Assessing reference sites and ecological quality of river plant assemblages from an Iberian basin using a multivariate approach

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This study attempts to develop and discuss the applicability of a method for using river plant assemblages as indicators of ecological quality in a southern Iberian basin, located in Portugal. Several relevant aspects contribute to make this purpose scientifically challenging. Two thirds of the basin are located in Spanish territory, a large part of the river system is intermittent, the historical reference data on river plants in the basin are non-existent and the distribution patterns of Iberian river plants are scarcely known. A survey of 40 sites was conducted in late spring 1999, covering all the basin. Vegetation data were treated by hierarchical classification and by canonical correspondence analysis, using both environmental (19) and anthropogenic (9) variables. Three plant ecotypes were identified, having different geological and geographical backgrounds. For the selection of reference sites of each ecotype, the environmental variables significantly explaining within-ecotype species distribution were determined by a forward selection procedure and latterly used as co-variables. A partial canonical correspondence analysis was then performed for each ecotype, using a unique variable for the anthropogenic influence, henceforth, leading to a sequence of less human disturbed sites being opposed to the more disturbed ones along the first canonical axis. A cluster analysis for the group of sites of each ecotype contributed to the final distinction of the reference sites, still presenting some intrinsic natural floral variability. The Bray-Curtis dissimilarity within the group of sites from each ecotype ranged between a natural similarity variation and a maximum dissimilarity (a community within the same ecotype but with a plant composition completely different from the floral reference). This range was partitioned into four classes of floristic quality corresponding to an increasing deviation from the pro-natural floristic state. The use of river plants as indicators of river integrity and the method used are discussed, considering the particular features of the river system studied and its plant assemblages.