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Evaluation de l'état écologique des cours d'eau de Guyane : Un nouvel indice diatomique pour la Guyane Française (l'IDGF): Rapport final (V1 du 30-04-2019)

Abstract : The Water Framework Directive (WFD) requires European Union Member States to monitor the ecological quality of their water bodies through the study of key biological compartments in the aquatic environment. French Guiana is an Overseas Department (DOM) located in the Amazonian part of South America. Despite its geographical remoteness, this department is a European territory like all the other French overseas departments and is therefore subject to the requirements of the WFD. Previous works over the past decade, notably on the diatom compartment, have led to the development of several new biological indexes specifically adapted to the biogeographic conditions (geological substrates, climate, local fauna and flora) of each of these overseas territories. However, work progressed at a slower rate on the diatoms of French Guiana, due to the particular natural conditions that lead to very specific and still largely unknown diatomic communities. Since no more appropriate system was available, an evaluation system based on a local adaptation of the Specific Polluosensitivity Index (SPI) has been previously set up to access the ecological state of French Guiana's watercourses (see the French Evaluation Decree of 27 July 2015), despite obvious weaknesses linked to the biogeographical mismatch of the ecological profiles of many taxa (based on the worldwide scale) inside this index. However, the regular increase in the quantity of data gathered through Guianese monitoring networks over the past 9 years, as well as the progress made on diatom taxonomy, allowed the development of the 2017-2018 present study leading to the development of the new French Guiana Diatom Index (FGDI or IDGF in French). Inspired by the experience of previous diatomic indices developed in other French overseas departments, a methodology adapted to the Guianese context has been developed based on (i) a reduction in the number of taxa taken into account in diatom communities and on (ii) the search for warning taxa in a multimetric approach. This approach leads to a WFD-compatible index (which integrates deviation from regional references) providing results that are relevant, according to the opinion of local experts, and consistent with the natural context and anthropogenic activities encountered in French Guiana. The French Guiana Diatom Index thus produced is a first version, stabilized in its conception but perfectible in the years to come, notably by the acquisition of more data: 1) under intermediate anthropogenic impact (where little information was available in this first study); 2) in fluvial lentic zones; 3) as well as by the continuation of calibration works for the relevant evaluation of the sites under intermittent haline influence. The improvement of the index's response stability and the resolution of its perfectible aspects are directly linked to the incrementation of new data coming from the future WFD monitoring networks, but also to the acquisition of additional data specifically acquired for a better illustration of the above described.
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D. Carayon, François Delmas, A. Eulin Garrigue, R. Vigouroux. Evaluation de l'état écologique des cours d'eau de Guyane : Un nouvel indice diatomique pour la Guyane Française (l'IDGF): Rapport final (V1 du 30-04-2019). [Rapport de recherche] irstea. 2019, pp.123. ⟨hal-02609634⟩

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