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
D'autre part, une etude phytosociologique sigmatiste est effectuee afin de comprendre le determinisme des phytocenoses isolees, doni !'association du Callitricho hamulatae -Myriophylletum altemiflori ainsi que deux aspects du Callitrichetum hamulatae. Ces deux approches son! complementaires, l'une insistant sur l'ecologie des especes, mais risquant de masquer les groupements reellement observes, l'autre mettant en evidence des groupements dont l'ecologie ne peut etre definie uniquement com me Ia somme des ecologies specifiques.
Mots-c/es: macrophyte -riviere -analyse multidimensionnelle -phytosociologie -Limousin.Abstract.-The distribution of aquatic macrophytes (phanerogams, bryophyta, lichens and algae) was studied in two rivers in the Limousin region of France, the River Vie nne and the River Gartempe. At each of 1 06 representative sites, environmental factors were recorded as well as a range of physico-chemical parameters. In this study, two different approaches were used to assess relationship between macrophytes and their habitat. Firstly, a multivariate analysis helps to point out the individual species ecology ; for instance, some species were found in areas of high ammonium concentrations (Callitriche hamulata and Ranunculus peltatus) whereas others were associated with raised nitrate 152 concentrations (Callitriche platycarpa, Leptodictyum riparium). Secondly a sigmatist phy1osociological study was undertaken in order to understand the determinism of plant communities and, in particular, an association between Callitriche hamulatae and Myriophyllum altemiflorum plus two communities belonging to Callitrichetum hamulatae. Both approaches were complementary, one concerned with the individual species ecology but with a risk of masking genuine groupings, whilst the other sought to record inter-plant communities whose ecology might not be said to be only the sum of species ecologies associated within each plant community involved.
La relation entre les plantes rudérales et les structures archéologiques n'a pas encore été étudiée de manière systématique et les observations sont jusqu'ici demeurées ponctuelles. A l'initiative de la Direction des Antiquités Historiques du Limousin, cette relation fait à présent l'objet de recherches m éthodiques conduites par les Facultés des Sciences et de Pharmacie de l'Université de Limoges dans les trois départements de la Corrèze, de la Creuse et de la Haute- Vienne.
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