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à‰tude de la flore vasculaire, de la végétation et des macrophytes aquatiques proliférants dans le delta du fleuve Sénégal et le lac de Guiers (Sénégal)

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par Abou THIAM
Cheikh Anta Diop de Dakar (Sénégal) - Doctorat dà¢â‚¬â„¢Etat ès Sciences Naturelles 2012

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The implementation of large dams in the late 80s, has led changes in hydrology, water quality in the delta of the Senegal River and Lake of Guiers. The new conditions caused changes in the flora and vegetation, and led the proliferation of aquatic macrophytes such as Typha domingensis, Potamogeton schweinfurthii, Pistia stratiotes and Salvinia molesta.This new situation has had some negative impact on ecology, people, socio-economic and health. The present work was undertaken to gather information that can contribute to a better knowledge of the flora, vegetation, biology and ecology of major macrophytes. These data are essential elements for sustainable management of aquatic macrophytes and environmental of the Senegal River delta and Lake of Guiers.

Systematic inventory of vascular flora has identified 151 species distributed in 102 genus and 47 families. Monocots represent 41.7% and dicots 56.2%. The most represented families are Poaceae, Cyperaceae, Fabaceae and Amaranthaceae. Species diversity is highest in low-lying areas liable to flooding, due to strong artificial and heterogeneity of these environments, compared to those of Lake of Guiers and the river. This flora consists approximately half (55%) of therophytes, phanerophytes and chamaephytes representing the land component, and half (44.5%) aquatic macrophytes including helophytes (helogeophytes and helotherophytes) (27%) are more important than hydrophytes sensu stricto formed by hydrogeophytes, submerged free hydrophytes, hydrotherophytes hydrohemicryptophytes (17.5%) in varying proportions depending on the lake, river and flood low-lying areas.These hydrophytes sensu stricto, although relatively less important, is nevertheless remarkable floristic elements with families and often monogeneriques, monospecific characteristics of aquatic environments such as Alismataceae, Hydrocharitaceaee, Lemnaceae, Najadaceae, Potamogetonaceae in monocotyledons, Avicenniaceae, Ceratophyllaceae, Lentibulariaceae, Menyanthaceae, Nymphaceae and Rhyzophoraceae in dicots.

A typology of 48 common species of wetland was established following four main types: fixed emerging macrophytes, floating macrophytes fixed, submerged macrophytes and free floating macrophytes. The presentation of each species could be a tool for identification and information on synonyms, botanical characters, distribution and ecology of the taxa. Species with wide geographical distribution, pantropical, paleotropical and cosmopolitan represent 3/5 of species, compared to strictly african taxa or Sudano-Guinean-Zambezian and Asian (38%). These species are relatively homogeneous characteristics and are well adapted to the aquatic environment which absorbs major ecological changes such as climate.

The study of vegetation, with the transect method, shows that there is a zonation of vegetation depending on salinity, slope of water level and duration of flooding: the typhaie on shore, in deeper water pondweed and between the two, in front of the nymphaea, Typha, direction of prevailing winds, waves or local mesological conditions, floating plants like Pistia and Salvinia or fixed plants as Phragmites autralis, Cyperus articulatus and C. alopecuroides.

The analysis of plant communities, by phytosociological technique has identified 9 plant groups, which are divided into vegetation of salty environment and fresh water, their floristic composition were specify and physionomy. The vegetation of saline environments includes Tamarix senegalensis and Arthrochnemum macrostachyum. That of freshwater has from deeper water towards upper parts of the banks, Azolla pinnata, Potamogeton octandrus, Cyperus alopecuroides, C. articulatus, Bolboschoenus maritimus, Phragmites australis, and T. domingensis.

The study of some aspects of the biology and ecology of proliferating macrophyte indicates that reproduction is primarily vegetative in P. stratiotes, and P. schweinfurthii whereas in T. domingensis, it is both vegetative and sexual. These modes give T. domingensis much power for colonization. S molesta has only vegetative multiplication.

In total, this work assess the actual situation of the vascular flora, vegetation and macrophytes in the Senegal river, various wetlands in the Delta and the lake of Guiers. This reference is essential for a scientific monitoring of biodiversity and vegetation dynamics. Data on the biology and ecology of key species could help the recovery, design and implementation of appropriate control methods of aquatic macrophytes and sustainable management of Senegal river delta and Lake of Guiers.

Key words: vascular flora, vegetation, macrophyte proliferation, Senegal river, lake of Guiers.

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