Description: ALKOR cruise 520-2 was part of the project ASKAWZ (Acoustic Seafloor Classification of the German EEZ) coordinated and funded by the Federal Maritime and Hydrographic Agency (BSH) and the Federal Agency for Nature Conservation (BfN). The main goal of the cruise was the full area acoustic mapping of the westernmost part of the German EEZ (so called “Entenschnabel”) by sidescan sonar and the detection of natural and man-made structures (trawl marks, etc.) on the seafloor. The full area mapping was achieved by 61 sidescan sonar profiles, covering an area of ca. 640 km². A single-beam echosounder with automatic seafloor classification (QTC 5.5) was running simultaneously to the sidescan sonar. The acoustic backscatter data was “ground truthed” using Shipek grab and Box-corer (Reineck-Type) samples. In addition an underwater video camera was used to gather further seabed information. Additionally sidescan data was recorded in the Helgoland-Reef pockmark area. Here 13 lines were surveyed (ca. 10 km²) by sidescan sonar in order to document the development of the pockmark field.

Description: (20.03. – 24.03.2019)

Description: (31.03. – 04.04.2019)

Description: (24.03. – 31.03.2019)

Description: Cruise period: 20.3.2019 - 4.4.2019 Alkor cruise 520-2 was part of the project ASKAWZ (Acoustic Seafloor Classification of the German EEZ) funded by the Federal Maritime and Hydrographic Agency (BSH) and the Feral Agency for Nature Conservation (BfN). The main goal of the cruise was the full area acoustic mapping of the westernmost part of the German EEZ (so called “duck bill”) by sidescan sonar and the detection of natural and man-made structures (trawl marks) on the seafloor. The full area mapping was achieved by 61 sidescan sonar profiles, covering an area of ca. 640 km². A single-beam echosounder with automatic seafloor classification (QTC 5.5) was run simultaneously to the sidescan sonar. The acoustic backscatter data was “ground truthed” using Shipek grab and Box-corer (Reineck-Type) samples. In addition an underwater video camera was used to gather further seabed information. Aditionally sidescan data was recorded in the Helgoland-Reef pockmark area. Here 13 lines were surveyed (ca. 10 km²) by sidescan sonar in order to document the development of the pockmark field.

Description: Marine habitats of shelf seas are in constant dynam- ic 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 as- sess seafloor hardness and roughness, and combine these pa- rameters 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 out- cropping bedrock to sandy and muddy sectors with mostly gradual transitions. RoxAnn data enabled to discriminate all seafloor types that were suggested by ground-truth informa- tion (seafloor samples, video). The area appears to be quite stable overall; sediment import (including fluid mud) was de- tected 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 sufficefor stakeholder issues, but the former classification based on ‘soft’ boundaries is preferred to meet state-of-the-art scientific objectives.

Description: 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-beamsystems, 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.

Description: A series of multibeam bathymetry surveys revealed the emergence of a large pockmark field in the southeastern North Sea. Covering an area of around 915 km2, up to 1,200 pockmarks per square kilometer have been identified. The time of emergence can be confined to 3 months in autumn 2015, suggesting a very dynamic genesis. The gas source and the trigger for the simultaneous outbreak remain speculative. Subseafloor structures and high methane concentrations of up to 30 mmol/l in sediment pore water samples suggest a source of shallow biogenic methane from the decomposition of post-glacial deposits in a paleo river valley. Storm waves are suggested as the final trigger for the eruption of the gas. Due to the shallow water depths and energetic conditions at the presumed time of eruption, a large fraction of the released gas must have been emitted to the atmosphere. Conservative estimates amount to 5 kt of methane, equivalent to 67% of the annual release from the entire North Sea. These observations most probably describe a reoccurring phenomenon in shallow shelf seas, which may have been overlooked before because of the transient nature of shallow water bedforms and technology limitations of high resolution bathymetric mapping.

Description: The joint research project WIMO (Wissenschaftliche Monitoringkonzepte für die Deutsche Bucht/Scientific Monitoring Concepts for the German Bight, NE North Sea) aims at providing methods for detection and analysis of seabed habitats using modern remote sensing techniques. Our subproject focuses on hydroacoustic techniques in order to gain information about seafloor environments and sediment dynamics. In a timeframe of four years, several key areas in the German Bight were repeatedly observed using different hydroacoustic gear (i. e. sidescan sonars, single/multibeam echo sounders and sub-bottom profilers). In order to ground-truth the acoustic data, hundreds of grab samples and underwater videos were taken. With these techniques it is possible to distinguish between different seafloor habitats, which range from muddy to sandy seafloors (esp. near the barrier islands) to rugged or vegetated/populated reefs around Helgoland. The conducted monitoring program revealed seasonal changes regarding the abundance of the sand mason worm (Lanice conchilega) and the brittle star (Amphiora filiformis) as well as ongoing sedimentary processes driven by tidal currents and wind/storms. It was also possible to determine relationships between sediment characteristics and benthos in some key areas. An essential part of our project included a comparison between the datasets obtained with different hydroacoustic devices, configurations, and evaluation methods in the same study areas. The investigation reveals that there could be distinct differences in interpreting the data and hence in the determination of prevailing seafloor habitats, especially in very heterogeneous areas and at transition zones between the habitats. Therefore, it is recommended to employ more than one hydroacoustic system (preferably a singlebeam device combined with a wide-swath sonar system) synchronously during a survey in order to gain more reliable and detailed information about the seafloor environments. The results of this project study form an important contribution to ongoing and future projects, in particular with regard to the technical configuration of the sonar systems, the workflows concerning post-processing and validation of the hydroacoustic data as well as the monitoring concepts that were worked out. However, a full automation of these workflows is not feasible. For the time being, measurements, post-processing and data evaluation still need supervision and expert knowledge.