Role of tide, light and fisheries in the decline of Zostera marina L. in the Dutch Wadden Sea
De Jonge, V.N.; de Jong, D.J. (1992). Role of tide, light and fisheries in the decline of Zostera marina L. in the Dutch Wadden Sea, in: Dankers, N.M.J.A. et al.Present and future conservation of the Wadden Sea: Proceedings of the 7th International Wadden Sea Symposium, Ameland 1990. Publication Series. Netherlands Institute for Sea Research (NIOZ), 20: pp. 161-176
In: Dankers, N.M.J.A.; Smit, C.J.; Scholl, M. (Ed.) (1992). Present and future conservation of the Wadden Sea: Proceedings of the 7th International Wadden Sea Symposium, Ameland 1990. Publication Series. Netherlands Institute for Sea Research (NIOZ), 20. Netherlands Institute for Sea Research (NIOZ): Texel. VIII, 301 pp., meer
In: Publication Series. Netherlands Institute for Sea Research (NIOZ). NIOZ: Den Burg. ISSN 0923-330X, meer
In the past, a sublittoral area of 65-150 km² and an eulittoral area of unknown size in the western Dutch Wadden Sea were covered with Zostera marina. This situation changed drastically in the early 1930s when all the eelgrass was killed by a 'wasting disease'. The eelgrass vegetation never recoverd to its former extent. In 1932 the dike closing off the Zuiderzee was completed, and as a result the hydrography and light conditions in the water changed. Calculations reveal that solely changes in tidal range and tidal curve could not have caused a deterioration of the light regime in the sublittoral zone and thus could not have caused the failing re-establishment of sublittoral Zostera marina beds. At many locations where Zostera marina used to grow prolifically, the current velocities have also changed dramatically (by a factor of 0.25 to 2.9) resulting in more sedimentation or erosion. This suggests that eelgrass was unlikely to re-establish at these new dynamic locations after the 'wasting disease' .The increased turbidity in the western Dutch Wadden Sea documented over the period 1950 to 1984 may also have hampered any re-establishment at other sublittoral locations. Calculations indicate that in 1983/1984, the period of greatest turbidity, Zostera marina was able to grow in a zone of only less than some decimetres around mean sea level, i.e. within the eulittoral zone. Improvement of the light conditions seems the main precondition for a successful recovery of the eelgrass vegetation. There is also evidence that human activities such as fishing for the spat of the blue mussel on natural intertidal mussel beds over the entire western Wadden Sea and the fishing for cockle severely damage local eelgrass stands. Improvement of the present situation can be achieved by protection of the remaining eelgrass stands, the reduction of intertidal fisheries by zonation and if possible measures to reduce the turbidity locally.
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