FR :

La composition, l'abondance et la biomasse de la communauté du picoplancton autotrophe (PPA, 0,2-2 m) ont été examinées dans un lac situé en Pologne et récemment considéré comme méromictique, le lac Czarne. Les échantillons d'eau ont été prélevés tous les mètres dans la colonne d'eau, en mars et en juillet 1998. Pendant ces deux dates, le PPA se caractérisait par des changements significatifs de densité dans la colonne d'eau. Au printemps, l'abondance la plus forte a été observée à 9 m (2,1·10⁵ cellules·mL^-1) tandis qu'en été elle est observée à 5 m (3,1·10⁵ cellules·mL-1). À toutes les profondeurs, le PPA était dominé par des Cyanobactéries. Au printemps, dominent les cellules libres de picocyanobactéries, contrairement à l'été où les cellules coloniales étaient prépondérantes. La contribution des cellules eucaryotes était faible en termes d'abondance (<20%) et bien plus élevée en termes de biomasse, avec une valeur moyenne de 33% au printemps. La biomasse moyenne du PPA était plus élevée au printemps (12 µgC·L^-1) qu'en été (8 µgC·L^-1) et des différences nettes sont observées entre les saisons dans sa distribution en fonction de la profondeur.

EN :

The composition, density and biomass of autotrophic picoplankton communities (PPA, 0.2-2 µm) were investigated in the meromictic and mesotrophic Lake Czarne (area 19.6 ha, water volume 2.2·10⁶ m³, max. depth 28 m, mean depth 11.2 m). It is located within the Drawa National Park (NW Poland), in extensive lowlands, at an altitude of 55 m. Water samples were taken in the deepest part of the pelagic zone, in 1-m intervals from surface to bottom, in late March and July 1998. The organisms were analysed by means of an epifluorescence microscope after collection by filtration on black Nuclepore filters of 0.2 µm pore size

Pelagic autotrophic picoplankton (PPA) taxa occurred in all water samples and the community was composed of cyanobacteria and eukaryotes (i.e. small chlorophytes). The cyanobacteria (Pcy) included Synechocystis, Aphanothece and Aphanocapsa species. Among eukaryotes (PAE), Choricystis minor (Skuja) Fott and Pseudodictyosphaerium jurisii (Hindák) Hindák were identified. These two chlorophytes were characterized by a larger mean cell size than Pcy. The most abundant species was the eukaryotic P. jurisii in spring and the prokaryotic Aphanothece bachmannii Komárková-Legnerová et Cronberg in summer (both species formed colonies). All PPA taxa in Lake Czarne are common, found in various lake types.

The total abundance of PPA varied within three orders of magnitude: from 1·10³ cells·mL^-1 to 3·10⁵ cells·mL^-1. During both sampling sessions, the PPA communities were characterized by a substantial variation in abundance within the water column and mean values were 8.5·10⁴ cells·mL-1 in spring and 5.4·10⁴ cells·mL^-1 in summer. In March, at a depth of 0 11 m, cell numbers were never lower than 105 cells·mL^-1. The highest density was observed at a depth of 9 m in March (2.1·10⁵ cells·mL^-1) but at 5 m in July (3.1·10⁵ cells·mL^-1). In the deeper, anoxic water layers, PPA abundance was lower and less variable with about 10³ cells·mL^-1 in March and about 10⁴ cells·mL^-1 in July. The presence of PPA deep in the pelagic zone (in the aphotic and anoxic zones) during both sampling sessions may result from sedimentation, although its importance in the case of the small cells of PPA was probably lower than in the case of the larger organisms. In the study lake, because of the stability of the lower layers of water (i.e. lack of mixing with the overlying layers), that layer was not a source enriching the upper layers with PPA.

At all depths Pcy were the most numerous components of the PPA community. This confirms the numerous reports about the dominance of Pcy within PPA. Single-celled picoplanktonic cyanobacteria (S-Pcy) dominated in spring, accounting for 82 98% (mean for the water column: approximately 90%) of the total Pcy abundance, whereas cells forming colonies (C-Pcy) prevailed in summer, reaching over 89% (mean for the water column: about 30%) of the total Pcy abundance. PAE were much less numerous or even absent in some water samples. In March, PAE abundance varied between 1.1·10² cells·mL^-1 and 1.5·10⁴ cells·mL^-1 (mean for the water column: 4.6·103 cells·mL^-1), and was the highest in the middle part of the water column. The contribution of PAE to the total PPA abundance ranged widely but did not exceed 20%. In July, PAE were less numerous and their presence was detected only in the bottom layer of water (9 m). Their contribution to the total PPA abundance did not exceed 10% (mean about 3%). Hence those results confirm that PEA prefer colder and nutrient-rich waters.

The total biomass of PPA varied in March from 0.2 µg C·L^-1 to 31 µg C·L^-1 (mean value for the water column: 12 µg C·L-1) and in July from 0.8 µg C·L^-1 to 49 µg C·L^-1 (mean: 8 µg C·L^-1). Thus the mean PPA biomass was 1.5 times higher in March than in July. During both sampling sessions PPA biomass was higher in the upper and middle layers of water than in the monimolimnion. During the first sampling session, values >10 µg C·L^-1 were recorded to the depth of 14 m, while during the second session biomass clearly concentrated in the upper part of the water column, to a depth of 7 m. Within PPA, usually Pcy dominated in terms of biomass. In March their contribution to the total PPA biomass varied from 35% to 88% (mean: 67%) and in July from 65% to 100% (mean: 88%). PEA biomass was higher in March than in July and mean values were respectively 3.0 µg C·L^-1 and 0.3 µg C·L^-1, accounting for 33% and 12% of the total PPA biomass. In July, PAE biomass was everywhere lower than Pcy biomass. In contrast, in March, PAE biomass exceeded Pcy biomass at some depths in the lower part of the water column. The contribution of PAE to the total PPA biomass was higher than to the total PPA abundance, especially in March, when PAE were the most numerous. Generally, the contribution of eukaryotic cells was smaller in terms of abundance and much higher in terms of biomass, because of the larger mean size of cells.

FR :

La lagune de Ghar El Melh, située au Nord de la Tunisie (dans le Golfe de Tunis), est caractérisée par une superficie de 28,5 km², une faible profondeur et une faible communication avec la mer. Actuellement, sous l'effet des actions anthropiques (les rejets terrestres), elle connaît une dégradation progressive de la qualité de ses eaux et sédiments dont les conséquences sont la chute de la richesse biologique et la réduction des ressources halieutiques de la lagune.

Dans ce travail, nous présentons les résultats de l'analyse des caractéristiques physico-chimiques et biologiques des eaux de la lagune. Ces analyses montent bien l'hyper-eutrophisation du milieu caractérisé par de fortes concentrations des eaux en azote total et en phosphore total. Les résultats de simulation du fonctionnement hydrodynamique, que nous avons réalisée à l'aide d'un modèle à 2 dimensions intégré sur la hauteur, montrent une stagnation des eaux dans environ 80% de surface de la lagune qui a fortement amplifié la dégradation de la qualité de l'écosystème. Parmi plusieurs scénarios d'aménagements simulés, nous proposons la création d'une nouvelle communication avec la mer dont la simulation hydrodynamique montre une nette augmentation des échanges d'eau mer-lagune. Nous présentons ensuite un modèle écologique homogène de la lagune de Ghar El Melh qui a été mis au point et calibré en s'appuyant sur les mesures de terrain. Les simulations, avec ce modèle, montrent que l'arrêt total des rejets terrestres et la création d'un nouveau grau réduisent considérablement l'eutrophisation du milieu ce qui conduiraient certainement à une nette amélioration de la production halieutique dans la lagune.

EN :

The Ghar El Melh lagoon is a Mediterranean water body, situated in Northeastern Tunisia, on the Northwestern side of the Gulf of Tunis. The Ghar El Melh lagoon is a vestigial part of the Utique Sea, and this lagoon was largely open at the time of the Roman invasion. Due to a combination of the shape of the coastline and alluvium deposits from the Medjerda River, this small gulf has become progressively closed from the Utique Sea, causing the lagoon to become progressively shaped to its present morphology. The coastal barrier separating the lagoon from the Mediterranean Sea was interrupted, allowing a permanent hydraulic communication across a local opening. The lagoon has an elliptical shape of approximately 28.5 km2 and an average depth of 0.8 m. Due to human activities within the lagoon itself and in the surrounding area, the lagoon ecosystem has suffered a progressive deterioration. This deterioration has led to a reduction in biodiversity resulting mainly in a decrease in fish resources and production.

In order to improve the water quality of the lagoon for ecological and economical purposes, a diagnostic survey was carried out to characterize the present site conditions and to provide data for calibration of hydrodynamic and ecological models. The main results of the annual survey of biotic and non-biotic parameters demonstrated the existence of zones within the lagoon, with some seasonal variation. Thus, a decreasing biotic and non-biotic gradient was measured from the area of the lagoon under marine influence towards the bottom of the lagoon, under the continental and anthropogenic influence of the west side. The lagoon could be considered hypereutrophic with an annual average concentration of total phosphorus of 350 mg/m³.

In order to conceptualize and optimise hydraulic structures for water quality improvement in the lagoon, a numerical model of the velocity fields (depth averaged) and water depth was used. The hydrodynamic model used was bi-dimensional, adapted for use in shallow lagoons. After model calibration using in-situ measurements, simulations were carried out to analyse the present hydrodynamic condition of the lagoon. Several stagnation zones were detected, which contributed to the altered water quality that was observed. Several management practices were proposed and simulated, aiming to control and improve the internal circulation and water exchange between the lagoon and the Mediterranean Sea. The main goal of these simulations was to improve water mobility inside the lagoon, and thus improve the water quality. The proposed hydraulic development measures consisted mainly of the creation of a new hydraulic communication in the south-eastern area of the lagoon, by dredging the south part of the lake or by channel creation in front of the new communication, reaching the stagnation zones of the lagoon. The impact assessment of the proposed development was verified with simulations using the bi-dimensional hydrodynamic model.

An ecological model based on nitrogen and phosphorus cycling, was also developed for Ghar El Melh lagoon. It took into account one ecological compartment, the macro algae, that included Cladophora sp., Ulva sp. and Enteromorpha sp. Algae (A), inorganic nitrogen (NA), inorganic phosphorus (PA), organic nitrogen (NEorg) and organic phosphorus (PEorg), were the main variables of this model. After calibration of the model, a 10-yr simulation showed that all variables demonstrated a steady behaviour and that the lagoon eutrophication level remained. Model sensitivity analysis allowed the choice of some restoration scenarios and the prediction of their impacts on the ecological behaviour of the ecosystem. The simulations showed that wastewater load deviation combined with an increase in the sea-lagoon water exchange, instead of a decrease of nutrient diffusion from the sediments, led to a substantial decrease in the eutrophication level of the lagoon. Indeed, the annual average nutrient concentrations decreased from 270 to 60 mg/m³ for total nitrogen and from 350 to 20 mg/m³ for total phosphorus in the Ghar El Melh lagoon.

FR :

Les sédiments utilisés dans cette étude sont prélevés dans deux milieux différents (bassin de maturation du lagunage de Boujaad et étang d'alevinage de la station Deroua, Béni-Mellal) dans le but de comprendre les conditions du milieu qui provoquent les échanges du phosphore à l'interface eau-sédiment et de comparer les échanges du phosphore à ce niveau.

Cette étude a montré une différence dans la composition chimique des deux sédiments. En effet, au niveau de l'étang d'alevinage, le phosphore inorganique représente une part majoritaire (92,5%) surtout liée au calcium. En revanche, le phosphore total est réparti en phosphore organique (49,2%) et inorganique (49,8%) au niveau du bassin de maturation.

Dans le bassin de maturation, le relargage du phosphore est plus important et il est sous forme inorganique lié essentiellement aux hydroxydes de fer. La fixation du phosphore par les sédiments est importante particulièrement en conditions aérobies, aussi bien dans le bassin de maturation que dans l'étang d'alevinage.

EN :

Sediment samples used in this work were taken from two different sites (maturation pond of Boujaad lagunage and fry pond of the Deroua fish farm) in order to understand and compare the conditions affecting phosphate exchanges across water-sediment interface.

This study showed a difference in the chemical composition of the two sediments. In the fry pond, the majority of the phosphate was present as inorganic phosphate (92.5%), especially in forms bound to calcium. On the other hand, total phosphate was divided equally into organic (49.2%) and inorganic (49.8%) phosphorus in the maturation pond. In this pond, phosphate solubility increased in the interstitial water under anoxic conditions. The phosphate released came principally from the inorganic fraction, particularly from Fe(OOH)-P. The maturation pond sediments, which had accumulated much organic matter, liberated more orthophosphate than did the fry pond sediments. Under air flushing, phosphate adsorption is significant particularly in the maturation and fry pond sediments.

FR :

Les zones méditerranéennes et semi - arides sont caractérisées par des régimes pluviométriques irréguliers dans le temps et dans l'espace. L'érosion hydrique des bassins versants et la sédimentation des cours d'eau constituent des phénomènes complexes très répandus qui posent de nombreux problèmes aux ingénieurs chargés des projets de mise en valeur. En raison de son ampleur, cette érosion constitue une contrainte majeure au développement de l'agriculture et à la promotion des activités rurales en Algérie. Une approche statistique de quantification des apports solides est développée dans cette étude. Les données utilisées sont recueillies au niveau de cinq sous-bassins du bassin versant de l'Oued Mina drainés par les stations hydrométriques, en amont du barrage de Sidi M'hamed Ben Aouda. La période d'observation s'étale sur 22 ans (1973 à 1995). Il existe une grande variabilité inter-annuelle et intrannuelle des apports liquides et solides. En plus de cette variation temporelle, il a été remarqué une variabilité spatiale des apports solides et liquides.

Les valeurs maximales du transport solide sont enregistrées au début d'automne et à la fin du printemps. Le transport solide d'automne reste le plus élevé, dépassant significativement les autres saisons. Cette variabilité trouve son explication dans la pauvreté du couvert végétal durant cette saison et la nature agressive des pluies d'automne. La charge spécifique du bassin versant de l'oued Haddad est de l'ordre de 212 t/km²/an. Le bassin versant de l'oued Taht a une charge spécifique inférieure à celle du bassin de l'oued Haddad (Ds=191 t/km²/an). Les bassins versants de l'oued el Abd présentent des charges spécifiques de l'ordre de 117 t/km²/an à Ain El Hamara et de 65 t/km²/an à Takhmaret. La charge spécifique de l'oued Mina au barrage de Sidi M'hamed Ben Aouda est de 160 t/km²/an.

Des modèles mathématiques liant l'apport solide au coefficient d'écoulement ont été proposés pour les cinq sous-bassins drainés par les stations hydrométriques.

EN :

Mediterranean and semi - arid zones are characterized by an aggressive climate and by irregular and violent rainfall events. Water-induced erosion in river basins and river sedimentation constitute a complex phenomenon, which poses many problems for the engineers responsible for development projects. This phenomenon constitutes a major constraint limiting agricultural development in Algeria. We propose a statistical approach to quantify the solid yield at different gauging sites.

The data were collected at five sites in the Oued Mina basin (upstream of the Sidi Me hamed Ben Aouda dam). The observation period covered 22 years, from 1973 to 1995. The results showed a high inter-annual and annual variability of the liquid discharge and solid yields; the coefficient of variation exceeded 60% in all cases. In addition to this temporal variation, we noted a variability of the solid yields and liquid discharge among the different sites. Analysis of the monthly solid yields shows that the autumn season contributes a large proportion of the annual solid yield (more than 40%) for the different basins. Tranport of solids in the Oued Mina and its effluents is highly variable in space and time. This variability is due primarily to the pluviometric regime, the vegetation cover and the lithology. In general, the seasonal variations of the solid contributions follow those of the liquid flow. The maximum values of the solid discharges, for all basins, are observed at the beginning of autumn and the end of spring. Generally, the monthly minimal solid yields are recorded in summer. This season is characterized by low rainfall and consequently a weak surface flow. However, the minimal solid yields do not necessarily correspond to the month characterized by the lowest liquid discharge. The autumn contributes 31% to the annual liquid flow and 51% to the annual solid yields of the Oued Mina area at Wadi El Abtal station; corresponding values are (26 %, 33 %), (33%, 53 %), (29%, 46%) and (36%, 48 %) at the stations of Sidi A.E.K Djillali, Ain Hamara, Kef Mehboula and Takhmaret, respectively. The autumn solid transport remains highest, significantly exceeding the other seasons. This variability is explained by the variation in vegetation cover (bare ground) during the year and the aggressive nature of the autumn rains. Because of these two factors, the first autumn flood transports considerable quantities of sediments after a long dry season characterized by high temperatures and destruction of the soil aggregates by these temperatures. The energy produced by the first rains and the low degree of vegetative cover rate in this season also contribute to this increase in autumn. The Oued Haddad basin presents a high specific degradation compared to the other basins (Ds=212 t/km²/an). This impoverishment of the soil is very visible at this site. It is due to factors favouring erosion : steep topography and discontinuous vegetative cover, which protects the ground poorly. The Oued That basin, which has a topography favouring surface flow and abundant rains, has a specific degradation lower than the Oued Haddad basin (Ds=191 t/km²/an). The basins of Oued El Abd present a specific degradation about 117 t/km²/an at Ain El Hamara station and about 65 t/km²/an at Takhmaret station. The Oued Mina at Sidi M'hamed Ben Aouda dam gives a specific degradation about 396 t/km²/an.

The runoff coefficient explains a large part of the variance of the solid yield (more than 60%). This coefficient depends intrinsically on lithology, geology, vegetative cover, topography and the hydrological and pluviometric regimes. By the introduction of this parameter, we sought to explain solid transport by the explanatory variables of this phenomenon. The result was very encouraging, considering the percentage of the explained variance (more than 60%). Following this analysis, mathematical models are proposed, connecting the solid yield to the runoff coefficient. The determinations coefficients are equal to : 0.79; 0.61; 0.64; 0.78; 0.65 for the stations of oued Al Abtal, Sidi Abdelkader Djillali, Ain Hamara, Kef Mehboula and Takhmaret, respectively.

FR :

Ce travail concerne l'étude de la qualité physico-chimique de la masse d'eau du réservoir Daourat au Maroc ; situé sur le cours inférieur du bassin Oum Rbia. Une vidange du réservoir a lieu le 23 décembre 2001, l'impact de cette vidange sur la qualité physico-chimique de l'eau a été déduit à travers l'augmentation du débit à l'entrée et à la sortie du réservoir en décembre ; l'élévation du taux de la matière en suspension ; des concentrations en éléments nutritifs et de la chlorophylle " a ". Cette étude inclue également une analyse de régression qui a permis de déterminer une relation entre le phosphore et la chlorophylle " a " ; ainsi qu'une évaluation du niveau d'eutrophisation du réservoir Daourat.

Les résultats fournis par les paramètres physico-chimiques ont révélé que la masse d'eau ne présente aucune stratification thermique ni anoxie et que le pH est alcalin. L'évolution des concentrations en éléments nutritifs dans la masse d'eau du réservoir dépend des apports exogènes et endogènes. Les apports exogènes sont dominés par les apports latéraux venant du bassin versant local peu boisé et caractérisé par un sol à vocation agricole responsable de l'enrichissement du milieu récepteur en éléments azotés. Les apports endogènes sont gouvernés par les interactions eau - sédiment suite à la minéralisation de la matière organique en période estivale et par le relargage d'éléments à partir du sédiment suite à la vidange et à la crue en période hivernale. L'impact de la vidange sur l'écosystème étudié a été déduit à partir des modifications des paramètres abiotiques et biotique enregistrés en décembre (période de vidange) dans le milieu du réservoir (S2) et son aval (S3). L'étude de la relation entre les orthophosphates et la chlorophylle " a " dégage une forte corrélation significative. Les données du phosphore et de la chlorophylle " a " classe le réservoir Daourat dans l'état hypereutrophe

EN :

The Oum Rbia River is the longest river in Morocco with a total length of about 550 km. Studies conducted on this river have shown that agricultural activity is the main factor affecting the water quality. The current study focused on Daourat Reservoir, a part of the Oum Rbia River.

The aim of this study was to investigate the water quality of the Daourat reservoir (physico-chemical parameters, nutrients and chlorophyll a) in order to estimate its eutrophication status. The influence of draining reservoir on water quality was also analysed. This survey also included a regression analysis that permitted the determination of the relationship between phosphorus and chlorophyll a, as well as an assessment of the level of eutrophication of the Daourat reservoir.

A monthly survey was conducted to monitor changes in the hydrological cycle as well as possible seasonal variations during the period from 2001-2002. Water parameters (T°, pH, dissolved oxygen, suspended solids), nutrients (NH₄ ⁺, NO₃^-, P-PO₄, Tot-P) and chlorophyll a were determined according to standard French protocols (AFNOR, 1994). Annual nutrient inputs and outputs were calculated from the monthly discharge from the river and the monthly mean nutrient concentrations. The annual nutrient output was calculated similarly.

The physico-chemical parameters and nutrient concentrations varied temporally in the Daourat Reservoir. The highest concentrations of nutrients were observed during summer and winter, with the lowest concentrations occurring in the spring. This evolution seemed to be influenced by the nature of the loads. Nutrient loading from the upstream regions of the reservoir was minimal compared to the contributions from the local catchment area and from internal loading from the sediments. Significant nutrient loading to the reservoir came from sediment resuspension on December 23, 2001 and by internal loading from sediments during the summer. The upstream reservoirs (Al Massira and Imfout) had minimal influence and were in fact responsible for the reduced fluvial contributions to the Daourat Reservoir. The increase in nutrient loads in the reservoir was responsible for high phytoplankton biomass, indicated by the high chlorophyll a values (mean 35 µg/L). The trophic state of the Daourat Reservoir was classified as hypereutrophic. Due to the large intra-annual variability of water quality, a properly designed monitoring program of reservoir water is recommended.

FR :

Au Maroc, plusieurs retenues de barrages connaissent des problèmes de la dégradation de leur qualité d'eau due essentiellement aux différentes sources de pollution. L'une des conséquences de cette situation est l'enrichissement de l'eau par les nutriments (N et P) indicateur d'un état avancé de l'eutrophisation. Ce phénomène provoque un déséquilibre de l'écosystème avec un développement anarchique d'algues et une consommation intense de l'oxygène au fond des lacs.

La Direction de la Recherche et de la Planification de l'Eau a conduit depuis la fin des années 80 un programme de surveillance de la qualité des eaux des retenues de barrage dans le but de proposer des solutions pour réduire ses conséquences.

Le présent travail consiste à présenter les résultats de l'eutrophisation des eaux de deux retenues de barrages : Smir située au Nord du Maroc qui sert à alimenter la zone côtière entre Tétouan et Sebta en eau potable, et Sahla située au Nord-Est du Maroc qui sert à alimenter la ville de Taounate en eau potable et protéger la grande retenue Al Wahda contre l'envasement et l'eutrophisation.

Les résultats concernent l'exploitation des données physico-chimiques et biologiques selon un profil vertical de profondeur. Entre 1993 et 1998 pour Smir et entre 1996 et 1998 pour Sahla.

Les paramètres physico-chimiques, transparence et phosphore total ne semblent pas être de bons descripteurs de la qualité des eaux de ces retenues se trouvant dans les zones semi-arides du fait de l'importance de la charge solide particulaire non bio-disponible pour le phytoplancton. Par contre la concentration en chlorophylle a semble être représentative du niveau trophique puisqu'elle donne une idée réelle du développement algale.

Un état trophique des deux lacs réservoirs est dressé, et une comparaison de la qualité des eaux est faite en tenant compte des différentes mesures prises par le gestionnaire de la ressource en eau pour atténuer le phénomène de l'eutrophisation.

EN :

In Morocco, many reservoirs have known water quality problems due to the different pollution sources to which they are subjected. One of the consequences of this situation is the enrichment of the water by nutrients (such as phosphorus and nitrogen), leading to eutrophication. This phenomenon causes ecological imbalance with an excessive development of algae and a high oxygen demand in the bottom of lakes.

Since the end of the 1980s, the Directorate of Water Research and Planning has been conducting a water quality monitoring program in reservoirs to understand the eutrophication phenomenon and to suggest solutions in order to reduce its possible impacts.

In this paper we present the results of a case study of eutrophication in two reservoir lakes: the Smir reservoir, located in northern Morocco with a capacity of approximately 43 million m3, and the Sahla reservoir, located in north-eastern Morocco with a capacity of approximately 62 millions m3. The Smir reservoir is used to supply potable water for the northern coastal region, whereas the Sahla reservoir supplies potable water to the city of Taounate and as well protects the reservoir Lake Al Wahda.

The results comprise a data set for a special experimental water quality monitoring program intended to monitor the evolution of physical, chemical and biological parameters along a vertical profile at the deepest point of the reservoirs. Samples were collected from 1993 to 1998 for Smir, and from 1996 to 1998 for Sahla.

The physical-chemical, transparency and total phosphorus parameters do not appear to reflect the trophic status for these reservoir lakes located in the hot regions of semiarid areas. Indeed the solid particles were not bio-available for phytoplankton. However the chlorophyll concentration appears to be representative and indicative of increases in algal growth. The trophic state of the two reservoir lakes was addressed and a comparison of water quality was carried out in relation to different considered measurements.

FR :

L'étude des variations des descripteurs physiques, chimiques et biologiques des eaux d'un réservoir de maturation du lagunage mixte (Ben Slimane, Maroc), a montré une augmentation des valeurs de certaines variables (oxygène dissous, transparence) après le 36ème jour de rétention en raison d'une remise en solution.

A 36 jours de rétention, la chute des teneurs de la DBO₅, de la DCO, des composés azotés (NH₄⁺, NO₃^-, NTK) et phosphorés (PO₄^3-, PT), et l'augmentation du pH, de l'oxygène dissous et de la transparence laissent présager une maturation à cette date de mesure. Les densités planctoniques enregistrées à ce niveau sont les plus basses.

Au-delà de ce stade de maturation, le phytoplancton prolifère avec dominance d'Euglena viridis qui montre un pic de densité (4,1.10⁶ Individus/m³) le 81^ème jour. Le début de sa phase exponentielle de croissance est observé à partir du 18^ème jour de maturation. La densité cellulaire est ensuite atténuée vraisemblablement en raison de la diminution de la charge organique (DBO₅ et DCO) après le 81^ème jour et de l'intensité de broutage par le zooplancton herbivore surtout Acanthocyclops robustus qui montre son maximum de croissance le 138^ème jour.

Après le 81^ème jour de maturation, Euglena viridis est remplacée par deux espèces de Chlorophycées (Pediastrum boryanum et Ankyra judai) dont le maximum de densité (91% de la densité algale totale) est atteint le 138^ème jour. La prolifération de ces espèces coïncide avec des valeurs en nitrates dépassant 10,6 mg N/L.

A la fin de cette période d'étude (le 138^ème jour), le zooplancton est à sa prolifération maximale (10⁶ individus/m3). Il est dominé par Acanthocyclops robustus avec présence de Nauplii et de Daphnia magna. Les Rotifères présents depuis le début de la maturation disparaissent à 138 jours de rétention. L'étude bactériologique a révélé un rendement épuratoire remarquable atteignant 100% pour les coliformes fécaux et les streptocoques fécaux.

EN :

A study of physical, chemical and biological descriptors of the water in the storage reservoir of a combined stabilization pond (Ben Slimane, Morocco), during maturation, showed an improvement of the purification output for some variables after 36 days of retention. Such an environment, rich with a range of different nutrients, allowed an optimal development of plankton characterized by a high species diversity that decreased with time. Zooplankton development seemed to be limited by poor phytoplankton biomass. However, trophic factors such as nutrients and temperature affected the evolution of planktonic species.

After 36 days of retention, the decrease in BOD₅, COD, NH⁴⁺, NO₃^-, total Kjeldahl nitrogen (TNK), PO₄^3-, total phosphorus (TP), and the increase in pH, dissolved oxygen and transparency, suggested that maturation had been reached at this level. Simultaneously, phyto- and zooplankton were present in low densities. Nevertheless, beyond this stage of maturation, phytoplankton proliferation occurred, dominated by Euglena viridis whose exponential phase of growth began at the 18th day of maturation and reached a maximum density after 81 days (4.1x10⁶ individus/m³). A low proportion of Chlorophyte species was observed, represented by Pediastrum boryanum and Ankyra judai. The zooplankton community was dominated by two species of Cladocera (Daphnia magna and Diaphanosoma sp).

The increase in Euglena viridis density up to 81 days coincided with zooplanktonic proliferation constituted exclusively by Rotifers (Brachionus calyciflorus and Polyarthra remata). At 111 days of retention, the zooplankton community consisted of Copepods (Acanthocyclops robustus), which reappeared at this stage, as well as Nauplii Cladocera (Daphnia magna) and residual Rotifers (Brachionus calyciflorus and Polyarthra remata). The remarkable decrease in Euglena density on day 111 and their complete disappearance on day 138 was probably linked to grazing by herbivorous zooplankton and to low organic matter concentrations. Euglena viridis was replaced by two Chlorophytes (Pediastrum boryanum and Ankyra judai) reaching their maximum density (91% of total algal density) on day 138. The proliferation of these species coincided with values exceeding 10.6 mg N/L. Zooplankton were dominated by Acanthocyclops robustus with the presence of Nauplii and Daphnia magna (106individuals/m3). Rotifers were present since the beginning of the maturation, but were absent by 138 days of retention.

The qualitative and quantitative changes in phyto- and zooplankton result from factors that control the growth and the loss of these populations. A low growth rate can lead to biomass accumulation if the growth exceeds loss. On the other hand, a low and stable biomass can result from a high growth rate that is compensated by important losses. Zooplankton grazing and algal sedimentation, which are the two main mechanisms related to phytoplankton decline, are two selective processes (POURRIOT et al., 1982). In stabilization ponds, food chains are established that allow water clarity and minimize algal proliferation. The bacteriological study showed a remarkable purification, with reductions reaching 100% for faecal coliforms and faecal Streptococcus.

FR :

Le bassin versant du fleuve Sinnamary est situé en Guyane française, au nord du continent sud américain, en zone néo-tropicale. Il s'étend entre 4 et 5°de latitude Nord et entre 52°50' et 53°30' de longitude Ouest. En 1994, la mise en eau du barrage hydroélectrique construit sur le site de Petit Saut occasionne l'ennoiement de 365 km2 de forêt primaire. Le remplissage s'effectue en 18 mois, et la demande en oxygène dissous nécessaire à la dégradation de la matière organique est telle qu'en quelques jours la masse d'eau se stratifie en un épilimnion oxygéné et un hypolimnion anoxique. Représentant quelques centimètres en 1994, l'épilimnion s'épaissit progressivement jusqu'en 1998. Depuis, cette progression s'est nettement ralentie et l'épilimnion oscille autour d'une valeur moyenne de 5 à 6 mètres. Il abrite la plus grande partie du zooplancton pélagique. L'hypolimnion est anoxique et riche en éléments réduits. Le zooplancton s'est rapidement installé avec des rotifères, des cladocères et des copépodes. Des ostracodes et des Chaoboridae y sont désormais associés. La retenue présente une zonation longitudinale, de la tête de la retenue vers la queue, aussi bien en termes de densités que de composition des peuplements. De même, il existe une zonation transversale, notamment marquée par un gradient croissant de la conductivité de l'axe vers les berges. Ces ressources endogènes ainsi que des apports complémentaires (végétaux, invertébrés terrestres, détritus…) sont utilisées par la faune ichtyque qui a su s'adapter au changement du milieu. Le temps de rétention des eaux qui fluctue saisonnièrement en fonction des pluies semble être le facteur prépondérant de variabilité pour la qualité physico-chimique des eaux et les communautés biologiques.

EN :

The Sinnamary River is located in a neotropical zone, in French Guiana, in the northern part of South America. Its flood basin extends between 4° and 5° N latitude and 52°50' and 53°30' W longitude. The river is found in an equatorial climate, and is influenced by its proximity to the ocean. The temperature varies little, with an average of 25°C for the least hot month (January) and 27°C for the hottest month (October). The humidity is always high and averages more than 90%. The Sinnamary River bed is broad (> 50 m), and runs from south to north for approximately 240 km with a very weak slope (0.0003%). Petit Saut, the site of the hydroelectric dam, is located approximately 60 km from the mouth of the river, with a flood basin area of 5927 km² and an average flow rate 260 m³ /s.

The construction of the hydroelectric dam at Petit Saut in 1994 resulted in the flooding of 365 km² of primary forest in a neotropical zone (the average depth of the reservoir is 11 m). The dam reservoir was filled over a period of 18 months and the amount of dissolved oxygen necessary for the organic matter to decompose was such that, in a matter of days, the body of water became stratified into an oxygenated epilimnion and an anoxic hypolimnion. Only centimeters thick in 1994, by 1998 the epilimnion had progressively expanded. Since that time it has varied between an average thickness of 5 to 6 m and is home to various biological communities. The hypolimnion is anoxic and rich in reduced compounds (nitrogen and phosphorus minerals, dissolved organic matter, particles, metals and gas). The products of the degradation of the flooded organic matter, which still prevail at the bottom, are gradually relayed towards surface by phenomena of dilution, hydrolysis and oxidation.

The river fauna disappeared from the reservoir and was replaced after a few weeks by various pelagic fauna. The zooplanktonic communities, primarily confined to the epilimnion, were quickly settled with rotifera, cladocera and copepoda. Initially dominated by detritivores (Bosminidae) and Cyclopidae from February to May 1994, the settlements diversified with phytophagous zooplankton (Calanidae and Daphniidae) when the phytoplankton developed. This procession was quickly accompanied by Sididae, which has dominated the cladocera since mid-1996 and by ostracoda, which were particularly abundant from 1995 to 1997. Carnivores such as Chaoboridae larvae were also present in Petit Saut reservoir. In contrast to the other zooplankton species, Chaoboridae larvae occupy the entire water column. As a result of great quantities of organic matter available at the time the reservoir was filled, the biomasses were initially large and have gradually decreased with decreasing nutrient concentrations.

The reservoir was not homogeneous in terms of zooplankton density, with a longitudinal zonation from the dam towards the upstream tail. In the tail, fauna were identical to those of the river. In the lake zone, phytophagous Calanidae dominated. In the intermediate zone, known as the zone of transition, the zooplankton biomass normalized to the volume of oxygenated water was less important than in the lake zone. Cyclopidae, well represented at the time of the reservoir colonization, dominated the communities in the transition zone. This distribution of zooplankton was likely connected with the dissolved oxygen concentrations, as well as with the quantities of phytoplankton and bacterio-chlorophyll. Similarly, there was a transverse zonation, marked by an increase in conductivity towards the reservoir banks. The density of the communities varied seasonally. The highest biomasses were measured along the central axis at the time of high waters and close to the banks at the time of low waters.

The time of water retention, which fluctuates according to the rainy season, seemed to be the factor dominating the physicochemical water quality variability and the biological communities. The rainy season is characterized by the dilution of various elements (e.g., conductivity is reduced). Conversely, during the dry season, the elements concentrate again in the water mass. Nevertheless, the peaks of pelagic invertebrate density were observed during times of high waters, when the hydraulic conditions favour the exchanges between the hypolimnion (anoxic and rich in nutritive elements) and the epilimnion (oxygenated and very low in nutrients).

These endogenous resources, as well as complementary contributions (vegetation, invertebrate, terrestrial, refuse), are used by the fish fauna, which adapted to the change in their environment. Predation contributes to the seasonal evolution of the zooplankton. Predation pressure was more important in the dry season than in the rainy season.

FR :

Quelques mois après le début de la mise en eau du barrage de Petit Saut, la mise en service normale de l'usine conduisait à une désoxygénation de l'eau du tronçon de rivière aval, le rendant incompatible avec la vie aquatique. La solution retenue a été la construction d'un seuil, afin d'apporter de l'oxygène et d'éliminer les gaz réducteurs produits au fond de la retenue, notamment le méthane, consommateur potentiel d'oxygène dissous.

Un seuil métallique à deux lames déversantes successives a été construit ; sa configuration prend en compte les principaux critères physiques jouant un rôle significatif sur l'oxygénation de l'eau (hauteur de chute, épaisseur de la lame déversante, le dimensionnement du bassin de réception des chutes, la présence de dispositifs favorisant l'éclatement de la lame d'eau).

Placé dans le canal de fuite de l'usine, à une centaine de mètres à l'aval du barrage principal, il est à l'abri des crues et ne crée pas d'obstacle supplémentaire en rivière.

L'article chiffre l'effet d'aération de ce seuil pour les deux gaz O₂ et CH₄ dans deux configurations : celles consécutives à l'abaissement partiel de la chute amont réalisé en deux étapes. Après décembre 2001, pour le débit moyen turbiné (près de 200 m³ /s), l'efficacité d'aération du seuil a baissé de près de 10 % (gain de 80 % en oxygène dissous et élimination de 70 % et 75 % du méthane dissous). Après février 2003, pour un débit de 100 m3/s, 75 % du déficit amont en oxygène dissous est comblé et près de 70 % du méthane dissous éliminé.

EN :

From the moment tropical reservoirs are impounded, climatic conditions cause rapid (within several weeks) and marked thermal stratification, especially during the dry season. This phenomenon is further exacerbated by the chemical and biochemical processes taking place in the reservoir due to the decomposition of submerged organic matter. In dense tropical forests, the overhead biomass is estimated at roughly 170 t(C)/ha, and the carbon contained in the soil is also not negligible since it is on the order of 100 t(C)/ha. The degree of biodegradability of the different compounds in the flooded biomass is variable, ranging from a few weeks for bacteria to several centuries for tree trunks.

The studies carried out at Petit Saut (French Guiana) show that, immediately after impoundment, only the epilimnion (a few dozen centimetres thick) was oxygenated whereas the hypolimnion was characterized by complete anoxia and a very high methane content (about 15 mg/L). Water quality in the river downstream from the reservoir was of course strongly linked to variations in the water quality in the reservoir as well as to its operating mode. The waters passing through the turbines, coming from the bottom layers, were anoxic and loaded with fixed or volatile reducing compounds (e.g., CH₄, H₂ S), and were responsible for a high immediate or progressive oxygen demand. At Petit Saut, despite an inflow of good quality water, there has been a progressive deoxygenation in the river downstream due to the high methane content (roughly 8 mg/L) of the turbined water. Thus, 40 km downstream from the dam, the oxygen content was less than 2 mg/L and therefore incompatible with most aquatic life. To solve this problem, it was necessary to build an aerating weir capable of reoxygenating the turbined waters and, more importantly, eliminating reducing gases such as methane at the same time.

The function of the overflow weir was to entrain air bubbles into the water and to give these bubbles a sufficiently long immersion time to ensure that they dissolve. At the time of its installation, only three examples of oxygenating weirs existed in the entire world, all located in the United States. The weir configuration was tested using a physical model to qualitatively examine the form of the flow both across the weir and downstream from it. The degree to which air bubbles were entrained in the water was also tested, but not the question of evaluating the flux of gaseous exchanges between the air and the water.

The system that was finally designed by EDF, in October 1994, was a metallic weir with two consecutive falls, the configuration of which respected the main physical criteria that play a significant role in the oxygenation of water, i.e.:

- the height of the falls (roughly 5.40 m, depending on the flow rate);

- the thickness of the water stream, the function of which is to entrain air bubbles and keep them in the water for a sufficiently long period of time for the oxygen to dissolve (between 12 and 25 seconds, depending on the flow rate);

- the dimensions of the receiving basin of the first waterfall where the air bubbles are held (5 hexagonal alveoli); and

- systems to promote the fragmentation of the flow.

This structure was placed in the tailrace channel of the plant, approximately 100 m downstream from the main dam. This location protected it from floods and did not create an extra obstacle in the river. In addition, it allowed the water to be re-oxygenated as soon as it left the reservoir.

The efficiency of the two waterfalls of the Petit Saut re-aerating weir was tested at two different turbine flow rates: 80 m³ /s and 230 m³ /s. In 1996, the results of the measurements showed that for a flow rate of 230 m³ /s, upstream of the weir the concentrations of CH₄ were around 5 mg/L and dissolved oxygen was 0.8 mg/L. Downstream from the weir CH₄ concentrations were 1.3 mg/L and dissolved oxygen concentrations were 6.8 mg/L. The dissolved methane elimination rate was approximately 75 per cent. At a flow rate of 80 m3/s, upstream of the weir the concentration of CH₄ was 5.5 mg/L and the dissolved oxygen concentration was 0.7 mg/L. Downstream from the weir concentrations of CH₄ and dissolved oxygen were 1.0 mg/L and 7.1 mg/L, respectively. The dissolved methane elimination rate was around 80%. The efficiency of the re-oxygenation was always greater than 90%.

These data prove that the efficiency of the Petit Saut weir installation was higher when the turbine flow rate was lower. This could be due to a greater waterfall height, the better entrainment of air bubbles per unit volume and/or a longer air bubble residence time in the downstream flow.

Between December 2001 and February 2003, for a flow rate of 200 m³ /s, the efficiency of the weir decreased by 10%, with the dissolved methane elimination rate at around 70-75%. The level of re-oxygenation was around 80%. Since February 2003, for a flow rate of 100 m³ /s, the efficiency of the weir has decreased by 10%, the dissolved methane elimination rate was around 70% and the level of re-oxygenation was around 75%.

On a local scale, the effect on the quality of the river water has been very positive, as aquatic life has been maintained. Without the weir, the methane contained in the turbined water would have been progressively transformed, along the course of the river, into carbon dioxide. In the absence of significant additions of good quality water and without the weir, a large part of the course of the river would have a dissolved oxygen content of less than 2 mg/L, the critical threshold for the maintenance of aquatic life.

At present time, the results of the current ecological survey are used to support studies on biogeochemical processes.

FR :

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.

EN :

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.

FR :

Les lacs Tanganyika et Malawi sont, de par leur volume, les deux plus grands lacs africains. Ces réservoirs semblent pour l'instant épargnés par la pollution en éléments en trace. Il est toutefois crucial, en raison de leurs caractéristiques hydrologiques, de poser la question du temps de réponse de ces systèmes à une pollution chronique potentielle véhiculée par les affluents. Cet article simule ainsi cette réponse dans la fraction dissoute suite à l'introduction pendant 50 ans de polluant par tous les affluents. Cette démarche s'appuie sur un modèle hydrologique intégrant les trois compartiments des colonnes d'eau (épi-, méta- et hypolimnion) et sur la prise en compte de la réactivité des éléments dissous dans ces compartiments par l'intermédiaire du taux de rétention élémentaire. Ainsi quatre types d'éléments sont considérés, (i) le type Cl, non réactif, (ii) le type Si, réactif-nutritif, (iii) le type Mn et (iv) le type V tous deux réactifs sensibles aux conditions d'oxydo-réduction. La réactivité de l'élément, l'efficacité du mélange vertical ainsi que la position de l'oxycline dans la colonne d'eau conditionnent l'amplitude et la cinétique de réponse des systèmes ainsi que le temps de retour à la situation initiale après l'arrêt des apports polluants. Ces caractéristiques propres à l'élément et au lac influent sur le risque potentiel encouru par l'écosystème et l'homme. Ainsi la pollution affecte principalement les eaux de surface (types Cl et V), les réseaux trophiques (type Si), les eaux profondes (types Si et Mn) et le compartiment sédimentaire (types Mn et V).

EN :

Lakes Tanganyika and Malawi are the largest African lakes as measured by volume. They constitute essential water and protein resources for the surrounding populations. These aquatic systems have become stressed due to high human population density, growth and associated activities. While eutrophication was apparent locally and organic pollutants were detected in fish and water, concentrations of several dissolved trace elements of potential concern corresponded to uncontaminated systems. However, due to their hydrological features, it was important to characterise the lake response time to chronic contamination loaded by the tributaries. This paper presents two simulations of this response, in the dissolved fraction, following 50 years of pollutant input by the tributaries. The first simulation corresponded to an annual pollutant input that was the same for both lakes, resulting in mean river input concentrations of 5.0 U L-1 and 3.7 U L 1 (where U is a weight or molar unit), respectively, for lakes Tanganyika and the Malawi. The second simulation corresponded to an annual input proportional to the lake volume, with mean river input concentrations of 5.0 U L 1 and 1.5 U L 1, respectively, for lakes Tanganyika and the Malawi. The polluted input was loaded by the dissolved fraction with the exception of Mn-type elements, which were carried by the particulate fraction. This approach was based on an annual hydrological model of three water column compartments (epi-, meta- and hypolimnion) of these meromictic lakes. In addition, the reactivity of dissolved elements in the water column was taken into consideration. The reactivity was characterised by the elemental retention rate that quantifies dissolved-particulate interactions linked to biological and physico-chemical processes. The reactivity of trace elements was assessed through their concentration distribution profile in the water column. Four element types were considered: the non-reactive elements characterised by homogenous concentrations in the water column (Cl-like); the micronutrient-type elements (Si-like) characterised by a strong positive concentration gradient below the thermocline; redox-sensitive elements (Mn-like) characterised by a strong positive concentration gradient below the oxycline and other redox-sensitive elements (V-like) characterised by a strong negative concentration gradient below the oxycline. Trace elements (F, Al, Fe, Mn, V, Ba, Sr, Mo, Cr, Ni, Co, Cu and Pb) in both lakes were associated with these element types but they did not necessarily belong to the same type in both lakes. Other elemental types likely occurred (e.g., carbonate type and Fe types) but they were not clearly identified. After 50 years, surface concentrations ranged from 0 to 1.15 U L 1 in Lake Tanganyika and from 0 to 2.40 U L 1 in Lake Malawi. The difference between the lakes was linked to the greater volume of Lake Tanganyika, mainly in its hypolimnion, and to the longer vertical water exchange time in Lake Tanganyika. For Cl-type elements the concentration response decreased for both lakes from the epi- to the hypolimnion with similar kinetics for the epi- and metalimnion and a delay for the hypolimnion. For Si-type elements the response decreased in Lake Malawi from the hypo- to the epilimnion and for Lake Tanganyika the maximal concentration was calculated in the metalimnion. The concentration range was higher in Lake Malawi than in Lake Tanganyika. For the Mn-type elements, the maximum concentration was calculated in the hypolimnion with a higher response in Lake Malawi. The metalimnetic water concentration of Lake Tanganyika increased slightly and epilimnetic and metalimnetic waters of Lake Malawi did not react. For V-type elements the epilimnetic waters were more sensitive to the increase, with a higher response for Lake Malawi. In Lake Malawi concentrations also increased in the metalimnion. Concentrations in the hypolimnetic zone of both lakes and metalimnetic zone in Lake Tanganyika remained zero. Depending on the element type and on the lake, the time required to return to initial conditions, when contaminant inputs stopped, varied from 30 to 7 300 years. In the epilimnetic zone of both lakes the intensity of reaction and the pollution persistence were higher for Cl-type elements. For Si-type elements, mainly in Lake Malawi, the vertical input from deep waters was sufficient to sustain productivity even after the input of pollutants was stopped. For these elements the dissolved contamination was mainly stored in deep waters. For Mn-type elements the contamination was also stored in deep waters with a relatively slow net transfer to the sedimentary compartment. V-type pollutants were transferred from the dissolved to the particulate phase in deep waters leading to a relatively rapid net transfer to the sediment. Once the pollutant was in the system and until its evacuation to the outlet or to sediment, the risk for the ecosystem and for the population was associated with its presence in the dissolved phase of the surface water. The risk was then higher for Cl- and V-type elements as well as for the Si-type elements that were introduced into the web food. For the Si- and Mn-type elements that were mainly stored in deep waters, the associated risk was linked to a breaking of the thermo-haline stratification or to a reinforcement of vertical mixing. For the V-type elements and also for the sedimentary fraction of the Mn-type elements, the risk was also associated with possible remobilization from the sediments due to physico-chemical changes at the water-sediment interface.

Element reactivity, efficiency of the vertical mixing and the depth of the oxycline control the importance and the kinetic response. They also controlled the time to attain initial conditions once contaminant inputs were stopped. These features, relative to the element and to the lake, were key parameters in the assessment of the potential risk for both the ecosystem and people that rely on these lakes. Even if the elemental typology was the same for both lakes, elements can be considered a different type from one lake to another. Contamination from the same pollutant would then have different consequences, for instance regarding the associated risk. Computed hydrochemical budgets were simple but realistic, illustrating the behaviour of elements in the water column. Computation of this budget requires the knowledge of global water column fluxes, which have to be improved mainly for Lake Tanganyika. The element's reactivity was mainly linked to liquid-solid reactions. It would be interesting in future studies to characterise particulate phases and their reactivity and to introduce such processes in hydro-geochemical models. Computations of chronic contamination response indicate that for both lakes, due to the inertia of the hydrochemical system, the lack of lake water contamination does not imply a systematic lack of pollution in the tributaries. Once pollution is detected, it will be persistent. A global watershed monitoring program should be organised in the near future. Monitored parameters should be relevant to metallic and organic pollutants, as well as eutrophication.

FR :

La lagune de Smir a été sujette à un aménagement (construction d'un barrage et d'un port de plaisance) qui a contribué à la modification de son système hydrologique notamment, la disparition de certaines espèces de flore et de faune et la réduction de superficies considérables de cet écosystème.

Une caractérisation des eaux de cette lagune a porté sur la mesure pendant le reflux, d'un ensemble de paramètres physico-chimiques. Une méthodologie a été suivie afin d'exprimer de façon cartographique les résultats. De même, des suivis d'une journée ont été réalisés pour les eaux entrantes et sortantes au niveau de deux stations fixes, dans une tentative de comprendre l'action de la marée sur l'hydrologie au sein de la lagune, de cerner les caractéristiques des eaux entrantes dans la lagune et celles sortantes de celle-ci et de suivre l'évolution des différents paramètres physico-chimiques au cours des cycles marégraphiques.

L'évolution spatiale des paramètres hydrologiques au niveau de la lagune, se réalise sous forme de gradients entre l'aval et l'amont. Elle est fonction des saisons, de la marée et des apports de l'Oued Smir et du Chenal principal venant des marais. La lagune subit le flux et le reflux de la marée d'un mètre d'amplitude environ. Combinée avec le déficit en eaux douces, la communication permanente avec la mer a eu pour conséquence l'augmentation générale de la salinité dans le plan d'eau lagunaire et dans les marais. Le fonctionnement hydrologique de la lagune de Smir se trouve actuellement régi par les facteurs hydrodynamiques liés à la marée (réguliers) et aux apports d'eaux véhiculés par le chenal des marais (irréguliers), auxquels peuvent être ajoutés des facteurs physiques (évaporation intense), bathymétriques (faible profondeur) et physiologiques (photosynthèse des macrophytes et des phanérogames dans la lagune et dans les marais limitrophes).

EN :

The Mediterranean Smir lagoon (35°43' N 5°20' W) is located in the extreme west of the Mediterranean basin, northwest of Morocco, 25 km south from the Gibraltar strait. The area of this lagoon is about 3 km² with a maximum depth of 2.5 m. The bottom is dominated by silty and fine substrates with some sandy zones close to the entrance. The bottom is extensively covered with macrophytes (e.g., Enteromorpha and Ulva) and phanerogams (e.g., Rupia maritima and Zostera noltii). The lagoon receives water from the Smir Wadi but this input has decreased. It also receives fresh water via another channel (the marshes channel) from swamps that lie between the water body and M'diq city. This ecosystem has been subjected to unplanned dam and pleasure port constructions, and these changes led to a modification of the hydrologic system. Currently, this ecosystem communicates with the Kabila harbour (marina) and the sea across a narrow gully and is regularly subjected to tidal movements.

This ecosystem, which functioned before as a lake, has been transformed into a coastal lagoon. This lagoon is changing and trends suggest a progressive evolution towards a neutral type lagoon, where water movement is exclusively influenced by the tidal rhythm. The salinity in the lagoon is comparable to sea salinity. This increase in salinity clearly influences the hydrologic conditions, and has altered the composition, structure and functioning of the biotic compartment. The biota has progressively changed and there has been a progressive colonization of halophyte plants within the marsh and a disappearance of fresh water species from the lagoon.

The physicochemical parameters (salinity, conductivity, dissolved oxygen, pH, water temperature and sediment temperature) of the lake water were characterized. The methodology used allowed the results to be expressed cartographically. Similarly, the measurements for one day were analyzed for incoming and retreating waters at two stationary points, thus increasing the understanding of tidal action on the lagoon-lake hydrology.

The salinity analyses clearly show a mixohalinity and strong spatial and temporal variabilities (9 - 40) within the lagoon. This parameter was influenced by tidal changes, rainfall and sunshine during the summer. The latter contributes to evaporation of lagoon water, which therefore increases salinity. Gradients established between the upstream and downstream regions are a function of tide and season. The salinity distribution is marked by maximum values recorded in the summer and minimum values recorded in the winter. The same evolution scheme evolution is also noticed for conductivity since it is a parameter closely linked to salinity.

The lagoon and the main channel waters had higher salinity in the summer (S‰=40) due in part to marine waters that pervaded this channel, and the evaporation of inflow currents. Low salinity aureoles were detected in the Smir Wadi mouth indicating that fresh waters dilute marine waters. Marine waters that enter the Wadi channel by the inflow currents blend with fresh rainfall waters or stagnant saline waters or with groundwater inputs.

Two periods were distinguishable based on the surface water temperature; hot summer conditions and cold winter conditions. The continental effect was compensated by the sea buffer effect that played an important role in the regulation of the temperature within the Smir lagoon. During the period of decreasing temperature gradients from the downstream region towards the upstream region (by stretching from sea towards continent), marine waters played a significant role by moderating the freshwater temperatures cooled down from the continent during cold periods (winter and autumn). In contrast, during decreasing temperature gradients from the upstream region towards the downstream region, the oceanic waters cooled the high temperatures of internal lagoon waters resulting from hot seasons (summer and spring). Furthermore, the shallow lagoon depth and the long exposure to sunshine, especially in the summer, resulted in water heating.

In addition, the water temperature was influenced by hot marsh channel waters that occupy the upper region and spread with the low tide up to the lagoon entrance. Waters brought by the marsh channel also were distinguishable by high oxygen values, which demonstrate the role played by the swamp vegetation in the oxygenation of these waters. The low oxygen values recorded at the Smir Wadi mouth reflected the effect of suspended elements (turbidity). The Wadi contribution to the oxygen distribution in the lagoon waters was notable as it brings less oxygenated waters because of their strong turbidity.

Nevertheless, the lagoon waters showed good dissolved oxygen saturation and no under-saturation was observed during the study period. This relates not only to the permanent exchanges with the sea, but also to the rather remarkable abundance of marsh vegetation and to the development of macrophytes and phanerogams within the lagoon. The dissolved oxygen isovalue distribution curves overlapped remarkably with isotherms, even as the temperature increased, because the entire zone that stretches up to the gully was occupied by a dense meadow. This biomass supplies oxygen to the central zone and can contribute to over-saturation. The lagoon inside was less covered with vegetation and showed comparatively low oxygen values. Besides the photosynthesis activity, the shallow depth and the uninterrupted lagoon waters mixed by the tidal currents explain the over-saturation sometimes observed within lagoon, especially in its downstream region.

Variations in pH appeared to be closely linked to dissolved oxygen and water temperature. The three parameters present comparable distribution patterns showing the influence of the marsh waters on the lagoon hydrology. The pH varied between 8.0 and 9.0, demonstrating an elevated alkalinity in some lagoon waters. This alkalinity trend was due to the constant seawater entrance and to the stopping of Smir Wadi fresh water supplies.

The sediment temperature measured at 5 cm deep gave insight into different daily and seasonal variations. The temperature of both air and marsh waters was comparatively well reflected in the sediments; the temperature distribution in the sediments overlaps with the water isotherms, with temperature values varying from 14 to 28°C. The inside part of the lagoon showed temperatures widely different from those recorded close to the entrance.

The spatial evolution of hydrologic parameters in the lagoon resulted from gradients between the downstream and upstream regions. These gradients are a function of seasons, tide, Smir Wadi inputs and also the main channel coming from the swamps. The lagoon is subjected to high and low tides. In addition to freshwater shortages, the permanent communication with the sea resulted in a general increase of salinity in lagoon waters and marshes. The lagoon hydrologic functioning is currently governed by hydrodynamic factors that are related to the tide (regular) and to the fresh water supplied by the marsh channel (irregular). In addition, physical (intense evaporation), bathymetrical (shallow depth) and physiological (photosynthesis by macrophytes and phanerogams in the lagoon and the neighboring marshes) variables also influence the functioning of the lagoon.

FR :

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 écosystèmes

EN :

Several 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.

FR :

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.

EN :

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.