WAX AND WANE OF ZOSTERA-NOLTII HORNEM IN THE DUTCH WADDEN SEA

The distribution, coverage and area of Zostera noltii Hornem. were studied in the four main seagrass stands in the intertidal zone of the Dutch Wadden Sea, two off the island of Terschelling and two in the sedimentation fields off Groningen. In addition, the actual seagrass stands and their close surroundings were mapped for several biotic and abiotic environmental factors, which were thought to be relevant to the past and present distribution of Z. noltii, viz, coverage by macroalgae, lugworm density and sediment composition. Compared with earlier findings on distribution of seagrass in the Dutch Wadden Sea, the total area of Z. noltii almost doubled between the beginning of the 1970s and the end of the 1980s. This increase is mainly due to the rapid re-establishment of seagrass in the outer sedimentation fields of the saltmarsh works off Groningen after a change of management. The total area of Z. noltii on the tidal flats off Terschelling decreased by approximately 20%. At the Terschelling seagrass stands, the coverage of seagrass was positively related to the clay content of the sediment and negatively to the density of lugworms (Arenicola marina L.). The presence of macroalgae, however, seems to play a minor role in the distribution of this seagrass species in the Dutch Wadden Sea. In the sedimentation fields off Groningen, almost no significant relationships were found between seagrass and environmental factors studied, which may be due to the fact that these seagrass stands are still expanding.