Hétérogénéité horizontale des peuplements microphytoplanctoniques et zooplanctoniques en relation avec les paramètres abiotiques dans la retenue El Kansera (Maroc)

La répartition horizontale du phytoplancton et du zooplancton a été étudiée dans le lac-réservoir El Kansera en période de stabilité thermique, en relation avec certains paramètres abiotiques (transparence, matières en suspension, conductivité, température, oxygène dissous, pH, nutriments azotés et phosphorés).La répartition longitudinale du phytoplancton montre un gradient décroissant, avec des maxima de biomasse (poids frais) situés au 2/3 de la longueur du réservoir, et des pics de chlorophylle a enregistrés vers l'amont de la retenue. L'abondance du zooplancton suit une évolution quasi-analogue à celle du phytoplancton avec en plus, une répartition hétérogène des Rotifères et des différents stades de développement des Crustacés. Les Rotifères et les petits stades de développement des Cladocères ou des Copépodes sont concentrés dans la zone médiane du lac, tandis que les stades adultes de ces Crustacés dominent vers l'aval. La prédation vertébrée des " Carpes Chinoises ", associée à la mauvaise qualité de la nourriture algale en période d'étiage (blooms à Microcystis), seraient à l'origine de cette répartition. Les résultats de cette étude permettent de distinguer 3 zones de la retenue : la zone d'embouchure de l'oued** Beht (station S1) agitée, turbide et peu oxygènée est la moins productive, la zone médiane (stations S2 et S3), constituant la transition entre le milieu lotique et lacustre, est la plus enrichie en peuplements algaux et la plus peuplée par le zooplancton. Enfin, la zone lacustre proprement dite (stations S4 et S5) est assez productive et constitue un site idéal pour les adultes des Crustacés.The goal of this study is to assess the degree of horizontal heterogeneity of phytoplankton and zooplankton communities in the lake-reservoir El Kansera (Morocco), in order to define the principal factors responsible for the plankton distribution in the lake.Five stations have been studied; station S1 is the most influenced by allochthonous inputs from oued Beht (a tributary), stations S2 and S3 represent a transition between the lotic and lacustrine environments, and stations S4 and S5 represent true lacustrine conditions.The phytoplankton and zooplankton samplings were carried out during two seasons in 1996 (spring and summer), with a plankton net which consisted of a silk cone (height: 120 cm, mesh size: 40 µm). An average sample was prepared by several vertical tows. The samplings of chlorophyll and abiotic parameters were performed at different depths with a polyvinylchloride bottle with a capacity of 5 L and an automatic closing device. The phytoplankton counting was done according to the method of Legendre and Watt (1972). The algal biomass (µg.L-1) was estimated by chlorophyll-a (method of Lorenzen 1967) and wet weight (method of Lohmann (1908). Zooplankton counting was done in a Dolfuss cell (method of Pont 1983). The biomass of Rotifers, Cladocerans or Copepods (µg.L-1) was calculated according to the method of Dumont et al. (1975) and Bottrell et al. (1976). The physico-chemical analyses were performed at LPEE (Public Laboratory of Essay and Studies). Temperature, pH and conductivity were measured in situ, with a sounding Horiba U7. Dissolved oxygen was measured by the method of Winkler (Rodier 1984). Water transparency was measured by Secchi disc (Reynolds). Suspended matter was determined according to the method of AFNOR (1994). Nitrates and total phosphorus were analysed by colorimetry, respectively after reduction on a cadmium column and mineralization in acid medium (AFNOR 1994).The lake-reservoir El Kansera presents a longitudinal gradient (oued Beht-dyke) of biotic and abiotic parameters in relation both to morphometric and hydrologic characteristics and to the importance of allochthonous inputs and the nutrient cycles (nitrates and phosphorus). The maximum phytoplankton abundance (wet weight) is observed at 2/3 of the lake's length, and the chlorophyll-a peaks are recorded in the upper zone. In the spring, two alga species are dominant in the lake, Scenedesmus ecornis and Cyclotella ocellata, whereas in summer, one species is abundant (Microcystis aeruginosa).Zooplankton abundance shows a similar evolution to that of the phytoplankton, with a longitudinal heterogeneity of Rotifers (Brachionus calyciflorus, Polyarthra vulgaris and Keratella tropica) and different development stages of Cladocerans (Bosminalongirostris and Diaphanosoma brachyurum) and Copepods (Acanthocyclops robustus). Rotifers and the small stages of Crustaceans are concentrated in the median zone, whereas the Crustacean adults are dominant in the deeper point of the reservoir. Vertebrate predation by the " Chinese carp " and the degraded quality of the algae (blooms of Microcystis) also seem to contribute to this distribution.The results of the study, confirmed by the statistical Student test, indicate three zones in the lake-reservoir: the upper zone (station S1), which is located at the mouth of the oued Beht, is the least productive due to its shallow depth, low oxygenation, high turbidity and frequent mixing. The intermediate zone, between lotic and lacustrine conditions (stations S2 and S3), is strongly enriched with nutrients (nitrates and phosphorus) and characterised by a higher productivity; this zone presents dense phytoplanktonic populations. The lacustrine zone (stations S4 and S5) is of intermediate productivity, and offers good conditions for large size classes of Crustacea