Fig. 1

The study area, approximately 400 m north of Helgoland’s coastline. At each of the three sites, in 5-m water depth by mean low-water spring (MLWS), the sampling was performed through SCUBA diving on a monthly basis using quadrats (1 × 1 m) along three 20-m-long transect lines. Depth data provided by Klaus Ricklefs (FTZ, Kiel)