Recent environmental change in North African wetland lakes: diatom and other stratigraphic evidence from nine sites in the CASSARINA Project

• Published in: Aquatic ecology Vol. 35; no. 3-4; pp. 369 - 388
• Main Authors: Flower, Rj, Dobinson, S, Ramdani, M, Kraïem, Mm, Ben Hamza, C, Fathi, Aa, Abdelzaher, Hma, Birks, Hh, Appleby, Pg, Lees, Ja, Shilland, E, Patrick, St
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Nature B.V 01-10-2001
• Subjects: Agriculture; Aquatic ecosystems; Bacillariophyceae; Biomonitoring; Dam construction; Egypt, Arab Rep; Environmental changes; Eutrophication; Fresh water; Freshwater; Lake sediments; Lakes; Land subsidence; Morocco; Plankton; Salinity; Sediment composition; Soil erosion; Tunisia; Water availability; Water diversion; Water inflow; Water resources; Water shortages; Wetlands; ASE, Morocco; MED, Tunisia; Africa, North
• ISSN: 1386-2588; 1573-5125
• DOI: 10.1023/A:1011984627760

All nine wetland lakes in the CASSARINA North African suite of sample sites have been disturbed strongly by human activity during the 20th century. Dated lake sediment core were used to provide evidence of the extent of recent environmental change at each site. Sedimentary diatoms at seven sites were useful for inferring salinity change trends during the last century. At two sites preservation problems severely degraded the sedimentary diatom record. Sediment core integrity was otherwise established. Lithostratigraphic measurements indicated some site specific changes in soil erosion and sediment composition but, for the Egyptian Delta lakes, no physical signal synchronous with Aswan High Dam construction was found. Sedimentary diatom assemblages were generally site characteristic and halophilous taxa were common. At two sites planktonic diatoms indicated some recent eutrophication but generally the assemblages were more indicative of salinity changes. Diatom-inferred salinity trends for the seven sites typically indicated that reductions in water salinity occurred sometime during the early or mid 20th century. Rather than climate, hydrological modification of water resources is implicated as the primary driver of salinity changes during most of the 20th century. In the western North African region these modifications were mainly local land drainage and water diversion programmes to alleviate winter flooding and/or promote summer water availability. In the Delta region, the Nile has been intensively exploited since antiquity and intensively so from the late 19th century to release more fresh water for agriculture. Here, diatom records indicate that freshening began well before the Aswan High Dam but salinity fluctuations have tended to diminish during the latter part of the 20th century. A small reversal in the water freshening trend in the 1980/90s was possibly a response to land subsidence/sea-level change or to reduced freshwater supply.