Long-term displacement of intertidal seagrass and mussel beds by expanding large sandy bedforms in the northern Wadden Sea

a r t i c l e i n f o On aerial photographs, sandy tidal flats display (1) large sandy bedforms (N10 m long, N 3 m wide), indicating effects of strong hydrodynamics on sediment relief, and (2) beds of seagrass and mussels, indicating stable sediment conditions. These physical and biogenic structures have been mapped from aerial photographs taken in a back-barrier tidal basin of the North Sea coast at low tide between 1936 and 2005. Fields of large intertidal sandy bedforms show a consistent spatial distribution in the central part of the basin, and have increased in area from 7.2 to 12.8 km 2 , corresponding now to 10% of the tidal flats. Areal expansion may be linked to a rise in average high tide level and an increase of the expansion rate from the 1960s to the mid 1990s might be traced back to an increased frequency of storm tides during this period. It is shown that expanding fields of large sandy bedforms have replaced mussel beds in the low tidal zone and displaced seagrass beds in the mid tidal zone. Fields of intertidal large sandy bedforms are expected to expand further with an accelerating rise in sea level, and it is recommended to monitor these physical indicators of sediment instability and disturbance of biogenic benthic structures by analysing aerial photographs. Seagrasses are the dominant vegetation of shallow sandy bottoms in coastal areas around the world (den Hartog, 1970) and they are of high ecological importance for coastal ecosystems (Cunha et al., 2005). Seagrasses are sensitive towards eutrophication, turbidity, strong hydrodynamics (currents and waves), sediment instability, desiccation as well as changes in temperature and salinity (de Jonge and de This has resulted in a worldwide decline of seagrass beds often due to human impact as observed since the early 20th century (Hemminga and Duarte, 2000; den Hartog and Phillips, 2001). From 1931 to 1934, the seagrass pathogen Labyrinthula zosterae caused the so called 'wasting disease' and severely eliminated seagrasses in the Wadden Sea (southeastern coastal North Sea) (den Hartog, 1987) while eutrophication caused another collapse of beds in the 1970s/1980s (van Katwijk et al., 1999). Today, seagrass beds dominated by Zostera noltii are occurring on about 10% of the tidal flat area in the northern Wadden Sea, mostly in the upper zone and where sheltered by islands and high sand bars against a rough sea (Reise and …