Merja

The term Merja indicates a shallow surface sheet of water with extensive mud flats. Deep mud is typical (1-2 m) and immersion can vary from daily, where there is tidal influence, to seasonal to permanent water. They are often brackish, high in suspended matter, but can be productive systems. The water level can exceed 4 m depth usually in March. The receding water level during summer leaves a large expanse of usually bare mud and salt deposits around the merja. The inundation limits vary not only according to the season but also between years. In summer, desiccation of exposed mud occurs and colonization by ruderal (weed-like) plants, depending on surface salinity, often proceeds. Merjas are few in Morocco, and they are generally found in the coastal zone occupying depressions on the landward side of coastal dune systems; they are usually above sea level but exceptionally they do communicate directly with the sea (e.g., Merja Zerga). They are often characterized by extensive stands of emergent plants, including Phragmites communis, Typha angustifolia, and Scir-pus with a broad marginal belt of Juncus maritimus and Juncus acutus (central parts being deprived of vegetation). Hence, the merjas are often associated with significant wetlands that may be extensive in the wet season. A typical merja receives stream runoff from the catchment but locally, springs often mark

Figure 2 Daya type 4 (Dayet Ifrah) in the Middle Atlas (Morocco). Note the typical circular shape and deforested landscape.

the emergence of artesian water. Generally, ground-water in the region of a merja is very near to the surface and also contributes to the maintenance of water level. These sites are however strongly threatened by the expansion of human activities, especially agriculture (where pumping and draining of water for crop cultivation lowers the water table, causing the merja and any associated wetland to shrink). One merja, Merja Bokka (northern Morocco), was desiccated completely by these processes in 1998 during environmental monitoring. Merja Bokka was formerly a substantial lake that in the early 1990s was still —250 m in diameter but very shallow. Before the 1980s, this permanent water body supported extensive emergent macrophyte communities. Aerial photographs showed several small low islands, which by the mid-1990s were covered with Panicum repens and Typha angustifolia. Several former higher lakelevel shorelines occurred around the lake, indicating that past water depths were over 2 m. Since the 1980s, the inflow stream became redundant as water was taken for agriculture and the formerly common Phragmites was reduced by burning and drainage. By 1997, the water level was a few centimeters and the lake possessed only a few introduced small carps but many amphibians, especially the newt Pleurodeles waltli and the Chelonia Mauremys leprosa. In 1998, the entire lake basin was dry and was prepared for agriculture. Since the 1960s, many shallow merjas and other lowland water bodies in agricultural regions of all the Maghrebian countries have followed a fate similar to that of Merja Bokka.

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