Intérêts et limites des « indices macrophytes » pour qualifier la mésologie et la physico-chimie des cours d'eau: Exemples armoricains, picards et lorrains

Abstract

There is an ever-increasing need to monitor the quality of watercourses for physical features and water quality. Macrophyte indices may be involved in this, as well as the more usual methods such as water analysis or a benthic invertebrate biotic index. Some « macrophyte indices » from the literature were applied to 5 rivers in Northern France, differing with respect to geological substratum, trophic level and/or pollution. These methods were: Phytoecological Diagnosis involving Ellenberg's indices, Harding's Plant Score, Newbold & Holmes's Trophic Index, Haslam's Damage Rating Method. A comparison was made with the results of water analysis. Diagnoses were convergent, but with differing precision. The floristic lists used by the authors are incomplete and raise many taxonomic queries. The level of bioindication (« trophy », chemical parameter of pollution,…) with respect to physical features of the watercourse is not often given precisely by the authors. In the case of the methods of Ellenberg, Harding and Newbold & Holmes, the indices are not based on a detailed study of relationships between ecological factors and plant distribution (« ecological profiles »). Haslam's method is holistic, but the floristic precision is very poor. Its application in France will be difficult because there are many unknown data: reference phytocenosis, degradation successions, precise relationships between geology and water chemistry in each ecoregion. The methods of Harding and Newbold & Holmes are simpler and lead to quite a good diagnosis. Nevertheless, macrophyte indices seem interesting and are expected by field engineers. Some improvements for existing methods and general comments for an integrating index are developed. © Gauthier-Villars, 1993.