Long-term effects of tidal restriction on fish assemblages in east Atlantic coastal marshlands

Anthropogenic pressures on coastal and estuarine wetlands are causing a decline in their ecological value and resources. The construction of hydraulic infrastructures stopping tidal influxes is among the most important historical and current threats to coastal wetlands. Yet, the potential effects of tidal restriction have been poorly studied in European marshlands. We analysed the long-term effects of tidal restriction on the fish assemblage of Atlantic marshlands. Multivariate and univariate analyses detected significant spatial and temporal differences in the structure and functioning of fish assemblages between an impacted marshland and control sites. Abundance and biomass values in the impacted marshland were 3 to 4 times higher than in controls, and the fish assemblage was dominated by exotic invasive species, while the control marshlands were dominated by native species. Generalist freshwater and estuarine guilds were dominant in the impacted marshland with respect to controls, where guilds were evenly represented. The diversity of feeding mode functional groups in controls was significantly higher than in the impacted marshland, which was strongly characterized by omnivorous species. This study highlights biotic homogenization as one of the main consequences of tidal restriction through the shift from native assemblages of specialized fish species towards assemblages of generalist, cosmo - politan and non-native species.