L'examen de 110 échantillons pélagiques el de 16 échantillons littoraux de plusieurs corps d'eaux du bassin moyen de l'Orénoque (lac de barrage, lacs d'inondation du système Orénoque et Caroni, rivières) a permis d'identifier 58 taxa de Cladocères, dont 24 sont nouveaux pour la faune vénézuélienne. 2 espèces, Alona ovata et Bosmînopsis macaguensis, sont nouvelles pour la science. Alona karelica, élément paléarctique, n'avait jamais été mentionnée sur le continent américain. 35 % de la faune étudiée sont représentés par des Formes néotropicales, dont 8 endémiques sud-américaines.Une partie taxonomique illustrée est consacrée à 13 espèces rares ou peu connues. La répartition des espèces dans les milieux prospectés et dans les deux catégories d'eaux blanches et d'eaux noires, a été étudiée. La faune la plus riche de Cladocères a été observée dans les lacs d'inondation de l'Orénoque, la diversité spécifique étant due aux formes littorales appartenant principalement à la famille des Chydoridae.En Cladocera of several water bodies in the middle basin of the Orinoco river (Venezuela).Keywords : Cladocera, taxonomy, ecology. Venezuela, Orinoco.The examination of 110 pelagic samples and 16 littoral samples from several water bodies in the middle basin of the Orinoco river (lakes formed by barrage or inundation of the Orinoco and Caroni river systems) revealed 58 taxa of Cladocera, of which 24 are new to the Venezuelan launa. Two species, Alona ovata and Bosmînopsis macaguensis, are new to science. Alona karelica, a palaearctic species, has never been recorded before on the continent of America. 35 % of the fauna studied are represented by neotropical forms, of which eight are endemic to South America.An illustrated taxonomic section deals with 13 species that are rare or little known. The distribution of species was studied in the environment examined and in the two categories of white and black waters. The richest cladoceran fauna was found in the lakes formed by inundation of the Orinoco, the species diversity being due to littoral forms, chiefly in the Family Chydoridae.In the pelagic environment, white-water bodies were characterised by the species association Moina minuta, Ceriodaphnia cornuta, Bosmina tubicen, and Bosminopsis deitersi, and black-water bodies by the dominance of Bosminidae : Bosmina hagmanni, Bosminopsis macaguensis, Bosminopsis deitersi, Ceriodaphnia cornuta. The case of Daphnia gessneri, generally considered endemic to the Amazon-Orinoco system and present in a single survey, is discussed. -IntroductionLes données taxonomiques, biogéographiques et écologiques sur les Cladocères néotropicaux sont très limitées (Collado & al., 1984 Brehm (1953Brehm ( , 1957. Margalef (1961), Herbst (1967, 1975

Zooplankton samples collected at low and high water in 14 and 21 stations respectively along some 900 km of the Orinoco and its Delta, revealed the presence of 100 rotifer and 48 cladoceran taxa. Of these, only 13 rotifer and 8 cladoceran species were frequent and numerically important, mainly Nauplii were dominant among the copepods. At low water, rotifers were by far the most abundant group (mean 49 org./l) followed by cladocerans (mean 2.8 org/1) and copepods (1.5 org/1). At high water, rotifer densities declined to a mean of 3,5 org/1, followed by copepods (mean 3.4 org/1) and cladocerans ( 1.2 org/1). Mean zooplankton densities at low water were eight times higher than at high water. At low water, longitudinal zooplankton densities seemed to be influenced by tributary river waters. At high water, densities were generally low up to the Delta where a longitudinal increase was observed. A high proportion of egg carrying cladocerans, particularly B. tubicen, B. deitersi and M. minuta, were observed at low water along the sampling sites, suggesting an ability of the species for growth and reproduction in the river.Klude longitudinale du zooplancton du Bas Orénoque et de son Delta (Venezuela)

Deux nouvelles espèces de Cladocères ont été découvertes dans du matériel pélagique et littoral de plusieurs corps d'eaux du bassin moyen de l'Orénoque. Bosminopsis macaguensis n. sp. est régulièrement présente dans le peuplement pélagique de Macagua, lac de retenue sur le fleuve Caroni, affluent de l'Orénoque. La population est comparée à une des populations de Bosminopsis deitersi qui colonisent les autres milieux pélagiques de cette région. La comparaison est effectuée tant au niveau des mâles que des femelles. Alona ovata n. sp. a été récoltée dans la zone littorale d'un lac du système d'inondation de l'Orénoque. Elle se distingue de toutes les autres espèces du genre par la forme et l'armature du post-abdomen.Bosminopsis macaguensis n. sp. and Alona ovata n. sp. (Crustacea, Cladocera) new cladocerans from Venezuela.Keywords : Taxonomy, Cladocerans, new species, Venezuela.Two new species of Cladocera have been discovered in material from the pelagic and littoral regions of several water bodies in the middle basin of the Orinoco river. Bosminopsis macaguensis n. sp. is present regularly in the pelagic population of Macagua, a man-made lake on the river Caroni, a tributary of the Orinoco. Its population is compared with one of the populations of Bosminopsis deitersi that colonises other pelagic habitats in this region. The comparison is made for both males and females. Alona ovata n. sp. has been collected in the littoral zone of a lake in the flooded part of the Orinoco. It is separated from all other species in this genus by the shape and armature of the post-abdomen.

Bosminopsis brandorffi, nouveau Bosminidae, se différencie de toutes les autres espèces du genre par l'importance de la région céphalique, l'absence de rostre, le grand développement de l'oeil et des antennules, la présence d'un fort talon abdominal. Mâles et femelles ephippiales sont inconnus. Bosminopsis brandorffi n.sp. (Crustacea, Cladocera), a new species of Bosminidae from the Amazon and Orinoco river systems.

Zooplankton samples were collected over a 21-month period in five fioodplain lakes of the Orinoco River in order to establish zooplankton species composition, abundance patterns, and biomass and their relationships with the patterns of inundations and lake morphometry. 60 rotifer taxa were identified. This group was generally more abundant than copepods and cladocerans (mean : 73,4 % of total zooplankton). spinulosum. Most frequent and abundant zooplankton species were euplanktonic with a dominance of filter feeders and microphagous detritivores. Mean zooplankton density in all lakes ranged from 340 ind./l to 3 486 ind./l. Biomass ranged from 71,1 ^g.l (dw) to 432,8 ^g/l. Rotifers accounted for 64,7 % of the total mean zooplankton biomass in three lakes while copepods accounted for 57,8 % of the biomass in two lakes. Both density and biomass were markedly seasonal with highest mean values at low waters. Mean density in the lakes was 100 times higher than in the Orinoco main stem. Lakes with highest variabilities in surface area and water depths showed highest zooplankton densities. The type of connection (direct or indirect) established between the lakes and the major source of the water also seemed important to interpret the productivity of fioodplain lakes.