Evolution of the macrophyte vegetation and trophic state in two streams of the Molignée catchment (Condroz, Belgium)
The aquatic, amphibious and supra-aquatic vegetation of a stream (called Molignée) and its main tributary (called Flavion) from a catchment in the south of Belgium was studied in 1994 and 2005 in the main stream and in 1995 and 2006 in the tributary. The stream channels were divided into 50 and 36 contiguous 500-meter long sections in the Molignée and Flavion, respectively. In each section, the vegetation was described and various mesological parameters of the stream channel and floodplain were recorded. In 2005 and 2006, the water and amphibious Bryophytes as well as the filamentous Algae were also recorded. The statistical data analysis was conducted from the tables of aquatic and amphibious macrophytes, by means of multivariate analyses such as non-symmetrical correspondence analysis. The dataset containing juxtaposed floristic and mesological tables was analysed using multiple factor analysis. The macrophyte biological index for rivers (IBMR) was calculated for each section. The results show, over a period of eleven years, a decrease in frequency of a lot of bank species and important changes in the vegetation linked to the water environment. There is an increase of species favoured by organic pollution and a reduction in species intolerant of that pollution. The presence of farms, cattle trampling, domestic pollution or urbanization are factors correlated with these changes. The contributions of the various techniques of data processing are discussed. Management suggestions are discussed.