PALEOECOLOGICAL AND ENVIRONMENTAL HISTORY OF LAKE MARIUT, EGYPT, BY MEANS OF DIATOMS

Abstract

Analysis of diatom assemblages preserved in Lake Mariut sediments provides a detailed paleoecological and environmental history for the lake during the Late Holocene. A total of 210 diatom taxa belonging to 57 genera were identified and counted from 111 samples obtained from three cores. The most commonly occurring species are Aulacoseira granulata, Cyclotella meneghiniana, Cocconeis placentula, Nitzschia sigma, Navicula cincta, Pleurosira laevis and Triceratium antediluvianum. The Twinspan program was used to identify the diatom groups, while the cluster analysis enabled classification of core samples into distinctive ecostratigraphic zones. Changes in the diatom composition, abundance and preservation were different throughout the examined cores, and this may be attributed to differences in local hydrology and basin morphology. The distribution pattern of the recognized diatom groups reflects changes in salinity and water depth. Sediments deposited in the northern area of the lake contain increasing numbers of diatom taxa associated with shallow, eutrophic brackish water conditions. The diatom assemblage deposited in the western area is dominated by the planktonic centric diatom, Cyclotella meneghiniana, which reflects slightly deeper freshwater conditions, with slight increasing salinity. However, the sediments of the southeastern region are characterized by diatom assemblages that denote fluctuations from a deep freshwater environment to shallow brackish water and ending by shallow, highly eutrophic saline water conditions dominated by saline diatom taxa. Most of the recorded diatom taxa in the core sediments are absent in the recent surface sediments and the phytoplankton. The poor preservation and reduction in the number of the diatom species are attributed to high pollution content of the lake.