Résumé
Coastal marshes are submitted to huge management due to anthropogenic pressure and thus, it is essential to preserve their biodiversity, their ecological functions and the ecosystem services they can provide. This study investigates the diversity and abundance of planktonic communities (heterotrophic prokaryotes, heterotrophic protists, microphytoplankton and metazooplankton). The aim of this study is to provide a first quantitative inventory on the plankton communities present in two marshes and to construct the different typologies of planktonic food webs in these marches. Seasonal samplings (4) for environmental variables, nutrient concentrations and planktonic communities were conducted at 2-3 stations on each marsh during a year. A total of five different types of planktonic food web were determined, three of them were found in both marshes. The saltmarsh phytoplanktonic communities were dominated by Cryptophyta, nanoflagellates and Cyanobacteria. The freshwater marsh was dominated by Cryptophyta in autumn and winter and by a diverse phytoplankton community in spring. Marine copepods (Calanoida and Harpacticoida) characterized the saltwater metazooplanktonic communities, Cladocerans and Rotifera the freshwater ones. Overall, the planktonic diversity was higher in the freshwater marsh (102 taxa) than in the saltwater marsh (54 taxa).Phytoplankton blooms represented nutrient and CO2 uptake in both marshes and this reification function seemed most efficient in autumn in the saltmarsh. Considerable zooplankton communities represented a potential for nursery. Of the three management actions performed, only periodic water renewals might affect the seasonal dynamics of planktonic communities.