Résumé
In the context of WFD implementation on Mediterranean lagoons, work has been carried out since 2010 by Ifremer, Tour du Valat and IRSTEA (Onema and AERM & C fundings), to assess the relevance of macrophytes and macrofauna indicators for oligohaline and mesohaline lagoons (mean salinity <18 PSU). The question of the relevance of indicator metrics and thresholds for the "phytoplankton" biological quality element also arises, especially due to the confined nature of olgo-mesohaline lagoons. The objective of this study is to characterize the abundance and composition of phytoplankton on 5 oligo and mesohaline lagoons of the French Mediterranean coast (Scamandre, Crey, Charnier, Bolmon and Grand Palun). Analyses of accessory pigments (chlorophyll and xanthophylls) performed for the five lagoons, highlight different phytoplankton composition profiles, related to total phytoplankton biomass levels. Grand-Palun lagoon, characterized by moderate phytoplankton biomass, has a high pigment diversity, with a similar contribution of the different phytoplankton groups. The lagoons of the Scamandre-Crey-Charnier complex, characterized by high phytoplankton biomass and dominance of nanophytoplankton (Scamander and Crey) or picophytoplankton (Charnier), predominantly present species of brown line and cyanobacteria. Finally, Bolmon lagoon, characterized by a very high phytoplankton biomass and dominance of picophytoplankton, is distinguished by the importance of species of the green line (Chlorophyta and Prasinophytes) and the possible presence of cyanobacteria. The current grids for phytoplankton or water physico-chemical quality have been developed and consolidated on a data set of poly-euhaline lagoons. The thresholds have to be consolidated for all oligo-mesohaline lagoons, taking into account the hydromorphological particularities and pressures on these ecosystems (confinement, sedimentary stock, continuous input through channels,…).