Rapid losses of intertidal salt marshes due to global change in the Gironde estuary (France) and conservation implications for marshland passerines

Abstract

We analysed coastline movements between 2000 and 2016 along the 24.5 km of the mesohaline region of the North bank of the Gironde estuary (France). This sector is identified as hosting the largest expanse of salt marshes of the estuary and as an important breeding and stopover site for different marshland passerines of conservation concern. Our results from the study area reveal an average shore retreat of 14.74 ± 0.50 m over the period, corresponding to a loss of 49.96 ha of intertidal wetlands (i.e. 2.04 ha per kilometer of coastline) and reaching on average of more than 30 m for 42% of the coastline. This erosion dynamic, explained by a significant perturbation of the estuary’s hydro-sedimentary dynamic (due to decreases in freshwater discharges and relative sea level rise) highlights the rapid disruption that can occur in estuarine eco-complexes in response to global change. Given the impacts that estuarine intertidal wetland losses have on carrying capacity for marshland passerines, experimental management approaches are being tested in the study area to compensate for losses already observed and to anticipate those expected. These approaches reveal in particular that partial reconnection of agricultural polders to tide influences with a regulation system for water ingress may allow interesting trade-off between maintaining polders with agricultural activities such as grazing and conservation plans for vegetation of intertidal salt marshes exploitable by marshland passerines.