Impact du zooplancton métazoaire sur le phytoplancton et les protozoaires ciliés dans le réservoir Sahela (Maroc)

L'impact du zooplancton métazoaire sur le phytoplancton et les protozoaires ciliés a été mesuré durant la période de juillet à décembre 1999 dans le réservoir Sahela sous climat méditerranéen semi-aride.Les expériences ont été réalisées à l'aide de chambres de diffusion immergées in situ pendant 7 heures en absence (chambres témoins) et en présence (chambres expérimentales) du zooplancton.Les résultats indiquent que la mortalité moyenne à 4 m des algues est de 0,13 + 0,03 h-1, et celle des protozoaires ciliés de 0,07 + 0,03 h-1. Cryptomonas ovata et Halteria grandinella ont subi la plus forte prédation, respectivement, 0,31 + 0,14 h-1 et 0,11 + 0,04 h-1 à 4 m. Toutefois, les algues de grande taille (Pediastrum sp, Ceratium hirundinella et Peridinium cinctum) n'ont été que très peu ou pas consommées.The Sahela reservoir, located in Taounate at 90 km from Fès, lying at an altitude of 325 m, was built to provide drinking water for the population of Taounate and to contribute to irrigate neighbouring farming perimeters.In order to assess the impact of metazoan zooplankton on phytoplankton and protozoan ciliates in the Sahela reservoir under semi-arid climate, we conducted experiments during the period from July to December 1999 at the deepest point in the lake (15 m).Sampling and measurements were carried out in diffusion chambers submerged in situ over a period of 7 h without (control chambers) and with (experimental chambers) zooplankton. During these experiments, counts were conducted on phytoplankton and ciliates to determine the abundance and the mortality of these organisms due to zooplankton in each diffusion chambers at t=0 and t=7 h incubation. The metazooplankton were counted and dry weight of each taxa was calculated.In summer the highest zooplankton biomass (150 µg·L-1) mainly composed of cyclopoid Tropocyclops prasinus, caused mortality of the small-sized ciliates, such as Halteria grandinella (0.10 h-1). In Autumn, the zooplankton biomass (75 µg·l-1), dominated by Daphnia longispina, induced a higher mortality for phytoplankton (0.10 h-1) than for ciliates (0.05 h-1). In Winter, the zooplankton biomass (100 µg·L-1), also represented by Daphnia longispina, had a low impact on ciliate mortality (< 0.02 h-1).The study showed that a heavy predation by the metazoan zooplankton was exerted on small-sized phytoplankton and ciliates and clearly demonstrated the relationships between protozoans and metazoan zooplankton to transfering the matter and energy in aquatic food webs