Seafloor Habitat Mapping and monitoring approaches for the German Bight


Contact
Finn.Mielck [ at ] awi.de

Abstract

The knowledge regarding seafloor environments in the German Bight is still sparse. To understand marine ecosystems and identify and evaluate these vulnerable habitats in a sufficient way, seafloor habitat mapping has become increasingly important to provide basic data for a sustainable environmental management. In this context, hydroacoustic systems offer a cost-effective and most widely reliable tool for investigation and monitoring of seafloor properties in a high spatial and temporal resolution. However, problems may occur when trying to compare hydroacoustic data sets obtained with different gear and evaluated by different approaches, especially in highly heterogeneous environments. This study analyzes two aspects regarding habitat mapping in the German Bight (SE North Sea): In the methodical part of this work, mapping routines were tested and evaluated in different habitats by comparing data sets obtained with hydroacoustic devices (i.e. sidescan sonar, acoustic ground discrimination system (AGDS), sub-bottom profiler and multibeam echo sounder). The research part of this study is dealing with the investigation and monitoring of occurring environments in the German Bight in terms of their spatial and temporal dynamics. Therefore, three investigation areas located east and west of the island of Sylt and north of the island of Helgoland were chosen. These study sites are holding a wide variety of habitats and seafloor features that are most widely representative for the whole German Bight. The first investigation area is located east of the island of Sylt in the North Frisian Wadden Sea sheltered by barrier islands. The area is affected by strong tidal currents and is typical for backbarrier tidal flats in the German Bight. The sandy surface material in this highly dynamic zone is often arranged in subaqueous dunes that indicate a high amount of sediment transport. The second investigation area west of the island of Sylt is located beyond the Wadden Sea and is hence more exposed to storm surges. Relatively large coast-parallel depressions in the order of ~2 m were detected that are known as sorted bedforms. These features are widespread in the North Sea and also on other continental shelves all over the world. The third study area north of Helgoland is characterized by rocky seafloor partially settled by macroalgae forests and reveals a completely different environment. The mapping approaches reveal that a deployment of single devices is generally not recommended. Sidescan-sonar images provided an inventory of occurring bedforms including their size, alignment and symmetry, while AGDS (RoxAnn) allowed to determine sediment classes and shell contents. Only by using both devices together, it was possible to reveal all habitat classes in the investigated areas. In order to monitor small-scale changes, sidescan sonar was the most convenient device, since multibeam and singlebeam echo sounders to not deliver a sufficient resolution to detect e.g. variations in small ripple alignment and small-scale oscillations of bedforms. However, multibeam data were useful to get a generally insight in the bathymetry. Sub-bottom profiles obtained with sediment echo sounder were very helpful to determine stratified sediments in seabed depths down to 10 m. In order to map the macroalgae forests off Helgoland, swaths devices were not very useful in these shallow waters, since the footprints on the seafloor were quite small and the long leaves of the kelp are disturbing the backscatter. The singlebeam AGDS, however, delivers good results which allow to discriminate between different dense kelp forests, procumbent vegetation zones and barren seafloor such as rocks and sand. Moreover, the mapping approaches standardized and evaluated in this study reveal new knowledge regarding sorted bedforms located west of the island of Sylt. Hydroacoustic data were collected within a time frame of five years in order to determine interannual variabilities of these coarse-grained seafloor features that are delimited by sharp borders to the surrounding fine-sand domains. The results suggest that locations and sizes of the sorted bedforms are most widely stable within this time frame although the region is generally subject to strong currents accompanied with a high amount of sediment transport. However, small-scale changes regarding the boundaries to the adjacent fine-sand domains are detectable. It can be concluded that these changes are due to storm events which periodically occur in the study area and erode and accumulate the fine sand. Wind directions and strengths of the storm events were manifested in the alignments and sizes of small oscillation ripples occurring on the bedforms and on the eroded fine-sand domains. Other forces seem to have only minor influence on these seafloor features. The study also reveals that it is often not necessary to obtain data at full-coverage to map homogenous study areas. The distribution of huge features such as submarine riffs and glacial valleys can nevertheless be determined by surveying with larger transect distances. Hence, vast areas can be mapped in a short period of time. However, in order to monitor sediment dynamics and to detect small-scale features, at least sidescan-sonar data at full-coverage are required. In conclusion, the wide variety of available hydroacoustic technologies and the diversity of seafloor conditions suggest that guidelines and requirements for survey are difficult to standardize. Yet, using different devices in combination (e.g. two sidescan sonar devices with different frequencies, AGDS, and multibeam echo sounder) generates positive results that are more reliable concerning an accurate seafloor classification.



Item Type
Thesis (PhD)
Authors
Divisions
Primary Division
Programs
Primary Topic
Peer revision
Not peer-reviewed
Publication Status
Published
Eprint ID
33954
Cite as
Mielck, F. (2013): Seafloor Habitat Mapping and monitoring approaches for the German Bight , PhD thesis,


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