THE DISTRIBUTION OF THE AMPHIPOD COROPHIUM-ARENARIUM IN THE DUTCH-WADDEN SEA - RELATIONSHIPS WITH SEDIMENT COMPOSITION AND THE PRESENCE OF COCKLES AND LUGWORMS

On the tidal flats in the Dutch Wadden Sea Corophium volutator is a do minant species of the upper intertidal zone; the closely related Corop hium arenarium is usually found in the lower zone, but only in low den sities (a few hundreds per m(2)). A survey in the Dutch Wadden Sea sho wed that this zonation pattern was only present when a muddy sediment was found in the upper zone and a sandy in the lower zone. C. arenariu m was restricted to sandy sediments, C. volutator to muddy sediments. Where a sandy sediment was found in the upper intertidal zone, C. aren arium locally occurred in relatively high densities (a few thousands p er m(2)). An aquarium experiment showed that C. arenarium actively avo ided muddy sediments. Field experiments were carried out to study the influence of other macrozoobenthic species (known to affect the relate d C. volutator) on the abundance of C. arenarium. Within large defauna ted areas small plots were stocked with different densities of the lug worm Arenicola marina and the cockle Cerastoderma edule. In small plot s within a natural benthic community densities of these species were a lso augmented or (in A. marina) reduced. Strongly negative density-dep endent effects of both A. marina and C. edule were found on the abunda nce of C. arenarium. In the natural situation, its densities showed A. marina to be the most important factor in determining the abundance o f C. arenarium. In particular the removal of lugworms caused a strong increase in C. arenarium densities. These results agreed with the dist ribution of these species along a transect perpendicular to the shore of Schiermonnikoog, where a significant negative correlation was found between the densities of A. marina and C. arenarium. Aquarium experim ents showed that the negative effect of cockles and lugworms must be d ue to migration rather than mortality in C. arenarium.