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SUMMARY
This study concerns a shallow aquatic ecosystem, Lake Guiers (Senegal) which the main
vocation is the distribution of tap water for Dakar City (30%). The main objectifs were to
understand the factors susceptible to control the development of the different
phytoplankton species, and particularly the cyanobacteria developing in the lake since the
operating of hydraulic dams in the 80s on the Senegal River.
The phytoplankton was analyzed during (1) an annual cycle from march 2002 to february
2003 in the central zone of the lake (4 representative stations) and (2) a north-south spatial
approach with 10 sampled stations to study the heterogeneity if the lake.
The evolution of the phytoplanktonic community (taxonomy, species richness, diversity
index, abundance and biovolume) as well as its interactions with the other components of
the ecosystem (17 environmental variables among which temperature and wind; 18
biological parameters including zooplankton) were studied using multivariate analyses.
Among the 111 species listed during the study, 24 are dominant constituting 11 functional
groups according to Reynolds et al.’s classification (2002). Three species form the bulk of
the phytoplankton community: two cyanobacteria (Cylindrospermopsis raciborskii and
Lyngbya versicolor) and a bacillariophyceae (Fragilaria sp.).
Experiments allowed to estimate the growth rates of one of the potentially toxic
cyanobacteria, Cylindrospermopsis raciborskii, as well as the impact of the grazing exerted by
the zooplankton on this dominant species.
We observed a clear alternation between the cyanobacteria dominance (July-October)
(temperature averages of 29,5°C) under dominant winds (maritime trade winds)
explaining the water column stability and the diatom dominance in response to the flood
and the change of wind direction (continental trade winds).
Toxicity tests performed on the strains of C. raciborskii isolated from the lake were all
negative. However, a permanent survey should usefully be established on Lake Guiers to
follow water quality and healthiness, and prevent future possible toxicity events in these
multi-purpose aquatic reservoirs which tend to become widespread in the tropical zones.
Keywords : phytoplankton, cyanobacteria, bottom-up and top-down factors, Lake Guiers,
Senegal.
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