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