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Programme de Recherche et d'Étude 2012-2016 « Diatomées des rivières de Nouvelle-Calédonie » : Conception d'un nouvel indice de bio-évaluation de la qualité écologique des cours d'eau à partir des diatomées benthiques (IDNC) t.1 Rapport final d'élaboration de l'IDNC

Abstract : New Caledonia, tropical archipelago of Oceania located in the Pacific Ocean, 17 000 km far from metropolitan France, consists of very specific natural environments whose biological communities have a high rate of endemism, that gives them a strong heritage value worldwide. Local anthropogenic pressures (urbanization, nickel mining activities, agricultural activities, settlement of tribes ...) lead to risks of alteration of the concerned natural environments. Due to its sui generis status, the WFD does not apply in New Caledonia. However, the Deliberations of the North Province and the South Province have settled-up specific monitoring and protection systems for natural environments, which require the implementation of monitoring tools adapted to the local conditions and the specific biological processes that occur there. It is in such a context that EIL and CNRT "Nickel and its Environment" set up a research and development program aimed at the development and transfer of a new diatomic index adapted to the evaluation of ecological status of the "Grande Terre" watercourses, the New Caledonia Diatom Index (IDNC). This program, supported by the ASCONIT-Irstea consortium, benefited from the support of a CIFRE PhD thesis from the ANRT and from important logistical and technical support from the local BIOEKO consultant. Its implementation, phased from October 2012 to mid-2017, relied on 4 field campaigns covering 2 different seasons, which finally allowed to collect 210 complete doublets of data associating chemical analyzes of water to diatom inventories, on 74 different field sites covering an assortment of local natural conditions (reference sites) and anthropized (trophic pollution, mining alterations). As a result of extensive taxonomic work on globally unknown flora, 466 different diatom species have been identified, of which 217 freshwater taxa actually contribute to the calculation of the IDNC. The bio-statistical part of the program first included exploratory analyzes of abiotic data (description of data and gradients covered by the study dataset) and biotypological analyses (typical diatom communities associated with specific environmental conditions). Then the TITAN tool was mobilized to select alert taxa for 7 anthropogenic descriptors, 4 of which representing a nutrient enrichment (BOD5, NH4, PO4, NO3) and 3 other ones more dedicated to typical mining alterations (nickel and dissolved chromium, lateritic clogging). The notes of each individual metric were re-normed in EQR (scale ranging from 0 to 1) in order to take into account the appropriate reference level for the natural area considered ("adapted" HER frame). The IDNC, which is a resolutely multimetric diatom index, uses the aggregation of these 7 metrics into 2 integrative metrics, a trophic metric and a metric of mining alterations. According to the One Out - All Out principle, the final index result is the one given by the most altered metric of both. The development of a new hydrobiological index in a so specific biogeographic zone (biological communities little described and economic and very specific societal issues, including nickel mining) was a risky project in terms of initial feasibility. Depending on the good range of skills covered and prior experience capitalized on comparable overseas sites, the consortium in charge of the implementation of this program successfully overcame the difficulties encountered at different levels. At the end, the IDNC is a both innovative and operational tool: in addition to taxonomic decoding of neo-Caledonian species previously unrecognized worldwide, as well as their ecology, that is to our knowledge the world's first diatom index, built on a multimetric basis, which uses the detailed composition of communities at the specific level to assess an integrated ecological status taking into account trophic, physical and toxic alterations. In addition, the present program largely took into account the methodological transfer of this new method with the elaboration of a methodological guide IDNC, of a taxonomic atlas of New Caledonia integrating the chorology and the self-ecology of taxa, and a project of on-site training session dedicated to interested local private consultants, managers and decision-makers. Following this program, it would still be useful, later on, to develop an index calculation application allowing the automatic calculation of the regionalized diatomic status (in EQR) based on IDNC, according to the geographic coordinates of the field sites.
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J. Marquié, Sébastien Boutry, E. Lefrançois, Michel Coste, François Delmas. Programme de Recherche et d'Étude 2012-2016 « Diatomées des rivières de Nouvelle-Calédonie » : Conception d'un nouvel indice de bio-évaluation de la qualité écologique des cours d'eau à partir des diatomées benthiques (IDNC) t.1 Rapport final d'élaboration de l'IDNC. [Rapport de recherche] irstea. 2018, pp.284. ⟨hal-02606920⟩

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