Environmental variablesExposed (erosion rates of gypsum blocks : 0.14-0.16 g/h; during westerly winds (force 4-5) which are prevailing around Helgoland rates reaches 0.28 g/h (de Kluijver, 1991). Mostly relative natural red sandstone; gently sloping substratra; average tidal range: ca. 2.5 m; geomorphological structure: channels (mean height to NN: ca. -1.50 m; width: ca. 2-20 m) and ridges (mean height to NN: ca. -1.00 m; width: ca. 5-25 m) showed a more irregular pattern as the semi-exposed study location: the ridges form discrete patches enclosed by channels which also submerged during low tide. Due to the seawall along the western part of the island the high- and mid-shore assemblages were basically not existent in the western study location.Sampling design At each study location all plots were sampled at 3-month intervals over 2 years (summer 2004 - spring 2006). All plots at each location were sampled at each 3-month interval within a period of a few days.Transect: 2 transects (7-10 m long, 45 m apart);plot: 2 and 3 plots respectively (4-10 m apart) along each transect;quadrat: 5 quadrats (50x50 cm, < 1 m apart) within each plot.All spatial scales were established at low-shore.Species list1. Structural layersTop layer (TL) , i.e. a structural layer usually consisted of thalli of large brown algae (e.g. Fucus spp., Laminaria spp.);encrusting layer (EL) , i.e. a structural layer which is formed by epilithic organisms adhering directly to the substratum (e.g. Phymatolithon spp., Ralfsia spp. or bryozoans).All other species which are not labelled with TL or EL be part of the middle layer, i.e. a structural layer which is represented by epiphytic, epizoic and endozoic species growing erect from the substrate, but not reaching the top layer.2. Species complexSome species were combined into a complex if the taxonomic position was tentative or if the morphological distinctions between two species were difficult to quantify (e.g. Monostroma grevillei/articum complex) tube-building organisms = a complex of polychaetes (mostly Polydora ciliata, Fabricia sabella).Sometimes higher taxa are listed, e.g. Ulva spp. or Actiniaria. All other species are identified to species level (full names are listed in the data sheet).
