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Over 3 successive seasonal cycles (April 1986 to October 1988), field experiments were established within 3 intertidal levels in the sheltered rocky intertidal of Helgoland (North Sea, German Bight). Competitors for space (Mytilus edulis, macroalgae), herbivores (Littorina spp.) and predators (Carcinus maenas) were either excluded from areas (0.25 m2) covered by undisturbed communities or enclosed at natural densities on areas that were cleared before of animals and plants. All the experimental fields (each 0.25 m2) were covered by cages with 4 mm gauze at the sides and a plexiglas top. The results of the experiments in the upper intertidal (occupied byLittorina spp. andEnteromorpha) showed that a natural density of herbivores could not prevent algal settlement and had only little influence on algal growth. Instead abiotic factors (storms, algae washed ashore) decreased the stock of the green algae. Experiments in the mid intertidal, dominated byMytilus (50% cover),Fucus spp. (20%) and grazingL. littorea (100 ind. m−2) showed that community structure was directly changed both by grazing periwinkles and by competition for space between mussels and macroalgae. WheneverLittorina was excluded, the canopy ofFucus spp. increased continuously and reached total cover within two years. In addition to the increase ofFucus spp., the rock surface and the mussel shells were overgrown byUlva pseudocurvata, which covered the experimental fields during parts of the summer in the absence of herbivores. As soon as perennial species (fucoids) covered most of the experimental areas, the seasonal growth ofUlva decreased drastically. Presence and growth of macroalgae were also controlled by serious competition for space with mussels. EstablishedMytilus prevented the growth of all perennial and ephemeral algae on the rocks. However, the shells of the mussels provided free space for a new settlement ofFucus andUlva. In the lower intertidal (dominated by total algal cover ofF. serratus, herbivores such asL. littorea andL. mariae, and increasing number of predators such asCarcinus), the feeding activity of herbivores can neither prevent the settlement of the fucoid sporelings nor reduce the growth of macroalgae.F. serratus achieved a total canopy on the rock within one year. Doubled density of herbivores prevented the settlement ofFucus and most of the undercover algae. Predation byCarcinus onLittorina spp. had little influence on the herbivore community patterns. However, the crabs supported the establishment of macroalgae by excluding the mussels from the lower intertidal. In summary, the community organization and maintenance in the mid and lower intertidal is influenced to a high degree by biological interactions. Whereas both the relatively important herbivory byL. littorea and competition for space between mussels and macroalgae dominate in the mid intertidal, predation reaches its highest relative degree of importance for community structure in the lower intertidal.

Biological interactions and their role in community structure in the rocky intertidal of Helgoland (German Bight, North Sea)