Found at four sites in the coastal waters of Belgium, the Netherlands and Germany, an encrusting cheilostomatous bryozoan unknown from Europe was identified by microscopical and scanning electron microscope studies as Schizobrachiella verrilli (Cheilostomata, Schizoporellidae). Until now, it was only known from the Atlantic coast of North America. Historic data confirms the existence of S. verrilli in the North Sea since 1905 and the few recent findings suggest that S. verrilli is a rare species living at refugial sites such as the sublittoral shell banks of the North Sea.

This is the first report about the arrival of the encrusting cheilostomatid bryozoan Watersipora subtorquata on floating seaweeds in German coastal waters of the North Sea. During summer 2012, drifting or stranded thalli of the non-indigenous brown macroalga Himanthalia elongata were investigated on the island of Helgoland showing numerous benthic species covering the holdfast of Himanthalia. Out of the 120 basal structures investigated, 8.3% had viable colonies of the invasive, non-native W. subtorquata attached to them. Himanthalia and W. subtorquata have both their nearest origin reported to be in the English Channel (La Manche), approximately 800 km away from Helgoland. Now we found W. subtorquata as a rafting species in the eastern part of the North Sea, showing its ability to distribute easily over very large distances using floating algae as a natural transport vector. This event might constitute the first step in the potential introduction of this alien species into German coastal waters.

The surroundings of the rocky island of Helgoland (German Bight, south-western North Sea) are one of the best-studied sites in European seas with species occurrence data available for nearly 150 years. As the area is strongly affected by global change (e.g. increase in mean sea surface temperature at Helgoland by 1.67°C since 1962), ecosystem structure and function are expected to change more than those of average marine systems. The paper presents a supplement to the local amphipod species inventory (5 nautical miles around the island). At least seven species of this ecologically important taxon seem to have newly established themselves at Helgoland since the late 1980s: Ampelisca typica, Amphilochus brunneus, Caprella mutica, Cheirocratus assimilis, Monocorophium acherusicum, Orchestia mediterranea and Orchomenella crenata. Most of them are not only new for the Helgoland area, but also for the German Bight; and two species (Amphilochus brunneus and Orchomenella crenata) are even new to the North Sea as a whole. Out of the seven new species, one (Caprella mutica) is a neozoon from the north-western Pacific. The other six species show clear warm water affinities (oceanic–Lusitanian species) which suggest a recent range expansion in the context of climate warming. The establishment of an increasing number of species formerly probably absent from the area does not seem to be compensated by losses of species, so that local species diversity is expected to increase.

A growing body of literature shows that benthic communities are hierarchically structured on spatial and temporal scales. In two study locations at Helgoland (North Sea), the northern and the western locations, we: (1) investigated the variation in abundance of specific algae and invertebrates at two spatial scales; and (2) evaluated the relationship between elevation and specific species at these scales. We were also interested in using this information about the spatial pattern of individual algae and invertebrates as well as the patterns of elevation to help develop a monitoring programme of the rocky intertidal. We examined the variation of individual algae and invertebrates by means of a hierarchical nested design. Data were taken from five replicates per plot, with plots located in transects (two transects per location).

At the northern location, the highest variability in cover of most algae and invertebrates occurred at the scale separated by about 50 m (scale: transect). This was a direct result of differences between the high- and the low-shore. Most species at high-shore showed a relatively low frequency of occurrence in contrast to a highest frequency of occurence (~100%) and maximal values of cover at low-shore. However, neither a linear nor a non-linear relationship between elevation and the specific species occurred. At the western location, the highest variability in most macroalgae and invertebrates investigated was among replicates (10s of centimetres apart). No relationship between elevation and individual species occurred at this location. Macroalgae at both locations were more consistent over time than invertebrate species. Our results suggest that the relevant processes shaping the individual macroalgae and invertebrates at the Helgoland rocky intertidal vary between locations and the specific species.

The potential causes of variation in macroalgal and invertebrate species at different spatial scales are discussed and suggestions for a future monitoring programme are given. Temporal inconsistency in the spatial patterns, and the fact that some individual algae and invertebrates comprising the benthic assemblages vary at different scales, speak in favour of a multiple-scale sampling approach for monitoring change in the intertidal communities at Helgoland.