Some Limnological Studies on the Lower Reaches of Tigris and Euphrates,Iraq

Some physical and chemical investigations of the lower reaches of Tigris and Euphrates as well as the upper reaches of Shatt al-Arab in Iraq were studied. The transparency showed local and seasonal variations. The Secchi values were markedly low due to the relative turbidity of Tigris and the maximum Secchi-disc values observed coincide with more sedimentation in Euphrates. The lowest Secchi values registered in April are attributed to the considerable increase in the amounts of suspended matter carried by the floods of the rivers. The slight decrease of water temperature with depth is related to the mixing processes. Considerable seasonal variations of air and water temperatures were observed. The lowest temperatures were recorded in January (winter) and the highest in August (summer). The temperatures gave considerably high values in April and October, since the region studied lies within an arid climate. Seasonal variations of pH were recorded and attributed to physical and biological changes in the region studied. The relatively high average pH values found in August and October give a good evidence for the phytoplankton abundance under better environmental conditions. The increase of the chlorosity content at the stations A, B, II and IV is mainly due to the effect of the brackish water from Hor Hammar on these localities. The dissolved oxygen showed remarkable seasonal variations and gave the highest average values in January and the lowest in August. It was concluded that the source of dissolved oxygen in the region studied is more related to the atmosphere than the photosynthetic activity, and the mixing processes as well as the water temperatures are the main factors affecting its concentration and distribution. The results obtained show strongly that the water characteristics at the stations I, II and III were different. This is a clear evidence that Tigris joins Euphrates at Qurna forming Shatt al-Arab.