Estuarine lateral ecotones shape taxonomic and functional structure of fish assemblages. The case of the Seine Estuary, France

Estuaries are complex ecotones including multiple lateral habitats that play essential functions for fish assemblages. However, the distribution of fish diversity and community assembly processes remain poorly documented in these habitats. This paper investigated the taxonomic and functional diversities of fish assemblages along lateral continuums of three salinity areas over the longitudinal gradient of the Seine Estuary. We examined fish diversity at a local scale (⍺-diversity) and between local scales (β-diversity) across the lateral habitats (estuarine main channel, tidal marsh, and diked marsh). Overall, the taxonomic diversity follows a typical ecotone distribution, with the highest richness reported at the system margins and high species dissimilarity between communities. The decrease of functional diversity in upper part of the lateral ecotone probably reflects the homogenisation of ecological niches. Changes in assembly processes between the three salinity conditions suggests that several lateral ecotones occur in estuaries with complementary functions for fish assemblages.