A REFERENCE SYSTEM FOR CONTINENTAL RUNNING WATERS - PLANT-COMMUNITIESAS BIOINDICATORS OF INCREASING EUTROPHICATION IN ALKALINE AND ACIDIC WATERS IN NORTHEAST FRANCE

Two bioindication scales of the degree of eutrophication based on aqua tic macrophyte communities were established in two types of running wa ters free of organic matter, the one in acidic ''soft'' waters (pH 5.5 -7.0, conductivity 40-110 mu S.cm(-1)), the other in alkaline hard wat ers (pH 7-8, conductivity 500-900 mu S.cm(-1)). We show that the main determining factor of the macrophyte distribution is the nutrient leve l (trophy), especially the level of phosphate and ammonia. The acidic scale, with increasing pH, includes four stages ranging from oligotrop hic to eutrophic level (traces to 300 mu g.l(-1) N-NH4+ and P-PO43-), while the alkaline scale at constant pH comprises six stages of a trop hic gradient. For the most part, the floristic composition found in th e two sequences is different and depends on conductivity and alkalinit y variation. However, some species occur in the two scales and may ref lect differences in the trophic level, depending on whether the waters are alkaline or acidic. This change of trophic level for these specie s is discussed.