Adative restoration of Mediterranean saltmarshes: Lessons on dispersal and colonization of ponds and wetlands

Mediterranean saltmarshes are characterized by the presence of a spatial mosaic of temporary waterbodies in a dynamic equilibrium between turbid-water and clear-water alternative stable states. Autumn flooding and high exposure to wind-induced waves generate initial conditions characterized by high water turbidity and elevated sediment resuspension. Water depth and seed-bank abundance determine the early growth of vegetation, which subsequently determines the reduction of sediment siltation and the generation of a clear-water, macrophyte-dominated state. Without macrophytes, waterbodies become algae-dominated and are prone to cyanobacterial blooms that may cause waterfowl mortalities. Inter-annual variability in flooding levels leads however to a changing mosaic of macrophyte- and algae-dominated waterbodies. This conceptualisation of Mediterranean saltmarshes indicates that maintaining the spatial diversity of waterbody size and depth may be crucial to increase their resilence to changes in water supply and quality. However, this approach faces considerable uncertainties linked to the role of dispersal and colonization in determining the yearly dynamics of the aquatic biota. In a recent restoration of a Mediterranean saltmarsh (Doñana national park, SW Spain), an adaptive approach was taken to the re-creation of waterbodies: the design of 96 re-created waterbodies combined factorially two depths and three sizes, and they were made with two degrees of isolation. Monitoring of wetland colonization by aquatic macrophytes, the analysis of propagule-bank composition and field observations on endo-zoochorous and wind-mediated dispersal indicate that, despite high levels of dispersal by waterfowl in the region, wind dispersal of small-sized propagules determines species arrival in the early stages of colonization, with two species of Riella, one charophye and one species of Ranunculus arriving first. The reduction of turbidity caused by the presence of these early colonizers creates a window of opportunity for the establishment of new species, which may however be offset by the preclusion of late colonizations owing to priority effects.