Experiments on epibenthic predation in the Wadden Sea
- K. Reise1
© Biologischen Anstalt Helgoland 1978
Field experiments were designed to evaluate the role of predators in the Wadden Sea, small predators like shore crabs, shrimps and gobies, and large ones like flatfish and birds. Exclosures, maintained in aCorophium volutator bed, an eelgrass bed, in a sandy and a muddy flat, protected the infauna from such epibenthic predation. The resulting changes in the macrofauna were recorded and compared with an unaffected control area. In sandy and muddy flats of the lower intertidal zone, cages (mesh size≤5 mm) altered abundance and composition of the infauna almost entirely. Nearly all species achieved higher population densities than in the control area, and in addition, the number of species increased as well. In the the scarcely populated mud flat a dense suspension-feeder assemblage emerged, associated with numerous tube-building polychaetes. The sand flat, normally dominated by deposit feeders, also became occupied by a dense suspension-feeder assemblage, mainly cockles. In contrast, narrowly meshed cages had only little effect in the beds of eelgrass and ofCorophium volutator. Both are positioned in the upper intertidal zone. Although a number of species still responded with significant increases in abundance, many remained indifferent or even tended to be less abundant within cages. Cages provided with a 20-mm mesh nylon net, excluding only birds, flatfish and the biggest crabs, increased significantly the survival of large-sized infaunal members. This was only apparent in the upper intertidal zone. It was concluded, that small sized epibenthic predators are the major determinants of the dynamic species abundance pattern of the lower intertidal flats. In the eelgrass bed, the meshwork of rootlets constitutes an important spatial refuge from these predators. The complex habitat structure causes a diversified faunal assemblage. TheCorophium bed in the uppermost intertidal zone is less accessable to predators like crabs, shrimp and gobies. The monotonous appearance of this faunal assemblage is assumed to be the outcome of competitive exclusion and of occasional harsh physical conditions.