Dredged-Sediment Islands as Temporary Ecological Refugia: Rapid Waterbird Colonisation and Fast Habitat Closure in a Baltic Lagoon

Abstract Artificial islands built from dredged sediments are increasingly promoted as a nature-based solution to replace lost early-successional coastal habitats, yet quantitative evidence on their early ecological trajectories and management needs remains scarce. We assessed five years (2021–2025) of breeding-bird colonisation and vegetation change on two dredged-sediment islands in the Szczecin Lagoon (southern Baltic). Using complete breeding surveys, spatial mapping, camera trapping and UAV-based orthophotos, we quantified (i) the speed and magnitude of colonisation by ground-nesting waterbirds and (ii) the rate at which open nesting substrates were lost to succession. Colonisation was immediate and assemblages increased rapidly, including nationally rare breeders (e.g. Pied Avocet Recurvirostra avosetta and the first confirmed breeding record of Caspian Tern Hydroprogne caspia in Poland). Despite a small land area (~ 3.7 km² combined), the islands supported large shares of regional breeding populations for several species. In parallel, succession progressed quickly: tall vegetation (reed and willow) expanded from < 1% in 2021 to 23% in 2025 on the completed island (W22). Under a no-intervention extrapolation based on observed annual growth, open ground is projected to largely disappear within approximately three years, threatening the persistence of open-habitat specialists. Our results show that dredged-sediment islands can deliver rapid, high conservation benefits, but these benefits are time-limited unless vegetation control and predator-surveillance are integrated into long-term dredging operations and site management.