INTERACTIONS BETWEEN ARENICOLA-MARINA AND ZOSTERA-NOLTII ON A TIDAL FLAT IN THE WADDEN SEA

On the tidal flats off Terschelling in the Dutch Wadden Sea, the seagr ass Zostera noltii Hornem. and adult lugworms Arenicola marina L. show strictly separate distributions with an abrupt border between the 2 c ommunities. Since seagrass and lugworms do not show different limitati ons regarding sediment type and emersion period, the border must be th e result of other environmental factors or plant-animal interactions. Transplantation experiments on the tidal flats showed that the seagras s was quite capable of growing on the lugworm tidal flat when worms we re absent. However, the seagrass density and biomass were severely inf luenced in the presence of lugworms, most probably due to the worms' s ediment-reworking activities. Lugworms quickly disappeared from the ex perimental sites on the seagrass tidal flat, irrespective of the actua l presence of seagrass. A local survey of the seagrass area showed tha t funnel formation by lugworms to sufficient depth was restricted by a dense day layer under the sediment surface, a remnant of a former sal t marsh. These results indicate that an increase of lugworm densities in the Dutch Wadden Sea may have contributed to the decrease of the ar ea covered by Z. noltii on the tidal flats off Terschelling over the l ast 20 yr.