Aquatic bryophytes for a spatio-temporal monitoring of the water pollution of the rivers Meuse and Sambre (Belgium)

Jaccard's distances between the aquatic bryophyte assemblages recorded in 1972-1973 and in 1997 in nine study sites along the Belgian course of the rivers Meuse and Sambre were correlated with the differences of concentrations of 13 physico-chemical factors (O2, N-NO2-. N-NH4+, N-NO3-, P-PO43-, K+, pH, Ca++, Fe, Cu, Pb, Zn and Mn) between 1972-1973 and 1996. The strong decrease in heavy metals pollution and the decrease of the trophic level are correlated with the recolonization of the most polluted sites in the 1970s by the pollution-sensitive species present in the valleys at the beginning of the century. Conversely, N-NO2-, N-N03" and K+ have increased between the two surveys. They currently reach much higher concentrations in the Sambre than in the Meuse, as well as N and micro-pollutants such as pyrene and anionic detergents. The different loads of these factors in the studied rivers might explain their different recolonization patterns. Although some species might have developed a tolerance to certain factors, an effort to improve the quality of the effluents coming from industrial areas still remains to be made, especially in the frame of the projects of river flooding of the riverine areas.