Survival strategy of Pistia stratiotes L. in the Djoudj National Park in Senegal

Abstract

The water level in the Senegal River is now regulated due to the construction of two high dams (1986 and 1988). In consequence, large parts of the floodplains have permanently dried up and the brackish water in the lower part of the river has become fresh. The Djoudj National Park, situated approximately 60 km upstream from Saint Louis in Senegal in the former floodplain, is artificially flooded when inlets from the river are opened during the rainy season. This change in ecological conditions has led to an annually recurrent, explosive development of the floating aquatic weed Pistia stratiotes 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 season (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 with drought during the second half of the dry season. This applies in particular to an area where the water is fertilised by excreta from a pelican colony. Flowering of Pistiaappears to be markedly enhanced when vegetative reproduction is inhibited by crowding.