High-resolution surveying techniques of subtidal soft-bottom seafloor habitats show higher small-scale variation in topography and sediment type than previously thought, but the ecological relevance of this variation remains unclear. In addition, high-resolution surveys of benthic fauna show a large spatial variability in community composition, but this has yet poorly been linked to seafloor morphology and sediment composition. For instance, on soft-bottom coastal shelves, hydrodynamic forces from winds and tidal currents can cause nested multiscale morphological features ranging from metre-scale (mega)ripples, to sand waves and kilometre-scale linear sandbanks. This multiscale habitat heterogeneity is generally disregarded in the ecological assessments of benthic habitats. We therefore developed and tested a novel multiscale assessment toolbox that combines standard bathymetry, multibeam backscatter classification, video surveying of epibenthos and box core samples of sediment and macrobenthos. In a study on the Brown Bank, a sandbank in the southern North Sea, we found that these methods are greatly complementary and allow for more detail in the interpretation of benthic surveys. Acoustic and video data characterised the seafloor surface and subsurface, and macrobenthos communities were found to be structured by both sandbank and sand wave topography. We found indications that acoustic techniques can be used to determine the location of epibenthic reefs. The multiscale assessment toolbox furthermore allows formulating recommendations for conservation management related to the impact of sea floor disturbances through dredging and trawling.

The tube-building polychaete Sabellaria spinulosa (Ross worm) can form conspicuous biogenic reefs that stabilize the seabed and increase biodiversity by providing a habitat for a multitude of other species. These reefs, however, are assumed to be vulnerable to human-induced physical disturbances of the seabed. In the Greater North Sea, S. spinulosa reefs are recognized to be under threat and worthy of protection. In August 2017, three S. spinulosa reefs with a minimum extent of 1016 m² were discovered in the Dutch Brown Bank area. This area comprises a large-scale sandbank and adjacent troughs. The reefs were found within the sandbank troughs, which have proven to be subject to high demersal fishing intensities (fished >5 times a year). Detailed bathymetry measurements showed that S. spinulosa reefs were mainly located within valleys of smaller-scaled sand waves, which have a perpendicular orientation compared to the large-scale sandbank structure of the Brown Bank. We hypothesize that the valleys in between sand waves offer suitable substrate for settlement and refuge from abrasion by fishing activities, enabling the S. spinulosa reefs to persist despite high fishing intensities. ROV footage of the reefs showed higher estimates of species abundances on the reefs compared with adjacent habitats, with some species present that are typical for hard substrate (rock gunnel, Pholis gunnellus; edible crab, Cancer pagurus; and velvet swimming crab, Necora puber). The information presented could be used for drafting management policies to protect these reefs, as Contracting Parties of the OSPAR Convention are committed to take measures and protect biodiversity.

High resolution maps of sandy seafloors are valuable to understand seafloor dynamics, plan engineering projects, and create detailed benthic habitat maps. This paper presents multibeam echosounder backscatter classification results of the Brown Bank area of the North Sea. We apply the Bayesian classification method in a megaripple and sand wave area with significant slopes. Prior to the classification, corrections are implemented to account for the slopes. This includes corrections on the backscatter value and its corresponding incident angle. A trade-off in classification resolutions is found. A higher geo-acoustic resolution is obtained at the price of losing spatial resolution, however, the Bayesian classification method remains robust with respect to these trade-off decisions. The classification results are compared to grab sample particle size analysis and classified video footage. In non-distinctive sedimentary environments, the acoustic classes are not attributed to only the mean grain size of the grab samples but to the full spectrum of the grain sizes. Finally, we show the Bayesian classification results can be used to characterize the sedimentary composition of megaripples. Coarser sediments were found in the troughs and on the crests, finer sediments on the stoss slopes and a mixture of sediments on the lee slopes.