Phytoplankton community structure and species assemblage succession in a shallow tropical lake (Lake Guiers, Senegal)

Phytoplankton species composition and seasonal changes were investigated monthly in Lake Guiers, one of the largest lakes in West Africa. Taxonomic composition, diversity, biovolume and abundance of phytoplankton were studied at 3 representative stations from March 2002 to March 2003, in relation to various climatic factors (wind, insolation), physical and chemical factors (temperature, conductivity, nutrients) and biological factors (zooplankton grazing). A total of 111 spe- cies of phytoplankton belonging to 7 algal classes were identified, predominately Chlorophyceae (32% of species) followed by Cyanobacteria (25%) and Bacillariophyceae (diatoms, 15%). From December to March, a period characterised by low temperature, solar insolation and north-easterly winds that mixed the water column, the community was dominated by the diatom Fragilaria sp. and cyanobacterium Anabeana miniata. From April to September, during higher temperatures, solar insolation and predominant north-westerly winds that mixed the water column less effectively, the community was dominated by cyanobacteria Cylindrospermopsis raciborskii and Lyngbya versi- color. A statistical approach based on co-inertia analysis was used to describe the phytoplankton/ environment relationships, in particular the factors governing the presence of C. raciborskii. The phytoplankton community was characterised by a clear succession of species groups: Fragilaria sp.-A. miniata (November to March) → Microcystis aeruginosa (March to May) → C. raciborskii- L. versicolor (June to November). Finally, a functional approach originally designed for temperate zones was applied to the algal groups (11 functional groups), which gave valuable information on algal strategies in a shallow tropical lake.

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