A reference system for continental running waters: plant communities as bioindicators of increasing eutrophication in alkaline and acidic waters in north-east France

Abstract

Two bioindication scales of the degree of eutrophication based on aquatic macrophyte communities were established in two types of running waters free of organic matter, the one in acidic “soft” waters (pH 5.5–7.0, conductivity 40–110 μS.cm⁻¹), the other in alkaline hard waters (pH 7–8, conductivity 500–900 μS.cm⁻¹). We show that the main determining factor of the macrophyte distribution is the nutrient level (trophy), especially the level of phosphate and ammonia. The acidic scale, with increasing pH, includes four stages ranging from oligotrophic to eutrophic level (traces to 300 μg.l⁻¹ N-NHinf4^p+ and P-PO₄ ³⁻), while the alkaline scale at constant pH comprises six stages of a trophic gradient. For the most part, the floristic composition found in the two sequences is different and depends on conductivity and alkalinity variation. However, some species occur in the two scales and may reflect differences in the trophic level, depending on whether the waters are alkaline or acidic. This change of trophic level for these species is discussed.