Microbial mats are often the dominant benthic biotype in desert streams, however information on such mats is very scarce. We investigated the gross morphology of microbial mats and their cyanobacterial species composition in a perennially flowing wadi in Northern Oman, in relation to current flow and other biotic and abiotic variables. Physical and chemical parameters at the six sites were generally similar with low concentrations of soluble phosphorus and nitrogen; however, flow rates varied greatly with the highest velocity being 50 cm/s. The mats varied greatly in their gross morphology with the four sites with the highest current velocities forming low laminated structures attached to cobbles, and the two sites with minimal or no flow forming complex three-dimensional structures or free floating balls 1 to 3 cm in diameter. The concentration of chlorophyll avaried between 2.6 (±0.02) and 1.4 (±0.11) mg chlorophyll a g-1 mat fresh weight, with the highest concentration detected in the free floating balls. Twenty one cyanobacterial morphotypes belonging to 14 genera (5 unicellular and 9 filamentous) were identified withCalothrix and Leptolyngbya spp. as the most abundant. Cluster analysis of the cyanobacterial species composition showed that mats could have very different morphologies despite similar species compositions suggesting a high degree of plasticity of mat form. We conclude that microbial mats are important elements in Wadi ecosystems and have great variety of structure and community composition.
Key words: Microbial mats, desert stream, cyanobacteria, nutrients, nitrogen fixation.
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