Cylindrospermopsis raciborskii is a toxic bloom-forming cyanobacterium that occurs at tropical and temperate latitudes. Despite several reports from Africa, no data were previously available about its dynamics or toxic potential there. We therefore carried out a 1-year survey of the dynamics of C. raciborskii in the main water reservoir in Senegal, Lake Guiers. Cylindrospermopsis raciborskii never formed a bloom in this lake during the period studied, but was dominant during the dry season. The only observed bloom-forming species was a diatom, Fragilaria sp., which displayed a seasonal pattern contrary to that exhibited by C. raciborskii. Principal component analysis applied to environmental and phytoplankton data showed that high C. raciborskii biomasses were mainly related to high temperature and water column stability. Tests for C. raciborskii species-related toxicity and/or toxin synthesis were performed on 21 isolated clones. All the strains isolated tested negative in mouse toxicity bioassays, toxin analysis (MS/MS) and tests for known cylindrospermopsin genes (ps, pks). The limited number of isolates studied, and the occurrence of toxic and nontoxic clones in natural cyanobacterial populations, mean that we cannot conclude that there is no C. raciborskii-associated health risk in this drinking water reservoir.

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 species 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 versicolor. 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. KEY WORDS: Phytoplankton · Cylindrospermopsis · Fragilaria · Functional group · West Africa Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 45: [147][148][149][150][151][152][153][154][155][156][157][158][159][160][161] 2006 pressure exerted by one or many predators, acting simultaneously within a community in a particular habitat or space. Cyanobacteria are more abundant than other classes of algae in most shallow tropical lakes, and are often competitors under conditions of high turbidity (Padisák 1997).Algal successions in tropical aquatic systems are generally characterised by a sharp contrast between the 2 main seasons (dry/wet-flood season): cyanobacteria/diatoms in Lake Victoria (Lung'ayia et al. 2000), cyanobacteria/chlorophytes-chroococcales in Lake Tanganyika (Descy et al. 2005) and cyanobacteria/ diatoms-chlorophytes-chroococcales in many reservoirs in north-east Brazil (Bouvy et al. 2000. According to Salmaso (2003), temporal changes in phytoplankton composition can represent a complex environmental gradient driving annual succession; thus, environmental variables may act jointly as a complex forcing factor that selects seasonal groups of species sharing similar requirements. In a given environment and for...

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Analyse du régime alimentaire de l'ichtyofaune dans les étangs piscicoles traditionnels de la Basse Casamance (Sénégal)

International audienceLa dynamique des particules dans les systèmes aquatiques est régit par l’interaction entre la resuspension et la sédimentation. Cette étude est consacrée à la détermination des processus responsables de la resuspension des particules dans le lac de Guiers situé au nord du Sénégal. Des échantillons ont été collectés au cours d’un cycle annuel au niveau de trois stations prises dans la partie centrale du lac. Dans ce site, la resuspension des particules est liée à l’action des vents mais aussi à la crue du fleuve Sénégal. La resuspension sous l’effet des vents se produit pendant l’harmattan, alors qu’en période d’alizés maritimes, c’est la crue du fleuve Sénégal qui est responsable de la remise en suspension des particules. Le taux de particule en suspension est plus important en période d’harmattan qui est marquée par des vitesses du vent plus élevées (moyenne de 3,6 m.s-1 fluctuant entre 1 et 7 m.s-1), mais surtout par un fetch fort (24 km). Cette évolution saisonnière de la resuspension affecte la turbidité de l’eau, les taux de matière en suspension, la biomasse phytoplanctonique, le coefficient d’atténuation lumineuse, le flux des particules et la structure de la colonne d’eau. La resuspension sous l’effet des vagues créées par le vent associé au fetch élevé constitue dans ce système peu profond le principal processus contrôlant la dynamique des particules

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