Succession of Egeria densa in a drinking water reservoir in Morbihan (France)

Located in western France, the Pen Mur dam of Muzillac (Morbihan) is mainly used for the production of drinking water and for fishing. Over the past f ew years, the upstream part of this eutrophic waterbody has been invaded by Egeria densa. To define suitable methods for the management of this plant, a 1996 study focused on the morphometric characteristics of the reservoir, the quality of its water and sediments, the diversity of its vegetation an d various aspects of colonisation by E. densa. Analyses of usage and pollut ion were also made, as well as a review of the techniques available to cont rol the development of this species. Based on the results of this study, a management plan was put forward. Studies in 1997 and 1998 showed a signific ant decline in the level of colonisation. This was probably due to the subs tantial floods which uprooted submerged plants in winter, followed by massi ve ice formation during winter and warm temperatures in spring leading to a development of cyanobacteria which are detrimental to macrophytes. Monitor ing the competition between E. densa and the cyanobacteria could lead to de termination of the causes of the predominance of one or the other.