High-intensity versus low-intensity restoration alternatives of a tidal marsh in Guadalquivir estuary, SW Spain

Dredged sediments from Guadalquivir River estuary were deposited on Algaida Marsh in the 1980s. Ecological restoration of 52 ha of Algaida Marsh was started in 2000. Intervention included the removal of dredged material, geomorphological reconstruction of surfaces, diversification of substrates and restoration of tidal flow. Biological colonization was achieved by natural mechanisms (water flow, active dispersal of fish, birds, and flying insects). Two intensities of intervention have been applied to adjacent sectors, and their effects on vegetation have been monitored for 5 years. The high-intensity sector was immediately colonised by fish (24 species, 30% of estuarine species) serving as a hatchery for the estuary. Both high- and low-intensity sectors attracted birds. Ninety-one species have been recorded, representing a large proportion of Guadalquivir Marsh species. Within 3 years, mudflats in the high-intensity sector were covered with marsh vegetation dominated by Spartina densiflora. The sector of limited intervention presents Sarcocornia perennis communities with limited Spartina incorporation. Results show a comparable vegetation succession on both sectors proceeding at a faster pace in the high-intensity sector where ecosystem connectivity was higher. Five years after restoration, species composition of the vegetation in both sectors was converging and showed only few differences to natural vegetation in tide marshes. The advantages of intensive versus extensive ecological restoration are discussed in terms of community biodiversity, successional changes, and costs.