Epilithic diatom assemblages and their relationship to environmental characteristics in an agricultural watershed (Guadiana River,SW Spain)

Large-scale patterns of benthic diatom assemblages were analyzed in an agricultural basin, the Guadiana River. The distribution patterns of epilithic diatom assemblages were analyzed at different spatial scales: the whole watershed, the upper calcareous subcatchment and the mid-lower siliceous subcatchment. At the whole watershed scale, two major ecological gradients were revealed. The first one summarized the diatom distribution throughout a nutrient concentration gradient, while the second gradient was related to the geological structure of the watershed. Variance partitioning allowed the effects of the different sets of environmental parameters related to every CCA gradient to be separated. Analyzing the subcatchment gradients with partial CCA allowed us to define specific key factors that affect diatom species composition. Although water chemistry consistently played the most important role in structuring diatom assemblages in the Guadiana, spatial factors such as altitude or geographic location also explained some variation in diatom distribution.