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@@@Spatial and temporal variations in the richness and abundance of rotifer (Brachionidae & Trichocercidae) and cladoceran communities in a small artifial eutrophic tropical lake

SUMMARY 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) represen- ting the upper, middle and lower parts of the lake, respectively. Water tem­ pérature was measured with a thermometer placed in the sampling bottle and pH was measured in the field with a portable pH-meter. Oxygen concen­ trations were determined according to the Winkler method. Water colour, suspended solids, and ammonium-nitrogen (NH 4-N) were analyzed spec- trophotometrically using standard methods. The transparency of the water column was determined with a Secchi disk. Zooplankton species were identi- fied alive under a phase contrast microscope using several manuals, and rotifers and cladocerans were counted in a Dolfuss chamber. Throughout the study, température 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 zéro in the bottom waters. Spatial and temporal variations of thèse variables indicated that