Environmental controls on foraminifera in Lake Qarun, Egypt

Foraminifera were investigated in Lake Qarun, an Egyptian inland saltwater lake, to determine their spatial distribution and the most common environmental factors that influence this distribution. Forty surface sediment grab samples from the lake were found to be rich in benthic foraminifera. Their faunal density ranged from 63 to 13,400 tests g(-1) with an average of 1700 tests g(-1) (s = +/- 2480). Faunal diversity ranged from 4 to 15 species with an average 10 species per sample (s = +/- 2.3). Both faunal density and diversity decreased in areas (biotopes 1 and 3) dominated by clay and silt (grain size <63 mu m) rich in organic matter. Degradation of organic matter by aerobic and anaerobic bacteria could lead to very low dissolved oxygen in pore and bottom waters where foraminifera live. Faunal data were used to divide the lake into three ecological biotopes. Samples from biotopes; 1 and 3 are characterized by high clay-and-silt content, with Ammonia tepida and Quinqueloculina seminula characterizing biotope 1 and A. tepida, Q. seminula and Cribroelphidium excavatum dominating biotope 3. Samples from biotope 2 are characterized by bioclastic sand-dominated substrates that are rich in Q. seminula, A. tepida, Quinqueloculina poeyana, Bolivina striatula and Rosalina globularis. Ammonia tepida and Quinqueloculina seminula compose similar to 70% of the total foraminiferal assemblage in Lake Qarun. The distribution patterns of these species indicate that each is controlled by distinct environmental factors. In biotopes I and 3, A. tepida positively correlates with high clay-and-silt content and Q. seminula correlates with high total dissolved solids. Cribroelphidium excavatum correlates positively with magnetic susceptibility and negatively with total dissolved solids (TDS). In biotope 2, Quinqueloculina poeyana, Bolivina striatula and Rosalina globularis correlate positively with coarse bioclastic substrates enriched in phytal remains, a condition consistent with the presence of the epiphytic taxon R. globularis. The occurrence of these halophilic species in Lake Qarun seems to be a consequence of rising "salinity" that has made the lake a suitable habitat for these foraminiferal taxa.