Hydrodynamics and hydroacoustic mapping of a benthic seafloor in a coarse grain habitat of the German Bight

Coarse-grained hard substrate areas with grain sizes up to very coarse boulder (> 2 m) are very rare in the German North Sea. The “Helgoländer Steingrund” is one of such highly biodiverse areas: it is characterized by a half-moon-shaped hard substrate ridge, which subdivides the site into a more exposed (westerly) and a less exposed (easterly) flank, characterized by a mixture of sand and gravel deposits. Sonar systems, underwater videos, and bottom samples were used for mapping and classifying the abiotic and biotic components in such very patchy and coarse-grained habitat. Three main seabed types (sand, gravel, and hard substrate) were identified, based on acoustic backscatter data. The additional information coming from underwater videos and sediment bottom sample analysis allowed the description of six different seabed types, which included both the abiotic (sediments, morphology, etc.) and biotic components. The flanks of the ridge and their transition to the surrounding soft-ground areas were characterized by a distinct dominance of the bryozoa F. foliacea and A. diaphanum on the western and on the eastern side, respectively. Morphology and hydrodynamics are likely responsible for such zonation. This is proved by the outcomes of the Acoustic Doppler Current Profiler data, which showed the general flow pattern across the ridge and even resolved the local variability of current pattern, dependent on the tidal stage and bottom relief.