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Along 200 km of the River Garonne (France), epilithon colonization of artificial substrates was followed over a four week period in eight sites with different human influences. Salinity and nutrient concentrations increased progressively downstream, but polluted sites and reservoirs acted as discontinuities with regard to physico-chemical features. Two experiments were carried out during the winter and the summer low water period. The relative abundance of each species was used to determine how community structure fluctuated in time and along the longitudinal stream gradient. Natural epilithon was investigated in the same way at the end of the experiments. Achnanthaceae, in winter, and Fragilariaceae, in summer, were the major colonizers after seven days and were then replaced by Naviculaceae and Bacillariaceae. The winter low water period was characterized by Diatoma vulgaris, Diatoma ehrenbergii, Fragilaria arcus, Gomphonema olivaceum and Navicula lanceolata; the summer low water period was marked by Achnanthes minutissima, Cocconeis placentula var. euglypta, Cyclotella meneghiniana, Cyclotella pseudostelligera, Navicula capitatoradiata, Navicula recens, Navicula subminuscula, Navicula saprophila, Nitzschia palea and Nitzschia paleacea. Along the longitudinal gradient, there was a progressive substitution of Achnanthaceae and Fragilariaceae by Naviculaceae and Bacillariaceae. The impact of pollution was more visible during the summer low water period. In summer, the epilithon communities architecture was probably partially due to grazing. The diatom flora (132 taxa) was typical of a limestone region river. The SHANNON Index and the analysis of the diatom communities on artificial substrates and on the natural rocks showed a great similarity. |
