In the arid northern Sahel, where people, plants, and animals all live on the edge of what is ecologically sustainable, wetlands stand out from their surroundings by the rich and diverse flora and fauna they support. As islands of resources, the ephemeral wetlands of eastern Mauritania fulfill key ecological and economic functions. However, because rainfall varies so much in eastern Mauritania, the water levels and vegetation in ephemeral wetlands fluctuate from year to year, and can even change significantly within a single year.
Landsat images of the department of Néma in eastern Mauritania from 1972 and 2013 show many wetlands that occupy channels and depressions. These wetlands stand out in imagery from the nearby strips of sandy and bare soils. In the middle of the sandy zones covered by Sahelian short grass savanna, localized eroded areas around boreholes and villages appear as bright aureoles — they are especially visible on the 2013 image. Using local terms, Mauritanian wetlands include tamourts (forested closed basins), gâats (herbaceous closed basins or flats) and oueds (seasonal watercourses). Tamourts are contained in deeper depressions, characterized by stands of Acacia nilotica, and have the longest mean water duration. Gâats are open wetlands present in a more shallow topography and typically cultivated during the dry season (Shine, 2011). These wetlands are fed by rainfall and runoff from their catchment areas and reach their maximum water level after the rainy season in October or November. On average, this region receives 280 mm of rainfall per year (Shahin, 2007). Because they rely on annual precipitation, the duration, depth, and size of these wetlands vary widely from year to year. A closer view of the Gâat Mahmoudé, the largest wetland in eastern Mauritania, illustrates this dynamic (see adjacent insets).
The runoff washes fine clay particles and organic matter into the wetland depressions, which makes the wetland soils more productive than the surrounding sandy soils. The increased nutrient availability and moisture in the wetlands attract a variety of economic activities. Indeed, livestock rely on the surface water and pasture in the wetlands for part of the year, bringing additional nutrients into the system. Flood recession agriculture is also practiced in some of the wetlands, as well as fishing and hunting. The tamourts provide wood for construction and fuel, wild foods and medicinal plants.
Wildlife also depends on water and vegetation resources offered by the wetlands and oftentimes competes with economic activities, sparking debate about conservation and management. The role of wetlands as stop-over, over-wintering sites and breeding grounds for migratory birds has received particular attention. Aerial surveys have been used to monitor Black Stork populations, whose concentrations in the wetlands vary from year to year in response to the variable and unpredictable surface water. In favorable years, Black Storks over-winter in the Gâat Mahmoudé; in other years they continue to more permanent wetlands in Mali, Niger, and Burkina Faso (Shine, 2001).