133 publications from this institution
Quantitative acoustic marine habitat mapping needs to consider the impact of macrobenthic organisms on backscatter data. However, the sensitivity of hydroacoustic systems to epibenthic life is poorly constrained. This study explores the impact of a benthic community with sparse abundance on seafloor microroughness and acoustic backscatter at a sandy seafloor in the German North Sea. A multibeam echo sounder survey was ground-truthed by lander measurements combining a laser line scanner with sub-mm resolution and broad-band acoustic transducers. Biotic and abiotic features and spatial roughness parameters were determined by the laser line scanner. At the same locations, acoustic backscatter was measured and compared with an acoustic scatter model utilizing the small-roughness perturbation approximation. Results of the lander experiments show that a coverage with epibenthic features of 1.6% increases seafloor roughness at spatial wavelengths between 0.005–0.03 m, increasing both spectral slope and intercept. Despite the fact that a strong impact on backscatter was predicted by the acoustic model based on measured roughness parameters, only a minor (1.1 dB) change of backscatter was actually observed during both the lander experiments and the ship-based acoustic survey. The results of this study indicate that benthic coverage of less than 1.6% is insufficient to be detected by current acoustic remote sensing.
Hydroacoustic surveys are common tools for habitat investigation and monitoring that aid in the realisation of the aims of the EU Marine Directives. Howeve
<p>The research project MGF-Ostsee deals with the consequences of the exclusion of mobile bottom-contact fishing in the southern Baltic Sea, specifically to assess its effects on the biogeochemistry of surface sediments and across the benthic-pelagic food chain. In Summer 2021, an in-situ monitored experiment was conducted at a coastal site in the region of Warnemünde/Rostock to investigate the short-term impacts of bottom trawling. Herein, we present first results on how this anthropogenic intervention affects biogeochemical processes and associated elemental cycling, as well as the resulting changes in geochemical mineral tracers. We analyzed porewater and sediment, as well as the water column for major, minor and trace elements, and the stable isotope composition (C, S, O) of dissolved and solid carbon and sulfur species. Porewater gradients are combined with lander-based oxygen-consumption- and radiotracer-based microbial sulfate reduction rates to elucidate how the disturbances by the fishing gear affect element (C, P, Mn, Fe, S) and mineral (re)distribution.</p><p>The controlled trawling experiment generated a re-suspension plume that reached up to 2 m above the sea floor, with 4 NTU in the lowermost portion. In the central trawled area, short cores were taken with a MUC prior and one to two hours after the experiment, and on the following day. In addition, sediment cores were recovered by divers from furrows and mounds of recent trawl marks. First results suggest that in the trawled area, the coupled Fe-Mn-P cycle reacts most sensitively, as expressed by altered porewater gradients and element diffusion. In the trawl marks, pore waters are affected differently whether sediments are removed, as in trawl furrows (erosion), or added/topped, as in trawl mounds (burial). In general, the tentative results point towards a Mn loss in the trawling area and in the furrows, whereas in the mounds Mn becomes enriched. The observed short-term changes in geochemical patterns from the experiment in the Warnemünde region are compared to data from a monitored region in the Fehmarn Belt. There, the observed patterns are tentatively associated to meso-scale areas with a history of low or high trawling impact.</p>
The Baltic Sea has a long history of anthropogenic disturbance, that started earlier than in most other coastal oceans and marginal seas. Especially in regions where shallow depths and limited space constrain the area that is available for anthropogenic use, conflicts of interest arise from the rising demand of multiple socio-economic players such as offshore wind, nature conservation, shipping, coastal protection, fishing, military, tourism and many more. The intensive use over many centuries has left long-lasting and partly irreversible traces on the seafloor and the benthic ecosystem. We aim to make the traces of different seafloor modulating processes such as bottom trawling, ship anchoring, propeller wake erosion, seabed constructions or material dumping visible using marine geophysical data of different resolution and spatial coverage. From this data, we can derive the spatial distribution and intensity of anthropogenic disturbances in the Baltic Sea and subsequently evaluate the pressures that they exert in certain areas. This approach is exemplified for propeller wakes that are generated by commercial ships, and that are not yet included in cumulative impact assessments. The results outline how single processes can exert pressures on the entire vertical sea, from the ocean-atmosphere boundary down to the seafloor and below, with likely impacts on ecosystem functioning and marine biodiversity. For propeller wakes, the broad spectrum of direct consequences suggests that the challenges associated with this anthropogenic stressor can only be met and moderated through intensive interdisciplinary research.
The deformational behaviour of ‘salt giants’ during and shortly after their deposition is difficult to decipher in ocean margin settings where the original evaporites have been deeply buried and strongly mobilized. Here, we examine seismic reflection data from the R ed S ea, where evaporites deposited until the end of the M iocene (~5.3 Ma), are generally covered by only 200–300 m of low‐density sediments and where the presence of an axial spreading centre allows us to observe how they have responded to a varied configuration of underlying basement. The regional morphology of the S ‐reflection, representing the evaporite surface, is mapped out from seismic data from 13 cruises. The S ‐reflection is locally rugged and commonly angular. It is either underlain by layered reflectivity, suggestive of layered evaporite beds, or by more transparent seismic character, suggestive of massive halite. On average, the depth of the reflection on the flanks of the axial rift systematically declines from 700 to 1100 m below sea level (mbsl) going northwards from 16 to 23°N. In the central R ed S ea, the S ‐reflection has 100‐ to 200‐m‐deep depressions, extending towards the coasts in places. In the southern R ed S ea, the S ‐reflection forms a surface at 300–800 mbsl that appears less disrupted. We suggest that the evaporites originally had a flat, horizontal surface at the end of the M iocene and have subsequently been distorted by isostatic effects and axial rifting, which in turn promoted evaporite flowage. Off‐axis evaporite depressions correspond with flows identified with multibeam sonar. Furthermore, across‐rift lows in B ouguer gravity anomalies represent valleys in the underlying basement. The off‐axis evaporite depressions overlie those valleys, as would be expected if halokinetic movements were greatest where the evaporites are locally thick, leading to deflation of the evaporite surface. The thickness of post‐ M iocene sediment, also mapped out as part of this procedure, confirms the generally pelagic nature of this interval and increases on average from ~250 to 300 m from the central to the southern R ed S ea, mimicking the variation in pelagic productivity observed in the present water column.
The raw data used for the study Seasonal Change of Multifrequency Backscatter in three Baltic Sea Habitats by Schulze et al.; currently under review at Frontiers in Remote Sensing. Files are stored in the s7k-Format, and sorted by date of acquisition and frequency. 200 and 400 kHz data were manufacturer-calibrated. Correct absorption values have been applied duirng the export. Refer to the paper for further dataset information. This upload stores the 700 kHz data recorded in October 2019.
Different types of along-track stripe noise are common artifacts in side-scan sonar mosaics, hampering both visual image quality and automatic seabed class
Since the Last Glacial Maximum, ice has retreated through the fjords of the South Shetland Islands leaving a valuable record of submarine landforms behind. In this study, glacial landforms and sub-bottom characteristics have been mapped to investigate the late Holocene retreat behaviour of the Fourcade Glacier and to delineate past environmental processes in Potter Cove, King George Island. The comprehensive datasets include high-resolution swath bathymetry, shallow seismic profiling and one sediment core. Moraines, moraine incisions and glacial lineations were mapped on the sea floor in the inner part of the cove, whereas pockmarks, ice scour marks and channel structures were identified in the outer part. Sub-bottom characteristics have been assigned to different acoustic facies types indicating different depositional settings. The results reveal glacial recessions as well as stillstands and potential readvances during the late Holocene. Furthermore, the sediment record indicates that the Fourcade Glacier was situated inside the inner cove during the Little Ice Age (500–100 cal yr bp ).
This study describes the spatial distribution of flow-parallel sand ribbons and flow-transverse large and very large subaqueous dunes in the south-western Baltic Sea offshore Fehmarn Island between 13m and 20m water depth, based on hydroacoustic and grain size data. The system of sand ribbons and dunes is intermittently active due to currents induced during major inflows of the North Sea water into the Baltic Sea. The sand ribbons are located on a lag deposit on top of glacial till, while the dunes rest on top of drowned Holocene nearshore deposits. The sand ribbons reach heights between 0.4m and 0.6m, with widths varying between 60m and several hundreds of metres. The observed dunes have heights between 0.09m and 2.35m, while their wavelengths range from 17m to 120m. Offshore Fehmarn Island, the transition from sand ribbons to dunes is most likely linked to a contrast in sediment supply, as reworked drowned nearshore deposits provide sediment available for transport in significantly larger amounts than glacial till. Similar to an earlier approach for river bed states, the dimensionless thickness of sediment available for transport is able to differentiate between the bed states.
This study documents seafloor morphology and sediments based on multibeam, side-scan sonar and boomer surveys, as well as sediment samples taken on the inner to mid shelf of the Andaman Sea after the 2004 Indian Ocean tsunami. Preservation of submarine relief in former underwater mining areas points to limited impact of the tsunami, while channel structures parallel to the observed tsunami backwash indicate a possible higher impact. Therefore, the tsunami impact seems to be focused on some areas. The impact was probably most effective during the backwash, when stiff mud deposits containing grass, wood fragments and shells were transported by high density backwash flows. Moreover, several boulders, which might have been deposited during the tsunami backwash flow, were found in the channels in front of Pakarang Cape.
The Baltic Sea is one of the busiest marine regions in terms of commercial shipping. Increased marine traffic over the last decades already led to increased number of bigger ships and more powerful propulsions systems. This development has put a number of environmental effects of shipping, such as air pollution, marine noise or accidental discharges of hazardous substances, on the discussion list. What has, however, only marginally been studied is the possible effect of commercial shipping on sedimentation patterns and seafloor morphology. Here we use AIS data from the last 20 years to identify hotspots of marine traffic in the Baltic Sea. Subsequently we collect multibeam bathymetric data from different sources and databases to investigate seafloor morphology in some traffic hotspots. We further collect seabed sediment samples and time-lapse bathymetric data in the Bay of Kiel, where Kiel Canal, one of the most heavily used artificial waterways on the globe, commences. First results indicate that ships can erode hard substrate such as basal till, most likely through interaction of their wake with the seafloor. In addition to eroding the hard seafloor, the wakes may also mobilize and locally redistribute mobile sands.
