Application of Remote Sensing to Site Characterisation and Environmental Change Analysis of North African Coastal Lagoons
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Hydrobiologia (2009) 622:147–171 DOI 10.1007/s10750-008-9682-8 NORTH AFRICAN COASTAL LAGOONS Application of remote sensing to site characterisation and environmental change analysis of North African coastal lagoons M. H. Ahmed Æ B. M. El Leithy Æ J. R. Thompson Æ R. J. Flower Æ M. Ramdani Æ F. Ayache Æ S. M. Hassan Published online: 22 January 2009 Ó Springer Science+Business Media B.V. 2009 Abstract This article describes the use of satellite (1972–2004). Following geometric correction and imagery for identifying key environmental character- enhancement, the interpretation of the most recent istics within three North African coastal lagoons image acquired for each of the three lagoons provides (Merja Zerga, Morocco; Ghar El Melh, Tunisia and important insights into their current conditions. For Lake Manzala, Egypt) and for detecting the major Merja Zerga, these include the distribution of the environmental changes within these environments. A largest channels which drain extensive inter-tidal combination of Landsat MSS, Landsat TM, Landsat mudflats and the two major depositional features ETM? and ASTER imagery was acquired for the associated with sources of freshwater. The distribu- three sites for a period covering the last three decades tion of marginal aquatic vegetation is highlighted as is the intensive use of the surrounding landscape for agriculture. Intensive agriculture around Ghar El Guest editors: J. R. Thompson & R. J. Flower Melh is also indicated. The influence of the Mejerda Hydro-ecological Monitoring and Modelling of North African River, which was diverted away from the lagoon over Coastal Lagoons 100 years ago, is shown to persist as a residual area M. H. Ahmed (&) Á B. M. El Leithy Á S. M. Hassan of deltaic deposits in shallow water that has been Department of Marine Sciences, National Authority for eroded over time. Coastal processes including the Remote Sensing and Space Sciences, 23 Josef Burrows direction of the alongshore sediment transport and the Tito Street, El Nozha, Al Gadida, PO Box 1654, Cairo, Egypt influence of engineering work associated with port e-mail: [email protected] construction can also be recognised. Within Lake Manzala, vegetated islands divide the lake into a J. R. Thompson Á R. J. Flower series of sub-basins which can be clearly distin- Wetland Research Unit/Environmental Change Research Centre, UCL Department of Geography, University guished. The large influence of human activities College London, Gower Street, London WC1E 6BT, UK within this lake can be identified and include reclamation for agriculture and the conversion of M. Ramdani parts of the lake bed for fish farms. The historical Department of Zoology and Animal Ecology, Institute Scientifique, University Mohamed V, Charia Ulan Batota, images available for the three lagoons provide Rabat BP703, Morocco important insights into decadal scale changes, which have been greatest at Lake Manzala. Since the early F. Ayache 1970s large parts of the lake, in particular in the Faculte´ des Lettres et Sciences Humaines de Sousse, De´partement de Ge´ographie, Universite´ de Sousse, southwest where the shoreline has migrated north- Cite´ Erryad, B.P. 547, 4029 Sousse, Tunisia wards, have been reclaimed. Major engineering 123 148 Hydrobiologia (2009) 622:147–171 works, such as the El Salam Canal and road coverage of a specific geographic area, 16 days in the embankments, are shown to have resulted in signif- case of Landsat TM) from such sources permits icant lake change. The distribution of emergent regular image acquisition over the same area enabling vegetation within the lake has also changed. Classi- change detection at relatively high temporal fre- fication of images for this lake into open water, quency (Lillesand et al., 2008). vegetation and land enables the quantification of Common types of detectable land cover change, these changes. Between 1973 and 2003, the lake which can be monitored using remotely sensed data, declined in area by approximately 50%. Changes at include vegetation disturbance and regeneration, Merja Zerga over the last three decades include agricultural intensification, urban expansion, spatial reconfiguration of the marine outlet and the expan- changes in aquatic ecosystems and hence surface sion of the internal delta at the end of the Nador water extent, as well as a variety of soil disturbance Canal. The images of this site clearly demonstrate the processes including overgrazing and soil erosion (e.g. intensification of agriculture around the lagoon. The Milne, 1988; Hall et al., 1991). The application of most marked changes evident within the images of remote sensing for biodiversity and conservation over Ghar El Melh concern the sand bars that separate the a range of spatial scales is reviewed by Turner et al. lagoon from the sea. Geomorphological processes (2003), who emphasised the great potential of the operating within the coastal zone have resulted in the approach. Remote sensing techniques can enable straightening of the bars with central sections ecologists and resource managers to monitor the migrating out towards the sea. Remote sensing is vegetation condition and detect environmental change established as a promising application for detecting trends in regions where difficult terrain, poor access, the quantitative surface cover changes in coastal and extreme climate conditions make regular field lagoons and their near landscapes. investigations difficult. Satellite data are also suitable for inventorying the type, quality, distribution, pro- Keywords Coastal lagoons Á North Africa Á ductivity and biomass condition of natural as well as Remote sensing Á Environmental change Á planted vegetation at a range of spatial scales (Hobbs, Image interpretation and classification 1990; Milne & O’Neill, 1990; Franklin, 1991; Hall et al., 1991;, Turner et al., 2003). Land cover change detection and overall land use classification (e.g. Introduction Ambrose & Shag, 1990; Teng, 1990; Petit & Lambin, 2001) have global applications especially regarding Remotely sensed images can be used as a tool to map assessments of the magnitude and rate of conversion ecosystems and to detect, monitor and evaluate of natural landscapes into high productivity agricul- changes within them thereby supporting the devel- tural or urbanised systems. Satellite and/or airborne opment of resource management strategies. Satellite data has been demonstrated to have particular use for and airborne systems offer major opportunities for monitoring water and vegetation changes within monitoring large scale, earth surface characteristics aquatic ecosystems including wetlands (e.g. Hardisky and provide a database for change detection studies. et al., 1986; Howman, 1988; Hess et al., 2003; Ozemi Remotely sensed data can be used to span temporal & Bauer, 2004,Co´zar et al., 2005). and spatial scales ranging from local to aggregated In Africa, remote sensing has made major contri- global systems (Graetz, 1990). Monitoring global, butions to monitoring a variety of wetland systems regional and local areas can be performed by (e.g. Fuller et al., 1998; Munyati, 2000; De Roeck restricting the analysis to a single sensor series or et al., 2008). Landsat data, for example, has enabled by using different satellite data. With the availability the identification of the impacts of both natural of data from remote sensing satellites such as the processes and human interventions on African lakes, Landsat series, it has become cost effective and freshwater ecosystems and wetlands (UNEP, 2005). convenient to acquire multi-date images over a Results from studies utilising data from these sources greater array of spatial and temporal scales than have revealed that pollution resulting from land-use was possible with aerial photography. The temporal changes, environmental modification and other prac- resolution of data (the ability to obtain repeated tices associated with rapid population increase, and 123 Hydrobiologia (2009) 622:147–171 149 water abstraction have caused or accelerated many The MELMARINA primary lagoons negative changes in the continent’s lakes (e.g. Randazzo et al., 1998, Scheren et al., 2000). The Three primary lagoons, one in each of the three North main threats to water quality in Africa are perceived African countries who participated in the MELMA- to include eutrophication, pollution, increasing water RINA project, were identified: Merja Zerga demand and the proliferation of invasive aquatic (Morocco), Ghar El Melh (Tunisia) and Lake Manz- plants such as the water hyacinth (e.g. Albright et al., ala (Egypt) (Fig. 1). These lagoons were the focus of 2004). One study of Lake Chad carried out within field survey and monitoring activities (e.g. Ramdani NASA’s earth observing system programme showed et al., 2009, Thompson et al., 2009) as well as hydro- that the lake shrank to one-twentieth of its original ecological modelling (e.g. Rasmussen et al., 2009a, size during the last 35 years (NASA, 2001). In North b) and the remote sensing reported in this article. A Africa, remote sensing has been used to monitor the series of secondary sites was also identified to development of Lake Nasser including the new delta provide context for the primary sites (see Flower & in its southern section and environmental changes in Thompson, 2009). They were not, however, subjected Lake Ichkeul, Tunisia, (NASA, 2005a, b). The use of to remote sensing analysis. remotely sensed data for monitoring North African Detailed descriptions of the three primary lagoons wetlands has much potential (Flower, 1998) when are provided by Ayache et al. (2009). They are combined with ground surveys and existing field strongly contrasted according to their physical and data. Turner et al. (2003) stressed the importance of biological characteristics. All three are shallow water obtaining validatory ground evidence to help to bodies (\2 m deep) linked to the sea through one or interpret remote sensing products. For the manage- more well-defined connections. They vary in size ment of North African coastal lagoons, remote from c.13 (Merja Zerga) to c.700 km2 (Manzala) and sensing techniques provide an exceptional means of differ in water quality.