Variations spatiales et temporelles de la richesse et de l'abondance des rotifères (Brachionidae et Trichocercidae) et des cladocères dans un petit lac artificiel eutrophe situé en zone tropicale

Abstract

Les communautés de rotifères (Brachionidae et Trichocercidae) et de cladocères peuplant les eaux du lac Municipal de Yaoundé situé en zone tropicale (Cameroun), ont été examinés en relation avec quelques variables physico-chimiques du milieu. Des 26 espèces et sous-espèces de rotifères (Brachionidae et Trichocercidae) et 15 espèces de cladocères inventoriées, environ 16 sont identifiées pour la première fois au Cameroun. Parmi les espèces quantitativement dominantes, les espèces fréquemment rencontrées sont Brachionus angularis angularis, B. falcatus falcatus, et B. calyciflorus chez les Brachionidae, Trichocerca elongata elongata, et T. bicristata bicristata chez les Trichocercidae, et Ceriodaphnia cornuta et Chydorus eurynotus chez les cladocères. Plus de 75% des espèces répertoriées sont des espèces périphytiques communément considérées comme littorales, dont le développement important a été observé dans la zone pélagique du lac qui, sans doute, offre de nombreuses ressources et constitue un refuge vis-à-vis notamment de la prédation. L'absence de corrélations entre les communautés zooplanctoniques étudiés et les quelques variables physico-chimiques suivies, nous a amenés à considérer que le développement de ces communautés est sous la dépendance d'autres facteurs. D'ailleurs, une analyse canonique de correspondance indique que les stations, profondeurs et mois de prélèvement expliqueraient respectivement 3%, 5% et 36% de la variance totale associée à l'abondance des espèces prises en compte. Ce qui nous a permis d'émettre l'hypothèse selon laquelle les principales conditions environnementales qui affectent le développement potentiel des espèces analysées et le déroulement de leurs cycles biologiques, seraient dépendantes des deux saisons (la saison des pluies et la saison sèche) caractérisant les climats tropicaux.The rotifer and cladoceran communities of a small tropical eutrophic lake (the municipal lake of Yaoundé, Cameroon) were examined from November 1996 to December 1997, in relation to some physical and chemical variables. Using a Van Dorn bottle, water samples were collected weekly from 5 depths (0 m, 0.5 m, 1 m, 1.5 m and 2.5 m) at three stations (I, II and III) representing the upper, middle and lower parts of the lake, respectively. Water temperature was measured with a thermometer placed in the sampling bottle and pH was measured in the field with a portable pH-meter. Oxygen concentrations were determined according to the Winkler method. Water colour, suspended solids, and ammonium-nitrogen (NH4 -N) were analyzed spectrophotometrically using standard methods. The transparency of the water column was determined with a Secchi disk. Zooplankton species were identified alive under a phase contrast microscope using several manuals, and rotifers and cladocerans were counted in a Dolfuss chamberThroughout the study, temperature and pH values of the water remained around 25°C and 7, respectively. The Secchi disk transparency did not exceed 1 m and the dissolved oxygen concentration decreased with depth to almost zero in the bottom waters. Spatial and temporal variations of these variables indicated that the middle zone (0.5 and 1.5 m) was the stratum with a critical role in the functioning of the lake. Indeed, it represented the transition zone between the trophogenic upper water layer (0 to 0.5 m) where photosynthetic activity occurred and the bottom water layer (1.5 to 2.5 m) formed by the microaerophilic to anaerobic tropholitic layer where reducing processes were important.The rotifer and cladoceran communities were relatively diverse, with 41 species collected during the study year (i.e., 26 species of Brachionidae and Trichocercidae and 15 species of Cladocera), of which 16 are reported for the first time in Cameroon. These organisms filter small organic particles (fresh algal detritus and bacteria) and their presence in the lake could be related to the relatively high algal biomass found there. In terms of relative abundance, rotifers appear to be more important than cladocerans. Among the quantitatively dominant species, the most frequent ones were the Brachionidae (Brachionus angularis angularis, B. falcatus falcatus, B. calyciflorus), the Trichocercidae (Trichocerca elongata elongata and T. bicristata bicristata) and the Cladocera (Ceriodaphnia cornuta and Chydorus eurynotus).Brachionus angularis was the most abundant and frequent rotifer observed during this study. According to several authors, a high abundance of Brachionus can be considered as a biological indicator of more eutrophic waters. This corroborates the hypereutrophic status assigned to the lake through previous studies dealing with bacterioplankton and phytoplankton in this system. Among Cladocera, Daphnia, an important bacterivore, was absent despite the high density of bacteria (~108 cell mL-1) in the lake. Previous studies in other Cameroonian aquatic systems reported the scarcity of Daphnia and these earlier authors argued that it was probably replaced by smaller sized species, which in the case of the present study would be Ceriodaphnia cornuta and Moina micrura, the only representatives of Daphniidae encountered. Up to 75% of the number of species identified were periphytic species that generally develop in the littoral zone. The massive occurrence of these species in the plankton sample of the lake was related to the high particle content. Abundance generally peaked in the middle zone of the water column, likely due to the availability of resources and the general potential for such zones to provide a refuge from predation.However, there did not appear to be any correlation between the rotifer and cladoceran communities and physico-chemical variables when the data set was examined as whole, or grouped by seasons. This suggests that the heterogeneity in species abundances was controlled by other factors, among them sampling points, depths and time of sampling. A multivariate test using canonical correspondence analysis suggested that about 3% and 5% of the variance associated with the species abundance heterogeneity could be explained by sampling point and depth, respectively. Up to 36% of this variance was attributed to the sampling months. We therefore hypothesize that the environmental conditions affecting the development and biological cycles of the studied species are highly dependent on the prevalent seasons found in a tropical zone, i.e. the rainy and the dry seasons