Geo ecology of the Lake Urema / Central Mozambique


Lake Urema is one of the most important ecological features of the Gorongosa National Park, Central Mozambique. It is located in the Urema Rift which is a part of the East African Rift System. The understanding of the geo-ecology of the lake and its tributaries is particularly important for the conservation of its floodplain habitats. There are recent concerns that the lake has changed during the last years. A multi-temporal and multi-disciplinary approach was applied to investigate the dynamics and control mechanisms of the lake. Principal methods adopted were: remote sensing analyses of time series of Landsat and ASTER data, geomorphological interpretations of SRTM (Shuttle Radar Topography Mission) data as well as field investigations such as water and sediment analyses and vegetation mapping. The study showed that the water of Lake Urema and its in- and outflow has very low concentrations of total dissolved solids (TDS) between 20 to 100 mg/l. The pH is on average about 7, regulated by a CO2 - HCO3 —- CO3 2_ buffering system. The water transparency is very low, mostly less than half a meter. During the period of investigation no outflow of the lake was observed, but the Vunduzi River, one of the tributaries to Lake Urema, had a considerable discharge. A decrease of the lake level height was detected over a period of two months. In addition to the evaporation from the open water surface of the lake, water is probably also lost through spreading into the adjacent floodplain grasslands, through evapotranspiration as well as through infiltration. A significant enrichment of the water constituents in the dry season did obviously not occur. The widespread distribution of clayey sediments over large parts of the lake and the evidence of sandy sediments in the narrowing part of the lake towards its outflow suggest a temporally and spatially constrained pattern of transport and deposition. It is supposed that the axial part of the lake is characterized by a more energetic flow and the lateral areas by quiescent conditions. The results of the supervised classification of the satellite images from 1979 to 2000 did not indicate a trend for the variations of the lake's size. The area of Lake Urema ranged from 17.4 km² (09/1995) to 25.1 km² (08/1979). A rainfall anomaly was responsible for the outstanding lake size in May 1997. Investigations showed that alluvial fans limit the Urema Basin from all sides and make Lake Urema a kind of “reservoir lake”. Further studies should focus on the enlightenment of the water balance of the lake system. Especially the contribution of groundwater to the water balance of the lake is not yet understood.
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