Environmental factors related to the establishment of algal mats in concrete irrigation channels

Abstract

The establishment of algal mats in concrete irrigation channels presents a serious weed problem in Mediterranean areas. These periphytic growths can reach two meters in filament length, 20 cm in mat thickness, 100% cover and 1 kg of fresh weight per square meter. These biological characteristics of the algal mats, and also of the drifting mats, present important temporal and spatial variation in the channel system. The species richness of the algae mats was low and only two macroalgae species were present. The diatom Biddulphia laewis was dominant in most channels. The green Oedogonium sp. was predominant in one channel characterised by lower conductivity and alkalinity, but with higher nutrient levels and this species also dominated in drift samples. Redundancy analysis showed that channel width and the residence time of the water in the channels were the variables that explained most of the variability in the biological data. Some water characteristics were of secondary importance, especially pH and temperature. Biotic factors, such as grazing, were only locally determinant. Water velocity did not present enough variation to create a gradient significantly affecting the biotic variables, except at specific locations, such as the control-gates.