This paper describes the macroflora and macrofauna associated with two bull kelp species, Laminaria hyperborea and L. digitata, at the island of Helgoland, North Sea. During a study period of seven months (March-September 1987), 29 macroflora species and 125 macrofauna species were found. The dominant taxonomic groups were Polychaeta (25 species), Bryozoa (17), Amphipoda (14), Hydrozoa (10) and Ascidiae (8). The species maximum was in July. In general, L.hyperborea was preferred as a substrate for settlement to L. digitata. Compositio n of the communities associated with kelp changed.during the season accordim:j to exposure to wave action, and according to location on the kelp thallus. The rhizoid community of both kelps bore more species at exposed locations. Wave-exposed L. digitata lacked obvious faunal settlement on both phylloid and cauloid. Phylloid and cauloid of L. hyperborea were chosen as an attractive substrate at both sheltered and wave-exposed locations, showing an association of encrusting bryozoan and hydrozoan colonies.

Macrofauna and its distribution in the northeastern rocky intertidal zone of the island of Helgoland. The macrofauna and its distribution in the sheltered, rocky intertidal zone of Helgoland (North Sea) was studied at 9 vertically and/or morphologically different stations from March to September in 1984. Seasonal variations in the communities were described based on each species' "conspicuousness". A total of 172 species was found. The macrofauna shows a zoned pattern, but also the different substrata, for example, affect its distribution. The number of species increases from the upper intertidal to the upper subhttoral zone from 23 to 133 species. The upper intertidal is characterized by Littonna saxatilis, Chaetogammarus marinus and Hyale nilssonii. Typical and abundant species of the middle and lower intertidal are Flustrellidra hispida, Littorina mariae/obtusata, Littorina Iittorea, Mytilus edutis and Spirorbis spirorbis. The upper sublittorat zone is characterized by Gibbula cineraria and increasing species numbers of Bryozoa, Nemertini and Opisthobranchia. Only few species (e.g. Dynamena pumila, Laomedea flexuosa, Polydora ciliata, Pabricia sabella, Jaera albifrons, Carcinus maenas) occur in the entire intertidal zone. In comparison to other very sheltered shores in Great Britain, which are also dominated by Fucaceae, the macrofauna in the Helgoland intertidal zone lacks several littoral species, such as Patella spp., Monodonta lineata, Gibbula umbilicalis, Littorina neritoides, Chthamalus spp., whereas Littorina littorea and Cibbula cineraria are highly abundant. EINLEITUNG

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 (A4ytilus edulis, macroalgae), herbivores (iittorina spp.) and predators (Carcinus maenas) were either excluded from areas (0.25 m 2) 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 m 2) were covered by cages with 4 mm gauze at the sides and a plexiglas top. The results of the experiments in the u p p e r i n t e r t i d a 1 (occupied by Littorina spp. and Enteromorpha) 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 m i d i n t e r t i d a 1, dominated by Myfilus (50 % cover), Fucus spp. (20 %) and grazing L. 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. Whenever Littorina was excluded, the canopy of Fucus spp. increased continuously and reached total cover within two years. In addition to the increase of Fucus spp., the rock surface and the mussel shells were overgrown by Ulva 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 of Ulva decreased drastically. Presence and growth of macroalgae were also controlled by serious competition for space with mussels. Established Nfytilus 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 of Fucus and Ulva. In the 1 o w e r i n t e r t i d a I (dominated by total algal cover of it:. serratus, herbivores such as L. littorea and L. mariae, and increasing number of predators such as Carcinus), 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 of Pucus and most of the undercover algae. Predation by Carcinus on Littorina 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 by L. littorea and competition for space between mussels and macroalgae dominate in the mid intertidal, predation reaches it s highest relative degree of importance for community structure in the lowe...

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