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

The distribution, coverage and area of Zostera noltii Hornem. were stu died in the four main seagrass stands in the intertidal zone of the Du tch Wadden Sea, two off the island of Terschelling and two in the sedi mentation fields off Groningen. In addition, the actual seagrass stand s and their close surroundings were mapped for several biotic and abio tic environmental factors, which were thought to be relevant to the pa st and present distribution of Z. noltii, viz, coverage by macroalgae, lugworm density and sediment composition. Compared with earlier findi ngs on distribution of seagrass in the Dutch Wadden Sea, the total are a of Z. noltii almost doubled between the beginning of the 1970s and t he end of the 1980s. This increase is mainly due to the rapid re-estab lishment of seagrass in the outer sedimentation fields of the saltmars h works off Groningen after a change of management. The total area of Z. noltii on the tidal flats off Terschelling decreased by approximate ly 20%. At the Terschelling seagrass stands, the coverage of seagrass was positively related to the clay content of the sediment and negativ ely 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 f ields off Groningen, almost no significant relationships were found be tween seagrass and environmental factors studied, which may be due to the fact that these seagrass stands are still expanding.