Effects of fish farm pollution on phytocenoses in an acidic river (the River Scorff, South Brittany, France)

Summary Effects of fish farm pollution were assessed by comparing upstream and downstream vegetation from three fish farms on the Scorff River system (Southern Brittany). Changes in the occurrence of several species including vascular plants and bryophytes (Myriophyllum alterniflorum, Octodice- ras fontanum), as well as overall variations of species abundance (increase in cover for Amblystegium riparium, Callitriche hamulata, C. obtusangula and decrease in cover for Scapania undulata, Chiloscyphus polyanthos, Ranunculus penicillatus) were observed. Principal Component Analysis showed the importance of physical features in governing distribution and cover of macrophytes for such a disturbance. The ecological response of the vegetation was greater in swifter and shallower reaches (or stretches) than in lower and deeper ones. A particular study of seven species led to the analysis of the respective impact of physical features and pollution on the distribution of species.