Populations of Myriophyllum alterniflorum L. as bioindicators of pollution in acidic to neutral rivers in the Limousin region

Myriophyllum alterniflorum D.C. (Haloragaceae) was studied in the acidic to neutral rivers Vienne and Gartempe (Limousin region, France). Two complementary levels of organisation were evaluated to determine their usefulness for pollution assessment: morphological traits of river populations, and eco-physiological responses to ammonium enrichment in indoor experiments. Inter-node length, stem diameter and leaf length were measured and their ratio calculated for 100 selected plants collected in 20 homogeneous areas. Using multidimensional analysis, their ecological significance for trophic assessment was demonstrated: stem diameter was linked to conductivity, and leaf length/stem diameter ratio to phosphates. Nitrate reductase activity was measured in three populations (upstream, median and downstream populations) after experimental enrichment with ammonium. Ammonium enrichment decreased activity starting at very low concentrations. However, inhibition levels depended on tested populations, with upstream populations being less sensitive due to natural ammonium content in water. The differences could be explained by an eco-adaptation to the trophic level of water. The relationships between the two levels of organisation are discussed. These results could help to distinguish different ecotypes using Myriophyllum alterniflorum