Finn Mielck

Tools to evaluate seafloor integrity: comparison of multi-device acoustic seafloor classifications for benthic macrofauna-driven patterns in the German Bight, southern North Sea

To determine the spatial resolution of sediment properties and benthic macrofauna communities in acoustic backscatter, the suitability of four acoustic seafloor classification devices (single-beam echosounder with RoxAnn and QTC 5.5 seafloor classification system, sidescan sonar with QTC Swathview seafloor classification, and multi-beam echosounder with QTC Swathview seafloor classification) was compared in a study area of approx. 6 km2 northwest of the island of Helgoland in the German Bight, southern North Sea. This was based on a simple similarity index between simultaneous sidescan sonar, single-beam echosounder and multi-beam echosounder profiling spanning the period 2011–2014. The results show a high similarity between seafloor classifications based on sidescan sonar and RoxAnn single-beam systems, in turn associated with a lower similarity for the multi-beam echosounder system. Analyses of surface sediment samples at 39 locations along four transects (0.1 m2 Van Veen grab) revealed the presence of sandy mud (southern and western parts), coarse sand, gravel and cobbles. Rock outcrops were identified in the north-eastern and eastern parts. A typical Nucula nitidosa–Abra alba community was found in sandy muds to muddy sands in the northern part, whereas the southern part is characterised by widespread occurrence of the ophiuroid brittle star Amphiura filiformis. A transitional N. nitidosa–A. filiformis community was detected in the central part. Moreover, the southern part is characterised by a high abundance of A. filiformis and its commensal bivalve Kurtiella bidentata. The high number of A. filiformis feeding arms (up to ca. 6,800 per m2) can largely explain the gentle change of backscatter intensity along the tracks, because sediment composition and/or seafloor structures showed no significant variability.

Seafloor monitoring west of Helgoland (German Bight, North Sea) using the acoustic ground discrimination system RoxAnn

Marine habitats of shelf seas are in constant dynamic change and therefore need regular assessment particularly in areas of special interest. In this study, the single-beam acoustic ground discrimination system RoxAnn served to assess seafloor hardness and roughness, and combine these parameters into one variable expressed as RGB (red green blue) color code followed by k-means fuzzy cluster analysis (FCA). The data were collected at a monitoring site west of the island of Helgoland (German Bight, SE North Sea) in the course of four surveys between September 2011 and November 2014. The study area has complex characteristics varying from outcropping bedrock to sandy and muddy sectors with mostly gradual transitions. RoxAnn data enabled to discriminate all seafloor types that were suggested by ground-truth information (seafloor samples, video). The area appears to be quite stable overall; sediment import (including fluid mud) was detected only from the NW. Although hard substrates (boulders, bedrock) are clearly identified, the signal can be modified by inclination and biocover. Manually, six RoxAnn zones were identified; for the FCA, only three classes are suggested. The latter classification based on ‘hard’ boundaries would suffice for stakeholder issues, but the former classification based on ‘soft’ boundaries is preferred to meet state-of-the-art scientific objectives.

High-Resolution Hydroacoustic Seafloor Classification of Sandy Environments in the German Wadden Sea

ABSTRACT Mielck, F.; Hass, H.C., and Betzler, C., 2014. High-resolution hydroacoustic seafloor classification of sandy environments in the German Wadden Sea. The knowledge regarding the distribution of seabed environments in the North Sea is still fragmentary. Hydroacoustic devices can provide rapid and reliable information on the acoustic characteristics of the seafloor. Data of a typical, exclusively sandy and shallow investigation area located in the German Wadden Sea are here presented. The study area includes shallow sublittoral areas and deep channels. The acoustic ground discrimination system RoxAnn was used to measures backscatter intensities, which indicate roughness and hardness parameters of seafloor. Sidescan-sonar data provide acoustic images that are instrumental in seafloor characterization. For ground truthing, surface-sediment samples were collected. The results reveal surficial sediments that range from fine to coarse sand. Finer material is rather restricted to the shallow patches, while coarser sediments characterize the deeper tidal channels. The determined roughness and hardness parameters also strongly increase within these inlets following the change in grain size. The sidescan-sonar imagery shows flow-transverse subaqueous dunes of different sizes. Both ebb- and flood-dominated structures are present. RoxAnn reflects these zones of subaqueous dunes basically through the formation of distinct clusters of data points in hardness vs. roughness scatter plots. Further, the results show that areas characterized by similar grain-size spectra and sonar imagery occasionally reveal completely different roughness and hardness properties. This is caused by varying amounts of shell fragments on the seafloor. Both in situ and hydroacoustic methods are suitable for identifying seafloor properties, but the combination of methods yields more information at higher precision, especially about shell content and grain-size distribution.