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Le bassin versant du fleuve Sinnamary est situé en Guyane française, au nord ducontinent sud américain, en zone néo-tropicale. Il s'étend entre 4 et 5°de latitude Nord etentre 52°50' et 53°30' de longitude Ouest. En 1994, la mise en eau du barragehydroélectrique construit sur le site de Petit Saut occasionne l'ennoiement de 365 km2 deforêt primaire. Le remplissage s'effectue en 18 mois, et la demande en oxygène dissousnécessaire à la dégradation de la matière organique est telle qu'en quelques jours la massed'eau se stratifie en un épilimnion oxygéné et un hypolimnion anoxique. Représentantquelques 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 valeurmoyenne 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 rapidementinstallé avec des rotifères, des cladocères et des copépodes. Des ostracodes et desChaoboridae y sont désormais associés. La retenue présente une zonation longitudinale, de latête de la retenue vers la queue, aussi bien en termes de densités que de composition despeuplements. De même, il existe une zonation transversale, notamment marquée par un gradientcroissant de la conductivité de l'axe vers les berges. Ces ressources endogènes ainsi quedes apports complémentaires (végétaux, invertébrés terrestres, détritus…) sont utilisées parla faune ichtyque qui a su s'adapter au changement du milieu. Le temps de rétention des eauxqui fluctue saisonnièrement en fonction des pluies semble être le facteur prépondérant devariabilité pour la qualité physico-chimique des eaux et les communautésbiologiques.The Sinnamary River is located in a neotropical zone, in French Guiana, in thenorthern part of South America. Its flood basin extends between 4° and 5° N latitude and52°50' and 53°30' W longitude. The river is found in an equatorial climate, and isinfluenced by its proximity to the ocean. The temperature varies little, with an average of25°C for the least hot month (January) and 27°C for the hottest month (October). Thehumidity 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 kmfrom the mouth of the river, with a flood basin area of 5927 km2 and anaverage flow rate 260 m3 /s.The construction of the hydroelectric dam at Petit Saut in 1994 resulted in theflooding of 365 km2 of primary forest in a neotropical zone (theaverage depth of the reservoir is 11 m). The dam reservoir was filled over a period of 18months and the amount of dissolved oxygen necessary for the organic matter to decompose wassuch that, in a matter of days, the body of water became stratified into an oxygenatedepilimnion and an anoxic hypolimnion. Only centimeters thick in 1994, by 1998 the epilimnionhad progressively expanded. Since that time it has varied between an average thickness of 5to 6 m and is home to various biological communities. The hypolimnion is anoxic and rich inreduced compounds (nitrogen and phosphorus minerals, dissolved organic matter, particles,metals and gas). The products of the degradation of the flooded organic matter, which stillprevail 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 byvarious pelagic fauna. The zooplanktonic communities, primarily confined to the epilimnion,were quickly settled with rotifera, cladocera and copepoda. Initially dominated bydetritivores (Bosminidae) and Cyclopidae from February to May 1994, the settlementsdiversified with phytophagous zooplankton (Calanidae and Daphniidae) when the phytoplanktondeveloped. This procession was quickly accompanied by Sididae, which has dominated thecladocera since mid-1996 and by ostracoda, which were particularly abundant from 1995 to1997. Carnivores such as Chaoboridae larvae were also present in Petit Saut reservoir. Incontrast to the other zooplankton species, Chaoboridae larvae occupy the entire watercolumn. As a result of great quantities of organic matter available at the time thereservoir was filled, the biomasses were initially large and have gradually decreased withdecreasing nutrient concentrations.The reservoir was not homogeneous in terms of zooplankton density, with a longitudinalzonation from the dam towards the upstream tail. In the tail, fauna were identical to thoseof 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 ofoxygenated water was less important than in the lake zone. Cyclopidae, well represented atthe time of the reservoir colonization, dominated the communities in the transition zone.This distribution of zooplankton was likely connected with the dissolved oxygenconcentrations, as well as with the quantities of phytoplankton and bacterio-chlorophyll.Similarly, there was a transverse zonation, marked by an increase in conductivity towardsthe reservoir banks. The density of the communities varied seasonally. The highest biomasseswere measured along the central axis at the time of high waters and close to the banks atthe time of low waters.The time of water retention, which fluctuates according to the rainy season, seemed tobe the factor dominating the physicochemical water quality variability and the biologicalcommunities. The rainy season is characterized by the dilution of various elements (e.g.,conductivity is reduced). Conversely, during the dry season, the elements concentrate againin the water mass. Nevertheless, the peaks of pelagic invertebrate density were observedduring times of high waters, when the hydraulic conditions favour the exchanges between thehypolimnion (anoxic and rich in nutritive elements) and the epilimnion (oxygenated and verylow in nutrients).These endogenous resources, as well as complementary contributions (vegetation,invertebrate, terrestrial, refuse), are used by the fish fauna, which adapted to the changein 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

revue des sciences de l'eau

Variabilités spatiales et temporelles de la qualité physico-chimique et des invertébrés pélagiques des eaux de la retenue hydroélectrique de Petit Saut (Guyane française). 