Microalgae play a fundamental role in the functioning of aquatic ecosystems. The diversity and structure
of the microalgae community are always influenced by the availability of nutrients in the environment, which is one
of the causes of water eutrophication. Microalgae are the best biological indicators of pollution and eutrophication
in aquatic environments. In this study, the diversity and structure of microalgae were determined and indicator species of eutrophic water were identified in the water of the Loumbila reservoir, a drinking water source in Burkina
Faso. Algae and water samples for physico-chemical analyses were collected monthly from January 2014 to June
2015 in three sampling points. Phytoplankton species were observed under a light microscope and identified using
standard methods. The results showed that pH, dissolved oxygen, water transparency, conductivity, nitrates and
orthophosphates levels showed distinct spatial and seasonal variations at p ≤ 0.05. The determination of the specific
composition and density of microalgae allowed the inventory of 205 species composed of 5 phyla with a higher
specific diversity of Chlorophyta (48.30 %) and a higher density of Cyanobacteria (43.71%). Analysis of microalgae diversity showed a Shannon-Wiener index varying from 3.11 to 3.60 between sampling points and from 1.70 to
3.73 between seasons. Microcystis aeruginosa, Pediastrum duplex, Pediastrum simplex and Aulacoseira granulata
were identified as indicator species for eutrophic water. This study provides basic information on the diversity and
structure of microalgae in water reservoirs. It also provides elements for a future assessment of the water quality of
drinking water sources using indicator algal species to prevent or reduce eutrophication phenomena.