A conceptual approach to the biomonitoring of freshwater: the Ecological Ambience System

The concept of ecological ambience (Ecological Ambience System, EASY) is based on the idea that biocenoses (BIO) are not only related to the input of organic and mineral substances (IN) but also to the way they are stored and processed by the ecosystem. Storage, assimilation and self-purification processes ("ecosystem defences": ED) are likely to vary among the different functional units (FUs) of the ecosystem. The functional units have been defined on the basis of a simple physical description of sites in an ecosystem, because the physical structure of these units is considered as being of prime importance in the ED processes. For example, mineral and organic substances may be preferentially stored in fine-sediment units, whereas the mineralization rate of organic matter is more likely to be highest in coarse permeable sediments. If the stream ecosystem is viewed as a mosaic, its overall ecological defences will depend upon: 1) the self-purification capacity of the different functional units; 2) their relative proportion within the ecosystem. The EASY concept is now used for ecological researches and also has several applications in the biomonitoring of running waters, illustrated by the study of the River Dore. Biological compartments, specific for each functional unit, are related to operational bio-indicators to build up a global harmonisation system for biomonitoring indices. Four main biological compartments were defined with their related bio-indicators: 1) general biological quality (IBGN biotic index), 2) biological sediment quality (IOBS oligochaete index), 3) biological water quality (diatom index IBD) and 4) biological fish quality (study of fish communities). The selected bio-indicators were adjusted to an ecological classification model (Typic concept). A weighting system of the general ecological quality at a site based on the percentage cover of fine sediments is proposed. This approach can be developed using several other compartments. Major difficulties and potential improvements are discussed.