Markus Diesing

Coastline evolution at different time scales - examples from the Pomeranian Bight, southern Baltic Sea

Sedimentological and morphological changes on the upper and lower shoreface during relatively stable sea-level highstand conditions have been investigated in the Pomeranian Bight, southern Baltic Sea, at time scales ranging from storm events to millennia. In order to cover that variety of time scales, different methods have been applied. Seasonal variations in the morphology of the upper shoreface were measured accurately using the tracer stick method. The ratio of breaking waves and energy dissipation due to wave breaking are the main forces controlling redeposition on the upper shoreface with the depth of disturbance up to three times the net change. The impact of single storm events can be observed from sidescan sonar mosaics to remain on the decadal scale. Aerial photographs covering the upper shoreface show that the location of gates, channel-like systems where water masses move offshore created during storm events, also remain stable over decades. Sedimentological and geomorphological variations and changes on the lower shoreface are only measurable on the century to millennium scale because the main driving forces are longlasting processes like sea-level fluctuations or neotectonics. Data on these scales have much more uncertainty in their relationship to forcing functions than data at shorter time scales. Because the effects of coastal processes active on different time scales can interact, comprehensive understanding of large-scale coastal behavior requires investigations from short events to long-term processes

Towards Quantitative Spatial Models of Seabed Sediment Composition.

There is a need for fit-for-purpose maps for accurately depicting the types of seabed substrate and habitat and the properties of the seabed for the benefits of research, resource management, conservation and spatial planning. The aim of this study is to determine whether it is possible to predict substrate composition across a large area of seabed using legacy grain-size data and environmental predictors. The study area includes the North Sea up to approximately 58.44°N and the United Kingdom’s parts of the English Channel and the Celtic Seas. The analysis combines outputs from hydrodynamic models as well as optical remote sensing data from satellite platforms and bathymetric variables, which are mainly derived from acoustic remote sensing. We build a statistical regression model to make quantitative predictions of sediment composition (fractions of mud, sand and gravel) using the random forest algorithm. The compositional data is analysed on the additive log-ratio scale. An independent test set indicates that approximately 66% and 71% of the variability of the two log-ratio variables are explained by the predictive models. A EUNIS substrate model, derived from the predicted sediment composition, achieved an overall accuracy of 83% and a kappa coefficient of 0.60. We demonstrate that it is feasible to spatially predict the seabed sediment composition across a large area of continental shelf in a repeatable and validated way. We also highlight the potential for further improvements to the method.

Rock Ridges in the Central English Channel

Publisher Summary The English Channel is a funnel-shaped, ENE–WSW-trending, shallow shelf sea between France and England. Hydrodynamically, it is a tide-dominated environment but is also influenced by long swell waves approaching from the open Atlantic Ocean. The Channel is situated at the boundary zone between Lusitanean and Boreal biogeographical provinces. The human impact on the benthic environment is assessed to be predominantly high to very high, including shipping, selective extraction (demersal fishing and aggregates), and obstruction (cables, wrecks), among others. Biological communities were compared across the three main geomorphic features, namely flat seabed, bedrock ridges and the palaeovalley, considering both the taxa recorded and the biotopes identified during the video analysis. Sixty-two taxa were identified to various levels of taxonomic precision and the analysis presented here is based on their relative frequency of occurrence. Geophysical and biological analysis revealed an extensive system of rock ridges located 30 km south of the Isle of Wight in water depths ranging between 40 and 80 m below Chart Datum. The feature extends 100 km in an east–west direction and 15 km in a north–south direction, covering ca. 1,100 km2 of seafloor. The rock habitat supports a substantial coverage of fauna including sponges, bryozoans, hydroids and anemones. Three major geomorphic feature types were identified, namely flats, rock ridges, and a palaeovalley. Surrogacy can be high for taxa that exploit niche habitats but is generally low when considering habitats and biotopes assigned according to the European Nature Information System (EUNIS) habitat classification scheme.

Identification of submarine hard-bottom substrates in the German North Sea and Baltic Sea EEZ with high-resolution acoustic seafloor imaging

Submarine hard bottoms (e.g., boulders, outcropping strata) are of particular ecological importance. They were investigated in the Exclusive Economic Zones (EEZ) of the German North Sea and the Baltic Sea, using high-resolution seafloor imaging techniques (i.e., sidescan sonar and multibeam echosounder). Examples are shown from the research areas Sylt Outer Reef (Sylter Ausenriff; North Sea), Kadet Trench (Kadetrinne), and Adler Ground (Adlergrund) (both in the Baltic Sea). There exist distinct differences between the two continental shelf seas regarding the distribution of boulders and the density (percent coverage) of boulders per unit seafloor. The observed differences are attributed to (a) different geological evolution of the seafloor, and (b) different forcing by waves, tides and currents, which are responsible for the redistribution of sediments.

Combining pixel and object based image analysis of ultra-high resolution multibeam bathymetry and backscatter for habitat mapping in shallow marine waters

Habitat mapping data are increasingly being recognised for their importance in underpinning marine spatial planning. The ability to collect ultra-high resolution (cm) multibeam echosounder (MBES) data in shallow waters has facilitated understanding of the fine-scale distribution of benthic habitats in these areas that are often prone to human disturbance. Developing quantitative and objective approaches to integrate MBES data with ground observations for predictive modelling is essential for ensuring repeatability and providing confidence measures for habitat mapping products. Whilst supervised classification approaches are becoming more common, users are often faced with a decision whether to implement a pixel based (PB) or an object based (OB) image analysis approach, with often limited understanding of the potential influence of that decision on final map products and relative importance of data inputs to patterns observed. In this study, we apply an ensemble learning approach capable of integrating PB and OB Image Analysis from ultra-high resolution MBES bathymetry and backscatter data for mapping benthic habitats in Refuge Cove, a temperate coastal embayment in south-east Australia. We demonstrate the relative importance of PB and OB seafloor derivatives for the five broad benthic habitats that dominate the site. We found that OB and PB approaches performed well with differences in classification accuracy but not discernible statistically. However, a model incorporating elements of both approaches proved to be significantly more accurate than OB or PB methods alone and demonstrate the benefits of using MBES bathymetry and backscatter combined for class discrimination.

Application of geobia to map the seafloor

Geographic Object-Based Image Analysis (GEOBIA) has been successfully employed to map terrestrial environments. However, 71% of Earths surface is covered by seawater and standard optical methods suitable for mapping the land surface have limited application in such environments. Application of GEOBIA to marine environments has nevertheless been attempted and can generally be subdivided into three domains: 1. The intertidal zone and shallow subtidal zone have been mapped with optical data and application of GEOBIA in such environments can be seen as a seaward extension of terrestrial approaches. 2. Photographs of the seafloor give very detailed but spatially limited information. GEOBIA methods have been applied to classify benthic species and habitats and estimate seafloor complexity among others. 3. Due to the rapid attenuation of light in water, the method of choice to map the seafloor employs sound. Modern multi-beam echosounders map the seafloor in high detail. Such sensors measure the topography (water depth) and the strength of the returning signal (backscatter), which can be used to characterise the seafloor substrates and habitats. This contribution will focus on the application of GEOBIA to marine acoustic datasets. A generic workflow for object-based acoustic seafloor mapping will be showcased and the current state of the application of GEOBIA to marine acoustic data will be discussed.

Seabed Characterization - Developing Fit for Purpose Methodologies

We briefly describe three methods of seabed characterization which are ‘fit for purpose’, in that each approach is well suited to distinct objectives e.g. characterizing glacial geomorphology and shallow glacial geology vs. rapid prediction of seabed sediment distribution via geostatistics. The methods vary from manual ‘expert’ interpretation to increasingly automated and mathematically based models, each with their own attributes and limitations. We would note however that increasing automation and mathematical sophistication does not necessarily equate to improve map outputs, or reduce the time required to produce them. Judgements must be made to select methodologies which are most appropriate to the variables mapped, and according to the extent and presentation scale of final maps.