Environmental Change in North African Lakes

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Exploitation of water resources and intensity of land usage have increased rapidly throughout the SMR during the twentieth century. This has occurred especially in Morocco, Tunisia, Egypt, Lebanon, and Israel, and on the Mediterranean islands and has paralleled regional population growth. Lakes are especially vulnerable habitats and many of North Africa's natural standing waters are now degraded by human activities. Although past human impacts on North African landscapes began in a major way during the Roman period, water has been managed in Egypt for over 5000 years. Nevertheless, shallow lake SMR ecosystems may have been the most intensively disturbed during the last 100 years following the introduction of mechanization of land management. Large-scale hydraulic modifications of most North African river catchments have been carried out since the 1950s (e.g., Sebou River in Morocco, the Mejerda in Tunisia, the Nile by the Aswan High Dam in Egypt). In all cases, these modifications were carried out to control annual floods and/or to improve water availability for people and for agriculture. Although people have benefited, many small lowland ponds and wetlands, especially those in the Maghrebian countries, have been a major casualty of this program as natural fresh water availability has generally declined. An exception is the Nile delta lakes, where improved water availability, especially following completion of the Aswan High Dam, in the Delta has lead to increased agricultural returns entering the lakes, causing a freshening of receiving standing waters. Elsewhere, there is evidence that, at least initially, improvements in water supply for agriculture led to local freshening of lakes and wetlands but since the 1980s many sites are showing a salinization trend. Furthermore, local eutrophication and pollution are also impacting many inland waters of the SMR, and, despite some improvements in environmental management in the latter part of the twentieth century, water resources and their aquatic communities have been generally diminished. Land drainage, afforestation, and intensified crop production arising from development programs as well as from effluents and water abstraction continue to threaten lake water quality in most North African regions. Unfortunately, without active management, all the current problems that affect water quality and availability now undoubtedly will be exacerbated by global warming.

The upland regions, where there are no major groundwater reserves to exploit (such as in Libya and Mauritania), are particularly prone to water shortages brought about by climate change. In the Middle Atlas Mountains of Morocco, a karstic landscape makes most natural lakes very prone to major water level fluctuations as a result of the inter-annual periods of low rainfall. The level of one such lake (Aguelmane Azigza) declined by over 11m between 1979 and 1984 following a period of low rainfall. Some recharge has occurred subsequently but this lake and others like it remain vulnerable to persistent climate change. At the other end of the spectrum of SMR lakes, many oases are declining because of a variety of reasons, including rural exodus and reduced or overmanagement of the semiartificial sites, increased water deficit, and siltation by the progression of the dunes. In the lowland plains, shallow lakes (dayas and merjas) occupy natural closed depression and all are threatened by climate, by encroaching agriculture as well as by sea level rise. Locally, afforestation offers a special threat to dayas in the Maghreb. In Morocco, ancient forests of cork oak were very extensive in the Gharb region, where dayas were abundant. Several recent processes have lead to the loss of most of these dayas. During the periods 1980-1981 and 1992-1996, rainfalls were only two-thirds compared with the normal, and the period of dryness often extended over 9 months. These also contributed to the on-going decline of cork oak in the Mamora and Bin Abid forests. Forest openings expanded, permitting access by livestock and facilitating plantation of the Eucalyptus forest which began after the 1900s. This intense pressure has lead to the widespread disappearance of dayas from these Moroccan forest areas by the twenty-first century. Apparently both human- and climate-induced changes have strongly impacted North African lake systems, but it is in the lowland regions where landscape modifications have been most important in diminishing sites during the recent past. Depending on the type and degree of water abstraction and the relationship of a lake basin with the sea, hydrological modification can either bring about freshening (cf. Nile Delta lakes) or salinization (cf. Garaet Ichkeul, Tunisia), or complete desiccation (Merja Bokka). Even where there has been a freshening trend (Nile Delta lakes), it is unlikely that this will persist into the future since the Mediterranean Sea level is rising and the Delta is subsiding.

Many of the North African natural lakes are con-servationally important habitats and some have been designated RAMSAR sites - internationally recognized bird reserves - and several support significant fisheries (such sites are Lake Burullus in Egypt, Merja Sidi Bou Ghaba (Figure 8), Aguelmane Afenourir, and Merja Zerga in Morocco, lakes Oubeira and Tonga in Algeria, and Ichkeul in Tunisia). Despite achieving this status many of these sites are already impacted by anthropogenic activities, but the extent of environmental stress is largely unknown except that habitat quality is generally declining and migrant bird populations are threatened. One way to track the development of a lake through time is by using palaeolimnology. Analysis of sediment cores for fossils and geochemistry is commonly used to answer questions about the rate and extent of changes in aquatic biota in wetlands and lake ecosystems on a variety of time scales. Sedimentary records can also help identify trends in pollution impacts and determine the rate of siltation. In the SMR, lake sediment

Figure 8 Upper: The nature reserve and Ramsar site Merja Sidi Bou Ghaba, a relatively undisturbed coastal lake in north-west Morocco in spring. Lower: Sidi bou Ghaba in late summer showing desiccation of the south part of the lake (photo 2002).

cores have been used to infer historical changes in several lakes in Morocco, Tunisia, and Egypt. Results generally show that nutrient enrichment, salinization, metal and pesticide contamination, and soil erosion have all impacted the investigated lakes during the twentieth century. Further, these sediment archives demonstrate major changes that have occurred in biodiversity with species of some aquatic organisms, especially plants and invertebrates, being lost as salinity and water availability have changed. However, compared with boreal zone lake sediments, sedimentary preservation of some organism groups such as diatoms is often poor so limiting the potential of some palaeolimnological techniques for reconstructing past environmental changes. Probably, the frequently warm and strongly alkaline conditions in SMR lakes do not favor preservation. Hence, little is known about the resilience of some aquatic communities to recent environmental changes, but where lake monitoring has not been undertaken or only sporadically, palaeoecology and lake sediment core studies do offer the only way of tracking lake changes through time.

It is anticipated that detrimental effects of environmental changes on SMR lake ecosystems will increase during the twenty-first century as a result of the demands of local human populations and of accelerating climate change. To track how lake ecosystems are responding to these changes, a program of well-coordinated environmental monitoring is needed, which involves remote sensing and hydrological modeling combined with on-site water and sediment quality assessments. Nevertheless, it is already clear that diminishing freshwater availability is the most widespread threat to the persistence of many lakes in the SMR. So while monitoring ecosystem health is essential, appropriate restoration and conservation measures need to be introduced. Not least is improved water management that takes advantage of both traditional methods as well as new innovations so that natural water resources can be used more efficiently. Such actions taken together will help inform and set policy for future management of fresh-waters in North Africa. These actions must also take account of aquatic biodiversity as well as the water demand.

See also: Lakes as Ecosystems; Mixing Dynamics in Lakes Across Climatic Zones; Saline Inland Waters.

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