AQUATIC WEED ASSEMBLAGES IN AN IBERIAN DRAINAGE CHANNEL SYSTEM AND RELATED ENVIRONMENTAL-FACTORS

The composition, cover and weed expression of the macrophytic assembla ges found in an Iberian irrigation system, as well as local environmen tal features, were studied in 100 sites. Species with more than 40% av erage cover included the exotics Myriophyllum aquaticum (Vell.) Verdc. , Eichhornia crassipes (Mart,) Solms. and Azolla filiculoides Lam., an d the natives Phragmites australis (Cav.) Trin, ex Steudel, Lemna spp. , Myriophyllum spicatum L., Potamogeton pectinatus L. and macroalgae. The environmental variables considered explained 20.1% of the total pl ant distribution and the species:environmental biplot explained 66.6% of the total variation. Site position, conductivity, pH and the amount of water used to irrigate contiguous crop fields were the most import ant abiotic contributors to the system's variability. Based on these r esults, significant differences were found within the study area for m ost of the abiotic variables considered. Species distribution appeared to be related to channel habitat diversity and channel age and degree of maintenance.