Suprabenthos of the upstream part of the Seine estuary (France)

The suprabenthos includes all bottom-dependent animals (mainly crustaceans, especially peracarids and decapods) that perform daily or seasonal vertical migrations with variable regularity. In the megatidal (tidal range > 8 m) Seine estuary, the suprabenthos of the downstream polyhaline and mesohaline zones have been well described, while this biological compartment have been neglected in the more upstream, oligohaline and freshwater zones. Seasonal sampling of the upstream part of the Seine estuary was conducted between 2019 and 2021 to investigate for the first time the abiotic factors influencing the structure and pattern of the suprabenthos sensu stricto in this part of the estuary. Two suprabenthic communities were distinguished, showing a contrasted spatio-temporal pattern in relation to environmental variables such as salinity, turbidity, water temperature and river discharge. The suprabenthos was characterized by a few numbers of species (13 in total), and was dominated by mysids (which represents 97.5% of total abundance), amphipods and decapods, which resulted in a low diversity of both communities. Species richness, abundance and biomass were the highest during summer at the most downstream stations. Some non-suprabenthic sensu stricto species (planktonic larvae, Teleostei) were also recorded, with major implications concerning the estuarine functions (spawning, nursery and foraging areas, etc.). The recruitment periods of some species are also discussed.