STRUCTURING ZOOBENTHOS - THE IMPORTANCE OF PREDATION, SIPHON CROPPINGAND PHYSICAL DISTURBANCE

In a set of field and laboratory experiments, the relative importance of (1) direct predation (measured as mortality in the prey populations ), (2) physical disturbance of the sediment (sand and mud), and the co mbined effects of disturbance and predation, and (3) sublethal siphon cropping, were tested on an infaunal food web in the low-saline northe rn Baltic Sea. The predators used (Saduria entomon and Crangon crangon ) are the major epibenthic crustacean predators of the northern Baltic coastal waters. and the prey organism in the experiments (the bivalve Macoma balthica) is, by frequency of occurrence and by biomass, the d ominant infaunal organism among the zoobenthos. In the field the preda tory effect of Saduria was tested on the entire infaunal community. Th e field experiment on direct predation by Saduria on the entire infaun al assemblage significantly reduced the density of the polychaete Mana yunkia aestuarina. Aquarium experiments show that although direct pred ation by Saduria or physical sediment disturbance alone had little eff ects on juvenile (< 3 mm) Macoma, the combined effects of these factor s were significant, and more important than the sum of the two single factors, demonstrating synergistic negative effects. Experiments with Saduria and Crangon in relation to adult (7-17 mm) Macoma showed suble thal effects on length-weight relationships and on a general index of condition in the mussels. The experiments illustrate that intricate co mbinations of biotic and physical (environmental) mechanisms may play important roles in the organization of zoobenthic communities.