Diatom Research qponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA (2002), Volume 17 (1), 243-266VUTSRQPONMLKJIHGFEDCBA DISTRIBUTION AND ORIGIN OF DIATOMS IN THE BOTTOM SEDIMENTS OF THE SUEZ CANAL LAKES AND ADJACENT AREAS, EGYPT Abdelfattah A. Zalat Geology Department, Faculty of Science, Tanta University, Tanta 31527, Egypt- The diatom assemblages in the bottom sediments of Timsah Lake and adjacent sites, including the Great Bitter Lake, the northern part of the Gulf of Suez and the Mediterranean Sea at the entrance of the northern canal, have been recovered and studied in detail. A total of 394 species and varieties belonging to 96 genera were identified. Of these, 263 diatom taxa were detected in Timsah Lake sediments, 151 species and varieties from the sediments of the Great Bitter Lake, 119 from sediments of the northern part of the Gulf of Suez, and 167 from the coastal sediments of the Mediterranean Sea. The distribution pattern of the diatom assemblages in these studied sites are related to a variety of changes in the palaeoecological conditions, including current direction, sea-level fluctuations, salinity and climatic changes. The Timsah Lake is considered to be a mixing zone characterized by the occurrence of both autochthonous and allochthonous diatoms, as its basin floor includes taxa transported from both the Mediterranean and Great Bitter Lake through the Suez canal. The composition of the diatom assemblages indicates that non-planktonic taxa dominate over the planktonic forms. The assemblages are composed mainly of temperate littoral and shallow cosmopolitan marine species, accompanied by a considerable number of non-marine taxa, which are normally allochthonous and probably originated from the adjacent areas. INTRODUCTION During recent years, the diatom composition of surface sediments has been used to explore and quantify the relationships between the diatom assemblages and a number of water chemistry variables, for instance pH, salinity and nutrient concentration, in order to evaluate the data for palaeolirnnological inference (e.g. Anderson et at. 1986, Birks et at. 1990, BatterbeeQPONMLKJIHGFEDCBA & Renberg 1990, Bennion 1995, Hakansson et al. 1998). For the present study, the diatom species composition in the surface sediments ofTimsah and Great Bitter Lakes, the northern part of the Gulf of Suez and the coastal sediments of the Mediterranean Sea at the entrance of the Suez canal near Port Said city have been assessed in order to expand our understanding of ecological and taphonomic factors, and to obtain more detailed information on the nature and origin of the diatom assemblages in both Timsah and Great Bitter Lakes. The Suez canal area, as well as Timsah and Great Bitter Lakes, have received attention from numerous workers, among them EI-Sabh (1967, 1968), Morcos & Messieh (1973), Hassan & EI-Sabh (1975), Meshal (1975), Mohamed (1979), Toth & Lerman (1975), Stanley et at. (1982), Gerges & Stanley (1985). Their studies dealt with the mechanisms of water flow through the canal, the changes in water chemistry through time, the water exchange between the Mediterranean Sea and the Gulf of Suez, the hydrography of the canal and the dispersal of its sediments. However, the previous studies