In order to find effective measures to control a Moroccan shallow reservoir (Sidi Abderrahmane), a better understanding of phytoplankton composition, abundance, spatial and temporal distribution it is necessary. Trophic level and the stability status were assessed upon the basis of Shannon diversity index (H'), species richness (S), and evenness (J) index. Statistical tests were used to evaluate the different relationships between phytoplankton and the concentrations of several physico-chemical parameters, and the main soluble nutrients. In surveys, the samples were taken fortnightly from May 2011 to December 2012. 64 taxa belonging to seven groups of phytoplankton were identified, including Bacillariophyceae (25 taxa), Chlorophyceae (22 taxa), and Cyanophyceae (9 taxa). Aulacoseira granulata, Nitzschia palea, Scenedesmus acuminatus, and Oscillatoria sp, were the main contributors to the dissimilarity in temporal distribution. Phytoplankton population never reached a monospecificity situation. Shannon and evenness indices were between (0.0001 < H' < 0.15; 0.003 < J < 0.085) and manifested a young phytoplankton community with high multiplying power. There were significant correlations between total phytoplankton (r = 0.015, p < .01) and water temperature. Significant negative correlations were observed between transparency and Cyanophyceae (r = -0.208, p < .05) and between the number of species and transparency (r = -0.206, p < .05), orthophosphorus (r = -0.377, p < .01), and nitrates (r = -0.301, p < .01). A negative correlation was found between Orthophosphorus and Chlorophyceae (r = -0.377, p < .01). Similar correlations were also observed with nitrates and Chlorophyceae (r = -0.297, p < .01), Silica and Bacillariophyceae (r = 0, p < .01) and total phytoplankton (r = -0.372, p < .07). The underwater light condition, as indicated by Secchi depth fluctuations, hydraulic process conditions (short residence time, short outflow/inflow ratio) were shown to be the limiting factors in regulating the density of phytoplankton. With reference to Palmer pollution index, test results indicated an oligotrophic or mesotrophic reservoir. The data presented provide the first contemporary account of the level of algal diversity present, the prominent environmental conditions and trophic status of Sidi Abderrahmane reservoir waters.
Keywords: Diversity; Morocco; Phytoplankton; Shallow reservoir; Trophic status.