The Seine estuary is structured by a strong salinity gradient influencing the distribution of biological communities. This gradient goes from salt water at the mouth (euhaline) to fresh water upstream (limnic), passing through zones of intermediate salinity (poly-, meso-, oligohaline). Biodiversity varies according to this gradient, reaching a minimum in the oligohaline zone, but with sometimes high biomass. The development of the estuary (damming, dredging) has reduced the diversity of lateral habitats, weakening their ecological functions. The restoration of these environments is therefore a priority, in particular to restore their productivity, purification capacity and value for wildlife.
In the intertidal mudflats, the benthic fauna plays a crucial role: oxygenation of the sediments, erosion control, support to the food chain and stimulation of primary production. These functions can be measured using bio-indicators.
The EVEREST project aims to test several hypotheses, including: the loss of functionality in the event of rapid plant colonization, the potentially higher productivity upstream, and the key role of river annexes in the hydrosedimentary balance. The objective is to define a compromise between environments that are too open or too closed to the fluvial regime, and to identify the favorable conditions for a successful ecological restoration.
The study includes the characterization of organic matter and oxygen penetration (PPO) in sediments, indicators of ecological status and biogeochemical functioning.
A follow-up of the Lillebonne mudflat (restored in 2017-2018) makes it possible to assess whether biological communities are stabilizing and whether ecological functions are taking place.
Finally, EVEREST will propose bioindicators to evaluate the ecological success of restorations, by measuring community responses to natural and anthropogenic stressors.
