Composition, structure and stratification of laminated cyanobacterial matsin an irrigation canal (Al-Kharj, Saudi Arabia)

Three forms of algal mats : subaerial, emersed and submersed, and their spe cies composition, structure and stratification were studied for a period of twelve months (1996-1997) in an irrigation canal at Al-Kharj, Saudi Arabia . The development of the mats is a continuous process in a gradient fashion f rom the outlet of a piedmont cave-lake to the end of the canal. The growth maxima occurred at the first weir, which is subject to fast water-current a nd spray-wetted zone. Mats developed as discrete associations, recognizable by their colour and shape. Hard lithified, soft and floppy, and thin crust types of mats developed in their respective zones and comprised separate biotic communities. Ecologica l factors such as the nature of the microhabitat, water-chemistry, fluctuat ion in water-flow, depth of water column, splashing and temperature may hav e influenced their formation. The emersed mats with Phormidium tenue were p redominant from November to February, whereas the subaerial mats above with Tolypothrix distorta adhere to the canal wall as a permanent feature. The subaerial mat has stromatolite-like morphology and is considered a nuisance to the water and drainage maintenance.