la variabilité dans les caractéristiques des bassins versants et des lacs, en particulier le rapport de drainage et le temps de résidence des eaux, a des effets confondants sur les réponses des écosystèmes lacustres aux perturbations par les feux et les coupes.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

Les réseaux trophiques lacustres: structure, fonctionnement, interactions et variations spatio-temporelles

L'analyse comparative des réseaux trophiques lacustres est d'un grand intérêt pour le développement de la limnologie contemporaine et l'aménagement des lacs. L'analyse des mécanismes écologiques déterminant la structure et le fonctionnement des réseaux trophiques dans les lacs tempérés a permis l'émergence de plusieurs modèles, souvent contradictoires, et suscité d'intenses débats sur le rôle respectif des ressources et des prédateurs. Par contre, dans les lacs tropicaux, les études sont en majorité descriptives et la recherche de principes généraux et de concepts unificateurs y est rare. Cette synthèse présente l'état des connaissances, les approches méthodologiques, les modèles de régulation concernant la structure et le fonctionnement des réseaux trophiques lacustres. Les réseaux trophiques semblent varier selon un gradient de situations intermédiaires entre deux modèles extrêmes : (a) les milieux à cascades trophiques intenses et à effet atténué des ressources (lacs tempérés oligo-mésotrophes) caractérisés par la présence de poissons piscivores et de zooplancton herbivore de grande taille (tels Daphnia spp.) et (b) les milieux à régulation intermédiaire (lacs tempérés méso-eutrophes et la plupart des lacs tropicaux), caractérisés par la présence de poissons filtreurs microphages omnivores et de zooplancton herbivore de petite taille. Notre synthèse souligne aussi l'importance d'allier les approches expérimentales en enceintes ou par biomanipulation à des suivis à long terme et des modélisations pour avoir une bonne compréhension et des prédictions précises du fonctionnement des écosystèmes lacustres à différentes échelles spatiales et temporelles et pour différentes conditions climatiques, géographiques ou trophiques.Comparative analysis of lake food webs is a focal point of research in contemporary limnology and lake management. The study of ecological processes determining foodweb structure and function lead to the emergence of constrasting hypotheses and intense debates on the relative role of nutrients and food web structure in regulating temperate lake ecosystems. In contrast, studies in tropical lakes are in general descriptive and the search for integrate concepts and models is yet in development. This review paper presents an overview and a critical analysis of actual knowledge, methodological approaches, regulation models and controversies on foodweb structure and function in temperate and tropical lakes. Our synthesis suggests that the apparent diversity in models of lake foodwebs could reflect a gradient (or contiuum) of intermediate foodweb situations, regulated by various environmental factors. The differences among lakes could be related to three main biotic factors, independently of the climatic, geographical and trophic conditions: 1. the important cascading effect of strictly piscivorous fish in temperate lakes compared to the weak cascades induced by opportunistic omnivorous fish in tropical lakes, 2. the primacy of omnivory and opportunistic feeding behaviour of tropical fish, 3. the key role of herbivorous macrozooplankton (cladocerans, mostly Daphnia spp.) in temperate lakes where they are both selective preys of planktivorous fish and efficient grazers of nanophytoplankton, and 4. the synchronous reproduction of fish with seasonal plankton succession in temperate lakes, compared to continuous reproduction of fish and lack of seasonal coupling in tropical lakes. Consequently, food webs regulation ranges along a gradient of situations with two extreme models: 1. a model with intense cascading (top-down) regulation and attenuation of bottom-up effects typical of oligo-mesotrophic temperate lakes, characterized by the dominance of piscivorous fish and large herbivorous zooplankton (Daphnia spp.), and 2. a model with intermediate regulation encountered in eutrophic temperate lakes and most of tropical lakes, characterized par the dominance of filter omnivorous fish and small size zooplankton. Our synthesis also emphasizes the importance of coupling experimental approches in mesocosms or whole-lake biomanipulation with long-term monitoring and modelisation to fully understand and predict the functionning of lake ecosystems over different spatial and temporal scale

Réponses de trois espèces sentinelles aquatiques à la contamination en métaux traces dans deux rivières de l'Abitibi, Québec

La métallothionéine (MT) est un ligand protéinique intracellulaire important à cause de son implication dans les processus de régulation des métaux traces essentiels et non-essentiels. Bon nombre de travaux ont été conduits en laboratoire pour bien comprendre les mécanismes de synthèse de cette protéine, mais les études réalisées en milieu aquatique sont beaucoup plus rares, particulièrement dans les rivières (milieu lotique). Dans ce contexte, la présente étude avait deux objectifs principaux : (1) identifier en milieu lotique les métaux traces (argent "Ag", cadmium "Cd", cuivre "Cu", nickel "Ni", plomb "Pb" et zinc "Zn") susceptibles d'induire la synthèse de la MT chez trois espèces sentinelles, soit le mollusque bivalve Pyganodon grandis, la larve d'éphémère Hexagenia limbata et le grand brochet du nord Esox lucius ; (2) définir les variables physico-chimiques ou toxicologiques pouvant influencer la prise en charge des métaux traces et conséquemment les concentrations en MT chez le mollusque P. grandis et l'éphémère H. limbata. Deux rivières de la région de l'Abitibi ont été échantillonnées à l'été 2001 afin de récolter les espèces sentinelles à 21 stations. Les concentrations en métaux traces, ainsi qu'en métallothionéine, ont été mesurées dans les branchies des mollusques, dans le rein des brochets et dans le corps entier des éphémères. La métallothionéine a été mesurée à l'aide d'une technique de saturation au mercure. D'après nos résultats, le cadmium tissulaire semble être le meilleur prédicteur de la concentration de MT chez les trois espèces sentinelles. Cependant, l'implication d'autres métaux est également possible. Dans le cas de P. grandis, la concentration de manganèse (Mn) dans les sédiments pourrait influencer la prise en charge du Cd et la synthèse de MT.Metallothionein (MT), an intracellular metal binding protein, is recognized for its role in the regulation of essential and non-essential trace metals. The mechanisms of synthesis of this protein have been studied in the laboratory but much less work has been carried out in the field, particularly in lotic environments. According to the literature, metallothionein protects tissues from metal damage, and may be used as a direct measure of the biochemical state of the affected tissue. Metallothionein shows high affinity for group IB and IIB metals ions (e.g. Cd, Cu, Zn), and consequently can sequester and detoxify metals. The use of MT as a biomarker for elevated levels of trace metals in aquatic environments has therefore been proposed as a tool in ecotoxicological studies. Changes at the biochemical level offer distinct advantages as biomarkers, since molecular alterations are normally the first detectable responses to environmental changes.The present study focused on two objectives, the first of which was to identify which trace metals (silver "Ag", cadmium "Cd", copper "Cu", nickel "Ni", lead "Pb" and zinc "Zn") may induce MT synthesis in three sentinel species collected in two northern Quebec rivers (Colombière River, close to Val d'Or, and Matagami River close to the town of Matagami) in a mining region (Abitibi, Québec). The three sentinel species were the adult bivalve Pyganodon grandis, the larval mayfly Hexagenia limbata, and the adult great northern pike Esox lucius. The second objective was to define variables that could influence the uptake of these trace metals and consequently the concentrations of MT in the sentinel species. The selection of these sentinel species was based on the following criteria: their relative abundance in the aquatic systems studied; their relative immobility; the ease of sampling; metal tolerance; metal bioaccumulation capacity; dynamics of metal accumulation; capacity to synthesize metallothionein; and availability of physiological and behavioural data.In 2001, physico-chemical conditions (e.g.: temperature, oxygen, transparency, phosphorus, chlorophyll a, etc.) were monitored during two sampling campaigns at 21 stations. Trace metals were measured in water and sediment in all stations, and in tissues of the sentinel species that were collected when present at the station (P. grandis - gills; H. limbata - whole body; E. lucius - kidney). Dialysis cells were used to collect water samples for the determination of concentrations of total dissolved metals, major anions and cations, humic and fulvic acids, and dissolved organic carbon. At each sampling site, divers collected three replicate sub-samples from two sediment cores for metal analyses. The cores were extruded in the boat and samples taken from the uppermost 0.5 cm, i.e. from the oxidized layer.The two rivers differ in their morphometry, their physico-chemistry, and their biology. The Allard River is larger (200 m vs. 7 m) and deeper (30 m vs. 3 m) than the Colombière River. The Colombière River is slightly more acid (pH: 6.2 - 6.7) and less productive (14 - 32 µg total P L-1) than the Allard River (pH: 6.7 - 7.4; 24 - 45 µg total P L-1). However, the rivers present some similarities such as slow current conditions and limited water transparencies. The range of Ag, Cu and Ni concentrations in water and sediment was greater in the Colombière River, whereas Pb concentrations were higher in the Allard River and Cd concentrations were similar in the two rivers. Zinc was the dominant metal in both rivers, but variations were more significant in water in the Colombière River. Although trace metal and MT concentrations were highly variable among species, stations and rivers, no consistent spatial gradients were detected in animals collected downstream from mining activities.As was the case for the ambient environment, Zn was also the dominant metal in organisms of both rivers. Zinc concentrations were similar in mayflies from the two rivers, but higher in molluscs and lower in pike collected from the Colombière River than in comparable animals from the Allard River. Following Zn, Cu was the second metal in importance in species from the two rivers, whereas the concentrations of Ag were low in all organisms and were undetectable in pike. Cadmium was more concentrated in mayflies and molluscs from the Colombière River than in those from Allard River, and also higher in the pike collected in the contaminated zone of the Colombière River. The concentrations of Ni in all species were comparable between rivers, but were more elevated in mayflies than in the other sentinel species.The range of Pb concentrations was comparable between rivers for all species, but concentrations were lower in the pike (kidney). The concentration of MT was higher in the pike than in the two other species. Mayflies had higher MT levels in the Allard River, while MT concentrations were higher in molluscs of the Colombière River. The relationships between metals and MT were very variable between rivers and species. Nevertheless, MT concentrations were most strongly related to those of accumulated Cd in the three sentinel species. The relationships between MT and Cd for all three species were stronger in the Allard River than in the Colombière River. In the case of P. grandis, because the number of stations where the bivalves could be collected from the Colombière River was low (n=4), no statistically significant relationship was observed between MT and Cd in the tissues, in spite of the high correlation coefficient (r=0.98). For the bivalves, manganese (Mn) concentrations in the sediment were negatively correlated with accumulated Cd concentrations and with MT levels. In H. limbata, in addition to Cd, other metals such as Cu (-) and Zn (+) also exhibited weak relationships with MT. Finally, in bivariate analyses, Pb concentrations in pike liver were significantly correlated with those of MT in this species, but based on multiple regression models, no variable other than Cd affected the level of MT in E. lucius