Vegetation changes induced by dam construction in a tropical estuary: the case of the Manamo river, Orinoco Delta (Venezuela)

This study documents the increased distribution of mangrove communities into the oligohaline regions of the Manamo rivet as a consequence of a dam constructed in 1965. The dam caused a major change in the discharge regime of the water, stopping the annual floodings and allowing an extensive upriver intrusion of saline waters. Penetration of salt water from the sea due to the daily tides, that would be observed only during the dry season, occurs now throughout the year. Three sites were chosen along the upper, middle and lower river course. The evolution of site geomorphology and vegetation was evaluated over 3 periods, from 1932 to 1996, using aerial photographs and recent field surveys. Soil and plant samples were taken in the mangrove communities of two sites nearer to the river mouth along 100 m transects inland from the river fringe. The influence of sea water in soils was measured by the concentration of Na, Mg, Cl and SO4 in the interstitial soil water. In plants, the impact of soil salinity was determined by the concentration of Na, K, and Mg in mature leaves. The geomorphological processes created new habitats (sand bars) for aquatic and terrestrial vegetation. In the middle and lower river course tidal penetration elf sea water due to lower river discharge salinized the soil, allowing mangrove establishment. This occurred mainly in the open sand bars or in areas with sparse herbaceous vegetation. The ion content was higher in the down river site and also higher in the subsoils compared with the topsoils. The apparent expansion rate of a plant community dominated by a mangrove species (Rhizophora mangle) increased from 1 ha yr(-1) Drier to the regulation of water flow, to 6-7 ha yr(-1) after the river closure. The dam influences the entire Manamo river basin, affecting the distribution of halophytic species along the river fringes.