Climate and de-eutrophication affect abundance of benthos-feeding waterbirds in the Wadden Sea during stop-over

Abstract Habitat quality such as food availability and physical structures impact abundance of bird species. During 1987–2019 we studied long-term changes in the quality of the habitat of 13 waterbird species in the Wadden Sea, an important stop-over site on the East Atlantic Flyway between arctic breeding areas and wintering grounds in West Europe and Africa. Monitoring of waterbirds revealed that several species feeding on macrozoobenthos increased or remained stable in the northern and southern sections of the Wadden Sea, while their abundance mainly decreased in the central areas. The Wadden Sea is influenced by freshwater discharge from rivers draining a large part of central Europe, by geomorphological dynamics driven by the tides, and by sea level rise and climate. We hypothesised that the abundance of waterbirds that are dependent on intertidal flats for feeding is influenced by (a) regime shifts in the southern North Sea, (b) climate affecting riverine discharge (the amount of nutrients) from rivers in the Wadden Sea area and breeding conditions at arctic and boreal breeding grounds, (c) changes in geomorphology, (d) sea level rise and (e) biomass of macrozoobenthos. The results reveal that abundance of staging waterbirds is affected by (a) the North Atlantic Oscillation index (NAO) and regime shifts, (b) annual discharge of nutrients (N and P have both positive and negative effects) and (c) biomass of macrozoobenthos. Accretion or erosion of intertidal flats and sea level rise caused local displacements of waterbirds. In a broader context we found that waterbirds in the Wadden Sea are influenced by an array of variables including de-eutrophication due to improved waste water treatment and a reduced use of fertilizer in central Europe, regime shifts in the southern North Sea including the Wadden Sea (partly driven by the Gulf Stream) and changes in climate conditions, which may affect breeding conditions of waterbirds in Northern Europe as well as precipitation in Central Europe.