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Abstract. The development of temporary Mytilus edulis beds and their associated macro fauna was studied on an intertidal sand flat in the German Wadden sea during 1986 by comparing mussel beds with bare sandy areas of the same tidal elevation. Macro fauna samples We taken with a 100 cm2 corer (penetration depth 27 cm, mesh size 0.5 mm, 5 parallel samples). Numbers of taxa and diversity were higher in the mussel beds compared to the sandy areas. The abundance of macro fauna organisms decreased with the age of the mussel bed, whereas the relative abundance of species was not strongly affected. Hydrobia ulvae, oligochaetes, and Heteromastus filiformis dominated the macro fauna inside as well as outside the mussel beds. Among the lower ranks of dominance changes in species composition were observed due to increasing densities of opportunistic polychaetes. About six weeks after the establishment of the mussel beds a higher abundance of Capitella spp., Polydora ligni, Tharyx marioni, and nemerteans occurred within the mussel beds while other species such as Hydrobia ulvae and Pygospio elegans showed a significantly lower abundance compared to the adjacent flat. Macoma balthica as well as Mya arenaria were not affected by the alteration of a bare sand flat to a mussel bed. O‐group Carcinus maenas reached their highest abundance in the mussel beds (over 1000 indiv. m‐2). In contrast juvenile Crangon crangon preferred the bare sand flat. After destruction of the mussel beds by an early autumn storm, species abundance and composition was similar to the situation before the mussel beds had become established.

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The present compilation is the first attempt to generate a comprehensive list of all macrozoobenthic species recorded at least once in the German regions of the North Sea and Baltic Sea including non-indigenous species and freshwater species which occurred in brackish waters (estuaries, bays, fjords etc.). Based on the data of several research institutes and consultancies, the macrozoobenthic species inventory comprises a total of 1.866 species belonging to 16 phyla including 193 threatened species. The most common groups were: malacostracan crustaceans (21%), Polychaeta (19%), and Gastropoda (12%). Even though the two major marine regions are separated by only 50 km of land, the composition of the respective communities was different. The two seas shared only 36.6% of the recorded species which should have profound and far-reaching consequences for conservation purposes. Considering all macroinvertebrates listed 96 species, or the equivalent of 5.2%, were introduced mainly during the last two centuries. Both seas are heavily affected by human activities and are sensitive to climate change displayed by effects on the faunal compositions. The present checklist is an important step to document these changes scientifically and may act as a base for political and management decisions.