Survival strategy of Pistia stratiotes L. in the djoudj National Park in senegal

The water level in the Senegal River is now regulated due to the constructi on of two high dams (1986 and 1988). In consequence, large parts of the flo odplains have permanently dried up and the brackish water in the lower part of the river has become fresh. The Djoudj National Park, situated approxim ately 60 km upstream from Saint Louis in Senegal in the former floodplain, is artificially flooded when inlets from the river are opened during the ra iny season. This change in ecological conditions has led to an annually rec urrent, explosive development of the floating aquatic weed Pistia stratiote s L. in the Park during the first half of the dry season. A rapid increase in the salt content of the water in the Park, in the course of the dry seas on (due to the high salt content of the soils and evaporation), eventually causes massive die back of Pistia. Rapid seed production by Pistia permits it to survive these unfavourable conditions, i.e. high salinity coupled wit h drought during the second half of the dry season. This applies in particu lar to an area where the water is fertilised by excreta from a pelican colo ny. Flowering of Pistia appears to be markedly enhanced when vegetative rep roduction is inhibited by crowding.