A practical concept for the ecological assessment of aquatic ecosystems: application on the River Dore in France

The ecological assessment of freshwater ecosystems necessitates to consider (i) the operational biological classification for defining a range of ecosystem alterations, (ii) operational biomonitoring tools fulfilling the requirements of the classification, (iii) ecological quality objectives to be preserved or restored. The development of this biomonitoring approach was illustrated by the study of the River Dore. Four biological qualities were defined with their related bio-indicators: (1) general biological quality (invertebrates), (2) biological sediment quality (oligochaetes), (3) biological water quality (diatoms) and (4) biological fish quality (fish communities). The selected bio-indicators were adjusted to an ecological classification model, based on a range of 8 ecological qualities, from `pristine' (S0) to extreme toxicity (S7). The model allows to use numerical index values as well as methods without index calculations, and to assess the general ecological quality of the river and its time-dependent evolution. At the sites downstream of industrial discharges, an improvement of the general ecological quality was observed from 1983 to 1995 in relation to sewage-treatment actions. A general ecological quality objective is proposed for each site of the river, taking into account the past and present ecological evolution of the river. The relevance of the approach greatly depends upon the accuracy of bio-indicator adjustment to the classification model and would be enhanced by taking into account other biological characteristics, like trophic status and hyporheic biocenoses.