Long-term displacement of intertidal seagrass and mussel beds by expanding large sandy bedforms in the northern Wadden Sea |
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| Authors: | Tobias Dolch Karsten Reise |
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| Institution: | 1. School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK;2. Centre for Environment, Fisheries and Aquaculture Science (Cefas), Pakefield Road, Lowestoft NR33 0HT, UK;3. Changing Ocean Research Unit, Fisheries Centre, The University of British Columbia, 2202 Main Mall, Vancouver, BC, V6T 1Z4, Canada;1. IMARES Wageningen UR, Institute for Marine Resources and Ecosystem Studies, PO Box 68, 1970 AB IJmuiden, The Netherlands;2. Marine Research Institute, Skulagata 4, PO Box 1390, 121 Reykjavik, Iceland;3. Aquaculture and Fisheries Group, Wageningen University, PO Box 338, 6700 XX AH Wageningen, The Netherlands |
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| Abstract: | On aerial photographs, sandy tidal flats display (1) large sandy bedforms (> 10 m long, > 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 km2, 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. |
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