Dogger Bank Research Articles

Rapid organic matter cycling in North Sea sediments

Coastal shelf seas are zones of intense nutrient cycling, where a strong coupling between the sediment and the water column enhances primary productivity. To identify factors that control the strength of this benthic-pelagic coupling we measured sediment characteristics, solute fluxes, and porewater nutrient profiles in spring along a south - north transect in the North Sea crossing distinct regions: the shallow Oyster Grounds closest to the Dutch shore, the shallow Dogger Bank, the 80-m deep central North Sea, and the 150-m deep Fladen Grounds between the north of Scotland and Norway. The data were used to constrain rates of different mineralization processes with the 1-D diagenetic model (OMEXDIA). Surprisingly, we found no major differences in the biogeochemical signature along the 670 km long North Sea transect, despite sediments ranging in median grainsize from 25 to 217 μm, and a permeability range >3 orders of magnitude. Total carbon mineralization ranged between 4 and 13.5 mmol C m−2 d−1, and decreased significantly northward. Oxic mineralization was the dominant mineralization process in all studied sites. Finest, least permeable sediments were found in the Fladen Grounds where highest denitrification rates were recorded, linked to high nitrate concentrations in the overlying water. The coarsest, most permeable sediments of the shallow Dogger Bank represented a transition area between the Oyster Grounds, where oxic mineralization was highest (75–90%), and the central North Sea samples, where anoxic mineralization increased relative to oxic mineralization due to higher bioturbation rates (oxic: 59–72%, anoxic: 27–39%). Overall, denitrification rates increased, while phosphorus removal tended to decrease northward. This contrasting behaviour in nitrogen and phosphorus removal was identified as a possible cause for decreasing DIN:DIP ratios in the water column towards the north.

Proximate underwater soundscape of a North Sea offshore petroleum exploration jack-up drilling rig in the Dogger Bank.

Little is known about localized, near-field soundscapes during offshore hydrocarbon drilling campaigns. In the Dogger Bank, North Sea, underwater noise recordings were made 41-60 m from the drill stem of the Noble Kolskaya jack-up exploration drilling rig. The aims were to document noise received levels (RLs) and frequency characteristics of rig-associated near-field noise. The rig produced sound pressure levels (SPLs) of 120 dB re 1 μPa in the frequency range of 2-1400 Hz. Over transient periods, RLs varied by 15-20 dB between softest (holding) and noisiest (drilling) operations. Tonal components at different frequencies varied with depth. Support vessel noise was significantly louder than the jack-up rig at frequencies <1 kHz, even in its noisiest "boulder-drilling" phase, though radiated noise levels were higher above 2 kHz. Rig SPLs fell rapidly above 8 kHz. Marine mammals, such as harbor porpoise (Phocoena phocoena) forage regularly near offshore oil and gas rigs and platforms, and it is predicted that animals experience different noise regimes while traversing the water column and can potentially detect the higher-frequency components of drilling noise to a distance of 70 m from the source; however, while levels were unlikely to cause auditory injury, effects on echolocation behavior are still unknown.

Acoustic backscatter analysis of ground-fishing activity in the German North Sea sector

The physical impact of demersal fishing was studied in three different areas of the German North Sea sector by use of a multibeam echosounder. The areas represent typical shallow seas siliciclastic habitats in variable distance to the coastline. Their seabed substrate is dominated by coarse silt close to the shore and by fine sand in the most distal area on the Dogger Bank. The study documents the utility of the multibeam as a suitable tool to map large seafloor areas, as well as to quantify and qualify the footprints of the fishing groundgear. Besides the depth and amplitude information, the multibeam data allows the analysis of the incidence angle-dependent variation of the backscatter signal that provides additional information on the seabed impedance, roughness, volume scatter and substrate type. Two main types of destructive seafloor pattern were observed in the three areas. The dominant type are pairs of parallel furrows exhibiting widths of 6–9 m and depths of 2–6 cm and is related to beam trawling. Separation distance of the pair furrows is 16–23 m. The second type are pairs of narrow furrows (3–4 m) with much larger separation distance of 110–120 m. They are attributed to otter board trawling.A comparison of their mechanical ramification shows that the fishing gear incises the seafloor and modifies seafloor structures and properties. Our investigation documents that beam trawling has the greatest physical impact and exhibits the highest furrow density in the three study areas. The created furrows can persist for at least 4 months in a substrate of very fine sand. Depending on the hydrodynamic regime, the furrows can be later refilled by finer sediments, or levelled out by storm wave-induced turbulence reaching the seafloor. The hydrofoils used in otter trawl to spread the net likewise incise the seafloor, but the furrows are much narrower. However, the ground rope of the net towed between the boards scratches the seafloor and levels the microscale topography along track widths of ca. 100 m.

Identifying Trawl Marks in North Sea Sediments

The anthropogenic impact in the German Exclusive Economic Zone (EEZ) is high due to the presence of manifold industries (e.g., wind farms, shipping, and fishery). Therefore, it is of great importance to evaluate the different impacts of such industries, in order to enable reasonable and sustainable decisions on environmental issues (e.g., nature conservation). Bottom trawling has a significant impact on benthic habitats worldwide. Fishing gear penetrates the seabed and the resulting furrows temporarily remain in the sediment known as trawl marks (TM), which can be recognized in the acoustic signal of side-scan sonars (SSS) and multibeam echo sounders (MBES). However, extensive mapping and precise descriptions of TM from commercial fisheries at far offshore fishing grounds in the German EEZ are not available. To get an insight into the spatial patterns and characteristics of TM, approximately 4800 km2 of high-resolution (1 m) SSS data from three different study sites in the German EEZ were analyzed for changes in TM density as well as for the geometry of individual TM. TM were manually digitalized and their density per square kilometer was calculated. In general, TM density was highest in August and October. Moreover, different gear types could be identified from investigating individual TM in SSS data. Beam trawl marks were observed to have widths of up to 22 m whereas otter board marks showed widths up to 6 m. The persistence of TM was estimated to 2–7 days minimum for all three sites based on the SSS data from 2015–2019. A maximum persistence could be defined at one site (Dogger Bank) and it was five months for the investigation period 2016–2017. Besides the main factors driving TM degradation (wave-base impact, sediment-type), different methods for TM detection (SSS, MBES, under-water video) are discussed. The study provides valuable information on the physical impact of bottom trawling on the seabed and can support existing monitoring strategies.

Ice sheet and palaeoclimate controls on drainage network evolution: an example from Dogger Bank, North Sea

Abstract. Submerged landscapes on continental shelves archive drainage networks formed during periods of sea-level lowstand. The evolution of these postglacial drainage networks also reveals how past climate changes affected the landscape. Ice-marginal and paraglacial drainage networks on low-relief topography are susceptible to reorganisation of water supply, forced by ice-marginal rearrangement, precipitation and temperature variations, and marine inundation. A rare geological archive of climate-driven landscape evolution during the transition from ice-marginal (ca. 23 ka) to a fully submerged marine environment (ca. 8 ka) is preserved at Dogger Bank, in the southern North Sea. In this study, our analysis of high-resolution seismic reflection and cone penetration test data reveal a channel network over a 1330 km2 area that incised glacial and proglacial lake-fill sediments. The channel network sits below coastal and shallow marine sediments and is therefore interpreted to represent a terrestrial drainage network. When mapped out, the channel form morphology reveals two distinct sets. The first set comprises two low-sinuosity, wide (&gt;400 m) channels that contain macroforms of braid and side bars. These channels are interpreted to have originated as proglacial rivers, which drained the ice-sheet margin to the north. The second set of channels (75–200 m wide, with one larger, ∼400 m wide) has higher sinuosity and forms a subdendritic network of tributaries to the proglacial channels. The timing of channel formation lacks chronostratigraphic control. However, the proglacial rivers must have formed as the ice sheet was still on Dogger Bank, before 23 ka, to supply meltwater to the rivers. Ice-sheet retreat from Dogger Bank led to reorganisation of meltwater drainage and abandonment of the proglacial rivers. Palaeoclimate simulations show a cold and dry period at Dogger Bank between 23 and 17 ka. After 17 ka, precipitation increased, and drainage of precipitation formed the second set of channels. The second set of rivers remained active until marine transgression of Dogger Bank at ca. 8.5–8 ka. Overall, this study provides a detailed insight into the evolution of river networks across Dogger Bank and highlights the interplay between external (climate) and internal (local) forcings in drainage network evolution.

Here are the polyps: in situ observations of jellyfish polyps and podocysts on bivalve shells.

Most Scyphozoan jellyfish species have a metagenic life cycle involving a benthic, asexually reproducing polyp stage and a sexually reproducing medusa stage. Medusae can be large and conspicuous and most can be identified using morphological characteristics. Polyps on the other hand are small, live a cryptic life attached to hard substrates and often are difficult or impossible to distinguish based on morphology alone. Consequently, for many species the polyp stage has not been identified in the natural environment. We inspected hard substrates in various habitats for the presence of Scyphozoan polyps. Three polyps were found on Dogger Bank, Central North Sea, attached to the inside of the umbo of empty valves of the bivalves Mactra stultorum and Spisula subtruncata. One polyp was accompanied by four podocysts. With this knowledge, the inside of bivalve shells washed ashore in Oostende (Belgium) was inspected and supposed podocysts on the inside of empty valves of Cerastoderma edule and Spisula elliptica were found. Polyps and podocysts were identified to species level by 18S rDNA and mitochondrial COI sequencing. The three polyps found on Dogger Bank all belonged to the compass jellyfish Chrysaora hysoscella. One podocyst from the Oostende beach also belonged to this species but another podocyst belonged to Cyanea lamarkii. These are the first in situ observations of C. hysoscella and C. lamarckii polyps and podocysts in the natural environment. Mactra, Cerastoderma and Spisula species are abundant in many North Sea regions and empty bivalve shells could provide an abundant settling substrate for jellyfish polyps in the North Sea and other areas. Several new strategies to increase the detection of polyps on bivalve shells are presented.

Shifts in trait-based and taxonomic macrofauna community structure along a 27-year time-series in the south-eastern North Sea.

Current research revealed distinct changes in ecosystem functions, and thus in ecosystem stability and resilience, caused by changes in community structure and diversity loss. Benthic species play an important role in benthic-pelagic coupling, such as through the remineralization of deposited organic material, and changes to benthic community structure and diversity have associated with changes in ecosystem functioning, ecosystem stability and resilience. However, the long-term variability of traits and functions in benthic communities is largely unknown. By using abundance and bioturbation potential of macrofauna samples, taken along a transect from the German Bight towards the Dogger Bank in May 1990 and annually from 1995 to 2017, we analysed the taxonomic and trait-based macrofauna long-term community variability and diversity. Taxonomic and trait-based diversity remained stable over time, while three different regimes were found, characterised by changes in taxonomic and trait-based community structure. Min/max autocorrelation factor analysis revealed the climatic variables sea surface temperature (SST) and North Atlantic Oscillation Index (NAOI), nitrite, and epibenthic abundance as most important environmental drivers for taxonomic and trait-based community changes.

Left High and Dry: Deglaciation of Dogger Bank, North Sea, Recorded in Proglacial Lake Evolution

Reconstructions of palaeo-ice sheet retreat in response to climate warming using offshore archives can provide vital analogues for future ice-sheet behaviour. At the Last Glacial Maximum, Dogger Bank, in the southern North Sea, was covered by the Eurasian Ice Sheet. However, the maximum extent and behaviour of the ice sheet in the North Sea basin is poorly constrained. We reveal ice-marginal dynamics and maximum ice extent at Dogger Bank through sedimentological and stratigraphic investigation of glacial and proglacial lake sediments. We use a large, integrated subsurface dataset of shallow seismic reflection and geotechnical data collected during windfarm site investigation. For the first time, an ice stream is identified at Dogger Bank, based on preserved subglacial bedforms, eskers and meltwater channels. During ice-sheet advance, a terminal thrust-block moraine complex formed, whose crest runs approximately north-northeast to south-southwest. Subsequent ice stream shutdown caused stagnation of ice, and rapid retreat of the ice-sheet margin. The moraine complex, and outwash head from an adjacent ice-sheet lobe to the west, dammed a large (approximately 750 km2) proglacial lake. Subsequent sedimentation infilled the lake with 30 m of glacial outwash sediments. A lobate subaqueous fan formed at the ice-sheet margin, which thins towards the southeast with iceberg scours and ice-rafted debris at the base, and is onlapped by lake sediments calibrated to core as alternating clay and silt laminae, interpreted to be varves. The lake became isolated from the retreating ice-sheet margin, and ice-sheet retreat slowed. Sediment-laden meltwater was supplied to the ice-distal proglacial lake for c. 1500-2000 years. Subsequent ice-sheet retreat off Dogger Bank was more rapid due to the negative subglacial slope. The stepped retreat of rapid downwasting, slow retreat, and a final rapid phase off Dogger Bank occurred after the LGM at around 27 ka and before formation of a ribbon lake, dated previously to 23 ka and approximately 60 metres lower in elevation, formed to the north of Dogger Bank. The complicated stratigraphic architecture revealed through these data improves forecasting of ground conditions for turbine footings at Dogger Bank, an important step in the provision of clean, sustainable energy.

Cyclic properties of dense to very dense silica sand

This paper presents results of a series of cyclic laboratory test performed on dense to very dense silica sand. Triaxial and direct simple shear tests were performed on reconstituted sand. The test program aimed to determine the cyclic shear strength and soil deformation properties for foundation design of offshore wind turbine structures on the Dogger Bank site, offshore United Kingdom, but is also applicable for foundation design of other offshore structures and other locations. Two sand batches were used, one with clean silica sand and one with 20% silt content. Normally consolidated specimens from both batches were reconstituted to a target relative density approximately equal to 100% and 80%. On the clean sand with a target relative density of 80%, tests were also performed on specimens with an overconsolidation ratio of OCR = 4. Various contour diagrams, presenting cyclic shear strength and average and cyclic shear strains for different number of cycles, are presented. These diagrams can be used in the front-end engineering design (FEED) stage of commercial design projects on similar soil conditions, to define soil behavior when limited cyclic data is available, or in the planning stage of a cyclic laboratory test program. Effects of precycling and fines content on the cyclic behavior of triaxial and DSS specimens are discussed.

Topographic and hydrodynamic controls on barrier retreat and preservation: An example from Dogger Bank, North Sea

Barrier retreat can occur due to in-place drowning, overstepping or rollover, depending on the interplay of controls such as sea-level rise, sediment supply, coastal hydrodynamic regime and topography. Offshore sedimentary archives of barriers active during rapid Holocene sea-level rise provide important records of marine transgression, which are vital analogues to support appropriate mitigation strategies for future coastal realignment under projected relative sea-level rise scenarios. This study analyses the sedimentary archive at Dogger Bank, which is a formerly-glaciated area in the North Sea. Dogger Bank experienced marine transgression due to Early Holocene rapid relative sea-level rise. An integrated dataset of vibrocores and high-resolution seismic reflection data permits a stratigraphic framework to be established, which reveals the buried coastal geomorphology of the southern Dogger Bank for the first time. A transgressive stratigraphy was identified, comprising a topographically complicated basal glacial and terrestrial succession, overlain by two phases of barrier and tidal mudflat deposition, prior to shallow marine sedimentation. Barrier phase A was a recurved barrier drowned in place, and discontinuously overstepped to barrier phase B, which experienced continuous overstepping. By linking barrier elevations to relative sea-level curves, the timing of each barrier phase was established. Both barrier phases retreated during periods of rapid sea-level rise with abundant sediment supply. Coastal hydrodynamics (increasing wave energy) and antecedent topography with spatially variable accommodation are suggested to be the main reason for differing retreat mechanisms, rather than the rate of sea-level rise. Antecedent coastal geomorphology plays a critical role in erosional and depositional patterns during transgression, and therefore on the timing, rate and location of marine inundation, which needs to be included in models that aim to forecast hazards in coastal areas.

Shifting south-eastern North Sea macrofauna bioturbation potential over the past three decades: a response to increasing SST and regionally decreasing food supply

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 609:17-32 (2019) - DOI: https://doi.org/10.3354/meps12802 Shifting south-eastern North Sea macrofauna bioturbation potential over the past three decades: a response to increasing SST and regionally decreasing food supply Julia Meyer1,*, Petra Nehmer1, Ingrid Kröncke1,2 1Senckenberg am Meer, Marine Research, Südstrand 40, 26382 Wilhelmshaven, Germany 2Institute for Chemistry and Biology of the Marine Environment, Benthic Ecology, Carl-von-Ossietzky-Straße 9-11, 26111 Oldenburg, Germany *Corresponding author: Julia.Meyer@senckenberg.de ABSTRACT: Bioturbation is one of the most important processes for benthic-pelagic coupling and biogeochemical fluxes in marine sediments, such as the intake, transport, and preservation of organic carbon. However, only little is known about the large-scale and long-term variability of community bioturbation potential (BPc) and trait diversity of south-eastern North Sea (NS) macrofauna communities in relation to anthropogenic and environmental parameters. Here we pooled macrofauna species with similar life traits into functional groups, revealing the main functionality of a benthic ecosystem. The BPc and trait diversity of south-eastern NS macrofauna communities, derived from the NS Benthos Survey in 1986, the NS Benthos Project in 2000, and a more recent study from 2010-2015, were analyzed and compared. Significant changes in spatial variability of BPc were found, simultaneously to regionally decreasing BPc, e.g. in the central parts of the Oysterground, while BPc increased in other areas, e.g. along the North Frisian coast. Contrastingly, the spatial variability of trait diversity has remained stable since 1986. Overall, the study area was dominated by the functional group ‘biodiffusors with slow free movement.’ During the 1986 study period, we identified 3 basically different trait-based communities, i.e. the Dogger Bank, Oysterground, and coast community. Long-term analyses based on these 3 trait-based communities revealed changes in dominance of functional groups within each of the communities up to 2010-2015, which were related to anthropogenic pressures such as fishery and seabed degradation, synergistic to increasing sea surface temperature, food limitation, and de-eutrophication. KEY WORDS: Long-term variability · North Atlantic Oscillation Index · Trait analysis Full text in pdf format PreviousNextCite this article as: Meyer J, Nehmer P, Kröncke I (2019) Shifting south-eastern North Sea macrofauna bioturbation potential over the past three decades: a response to increasing SST and regionally decreasing food supply. Mar Ecol Prog Ser 609:17-32. https://doi.org/10.3354/meps12802 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 609. Online publication date: January 17, 2019 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2019 Inter-Research.

North Sea demersal fisheries prefer specific benthic habitats.

IntroductionThe future protection of marine biodiversity through good conservation planning requires both the identification of key habitats with unique ecological characteristics and detailed knowledge of their human utilization through fisheries. Demersal fisheries are important disturbers of benthic habitats. They often have a heterogeneous spatial distribution, pressurizing particular habitats with high abundances of target species. For the North Sea, we quantified the commonness/rarity of habitats in relation to the environmental determinants of so-called fishing hotspots, to support better-informed conservation planning of benthic habitats in this intensively used continental shelf.MethodsWe first distinguished 9 main seascapes in the study area based on seabed morphology. Secondly, we determined average fishing intensity and fishing hotspots using VMS-data for the three dominant Dutch fisheries from 2008 to 2015: beam-trawlers targeting sole Solea solea (Beam-Sole), beam-trawlers targeting plaice Pleuronectes platessa (Beam-Plaice), and otter-trawlers targeting Norway lobster Nephrops norvegicus and demersal fish (Otter-Mix). Within the seascapes subjected to >80% of the fishing activity, nineteen environmental factors (summarized by PCA) were used to ecologically characterize fishing hotspot locations using MaxEnt response modelling.ResultsWe found that all three fisheries target highly specific, uncommon habitats. Beam-Sole fishers targeted warmer, shallow, dynamic, nearshore habitats, and within these specifically the depressions between sand ridges. Beam-Plaice fishers mainly targeted the exposed, non-muddy flanks of the Dogger Bank and similar large-scale elevations (50–75 km) where especially the ridges of smaller sand banks are used. Otter-Mix fisheries concentrated in areas with low bed shear stress, located in muddy, relatively deeper areas.ImplicationsThis study is the first to provide insight in benthic habitat types that are frequently targeted by fishers in the North Sea. We demonstrated unequal exploitation pressure between seabed habitats, with the majority of hotspots in the less common habitats. Our results hence contribute to a more effective, ecologically informed planning for the protection and monitoring of all seabed habitats and biodiversity of the North Sea.

How benthic habitats and bottom trawling affect trait composition in the diet of seven demersal and benthivorous fish species in the North Sea

Demersal and benthivorous fish depend on prey availability in benthic habitats for their diet. Prey availability may in turn be affected by bottom trawling. Our study attempted to link the prey consumed by fish directly to those available in the benthic environment. We test the hypothesis that bottom trawling significantly affected the trait composition of fish diets with a strong link to benthic habitats. Stomach content data were analysed for seven demersal and benthivorous fish species in the southern and central North Sea and were related to habitat fauna collected by grab and epi-benthic trawl data across the same spatial extent. Biological trait analysis was used to avoid taxonomic bias in large-scale comparisons across habitats and to quantify the effect of bottom trawling, which tends to shift prey diversity in favour of small, opportunistic and short-lived species and traits. The diets of two demersal omnivores (whiting, Merlangius merlangus and Atlantic cod, Gadus morhua) and two opportunistic benthivores (haddock, Melanogrammus aeglefinus and long-rough dab, Hippoglossoides platessoides) did not appear to reflect either infaunal or epifaunal traits that were abundant in the benthic environment. The diets of European plaice (Pleuronectes platessa) and Dover sole (Solea solea), however, showed strong links with infaunal prey trait composition in shallow sand to muddy sand habitats located in the eastern North Sea and the Dogger Bank. Such strong links were also observed for the diets of Common dab (Limanda limanda) and plaice in similar habitats in deeper waters. Plaice targeted small to medium sized infaunal burrowers and deeper dwelling infaunal species. Its diet was significantly affected by bottom trawling, which caused a reduction in prey biomass in their stomachs rather than a shift in trait composition. Our study showed that the diets of benthivorous fish across large spatial scales are strongly linked with the prey trait availability in their benthic habitats and that bottom trawling significantly affected the diet of plaice, a species with strong habitat-diet associations.

Quantitative analysis of fish and invertebrate assemblage dynamics in association with a North Sea oil and gas installation complex

Decommissioning of offshore infrastructure has become a major issue facing the global offshore energy industry. In the North Sea alone, the decommissioning liability is estimated at £40 billion by 2040. Current international policy requires removal of offshore infrastructure when their production life ends; however, this policy is being questioned as emerging data reveal the importance of these structures to fish and invertebrate populations. Indeed, some governments are developing ‘rigs-to-reef’ (RTR) policies in situations where offshore infrastructure is demonstrated to have important environmental benefits. Using Remotely Operated Vehicles (ROVs), this study quantified and analysed fish and invertebrate assemblage dynamics associated with an oil and gas (O&G) complex in the Dogger Bank Special Area of Conservation (SAC), in the North Sea, Germany. We found clear depth zonation of organisms: infralittoral communities (0–15 m), circalittoral assemblages (15–45 m) and epi-benthic communities (45–50 m), which implies that ‘topping’ or ‘toppling’ decommissioning strategies could eliminate communities that are unique to the upper zones. Sessile invertebrate assemblages were significantly different between structures, which appeared to be driven by both biotic and abiotic mechanisms. The O&G complex accommodated diverse and abundant motile invertebrate and fish assemblages within which the whelk Buccinium undatum, cod fish Gadus morhua and lumpsucker fish Cyclopterus lumpus used the infrastructure for different stages of reproduction. This observation of breeding implies that the structures may be producing more fish and invertebrates, as opposed to simply acting as sites of attraction (sensu the ‘attraction vs production’ debate). At present, there are no records of C. lumpus spawning at such depth and distance from the coast, and this is the first published evidence of this species using an offshore structure as a spawning site. Overall, this study provides important new insight into the role of offshore O&G structures as habitat for fish and invertebrates in the North Sea, thereby helping to inform decommissioning decisions.

Ice marginal dynamics of the last British-Irish Ice Sheet in the southern North Sea: Ice limits, timing and the influence of the Dogger Bank

The southern North Sea is a particularly important area for understanding the behaviour of the British-Irish Ice Sheet (BIIS) during the last glacial cycle. It preserves a record of the maximum extent of the eastern sector of the BIIS as well as evidence for multiple different ice flow phases and the dynamic re-organisation of the BIIS. However, to date, the known ice sheet history and geochronology of this region is predominantly derived from onshore geological evidence, and the offshore imprint and dynamic history of the last ice sheet remain largely unknown. Using new data collected by the BRITICE-CHRONO project this paper explores the origin and age of the Dogger Bank; re-assesses the extent and age of the glaciogenic deposits across the shallow areas of the North Sea between the Dogger Bank and the north Norfolk coast and; re-examines the dynamic behaviour of the BIIS in the southern North Sea between 31.6 and 21.5 ka.This paper shows the core of the Dogger Bank to be composed glaciolacustrine sediment deposited between 31.6 and 25.8 ka. Following its formation the western end of the Dogger lake was overridden with ice reaching ∼54°N where the ice margin is co-incident with the southerly extent of subglacial tills previously mapped as Bolders Bank Fm. This initial ice override and retreat northwards back across the Dogger lake was complete by 23.1 ka, but resulted in widespread compressive glaciotectonism of the lake sediments and the formation of thrust moraine complexes. Along the northern edge of the bank moraines are on-lapped by later phase glaciolacustrine and marine sediments but do not show evidence of subsequent ice override.The shallow seafloor to the west and southwest of the Dogger Bank records several later phases of ice advance and retreat as the North Sea Lobe flowed between the Dogger Bank and the Yorkshire/Lincolnshire coasts and reached North Norfolk. New optically stimulated luminescence (OSL) ages from Garrett Hill on outwash limit the arrival of the BIIS on the Norfolk coast to 22.8–21.5 ka. Multiple till sheets and chains of moraines on the seafloor north of Norfolk mark dynamic oscillation of the North Sea Lobe margin as it retreated northwards. This pattern of behaviour is broadly synchronous with the terrestrial record of deposition of subglacial, glaciofluvial and glaciolacustrine sediments along the Yorkshire coast which relate to post Dimlington Stadial ice marginal oscillations after 21.5 ka.With respect to forcing mechanisms it is likely that during the early phases of the last glacial maximum (∼30-23ka) the interaction between the southern margin of the BIIS and the Dogger Lake was critical in influencing flow instability and rapid ice advance and retreat. However, during the latter part of the last glacial maximum (22–21 ka) late-phase ice advance in the southern North Sea became restricted to the western side of the Dogger Bank which was a substantial topographic feature by this time. This topographic confinement, in addition to decoupling of the BIIS and the Fennoscandian Ice Sheet (FIS) further north, enabled ice to reach the north Norfolk coast, overprinting the seabed with late-phase tills of the Bolders Bank Fm.

Shifting south-eastern North Sea macrofauna community structure since 1986: A response to de-eutrophication and regionally decreasing food supply?

Since the second half of the 20th century drastic changes were found within the marine environment, mostly driven by anthropogenic and climatic pressures. Within this study, the long-term variability of south-eastern North Sea macrofauna community structure was analysed on a large spatial extent, to examine effects of increasing sea surface temperature, decreasing pelagic and benthic primary production and phytoplankton biomass, together with de-eutrophication effects since the late 1980s. Macrofauna data from the 1986 North Sea Benthos Survey and the 2000 North Sea Benthos Project were reanalysed and compared with present data from 2010 to 2015. A stable spatial distribution of four main macrofauna communities was found, based on the environmental parameters depth, sediment composition, and tidal parameters. Diversity and taxa numbers remained stable since 1986. However, in 2010–2015, mean abundance and biomass of offshore waters had decreased slightly, while at the coastal nearshore areas and parts of the Oysterground a significant increase was found. Simultaneously, a spatial shift in maximum mean abundance and biomass from the Dogger Bank and Oysterground in 1986 towards the North Frisian coast in 2000 and 2010–2015 was found. Changes were probably driven by de-eutrophication effects in line with food limitation caused by decreasing phytoplankton biomass and pelagic and benthic primary production that strengthen the dominance of opportunistic tube-living taxa such as Phoronis spp. and Spiophanes bombyx. Synergistic effects of increasing sea surface temperature, ongoing seabed degradation, and predation by small non-target fish species caused distinct regionally different changes in characteristic species and dominance structure.

Site-specific controller design for monopile offshore wind turbines

The fatigue life of offshore wind turbine (OWT) support structures is sensitive to variations in site-specific conditions such as the water depth and soil properties. Site condtions may vary significantly within a wind farm, and they may change throughout the lifetime of the OWT. This paper analyses how control strategies for fatigue life extension can compensate for differing fatigue loads due to varying site conditions. Control strategies applicable for both power production and idling situations are analysed, and methodology to reduce undesirable side-effects is proposed. The design case is a 10 MW monopile OWT located in 30 m water depth at the Dogger Bank in the North Sea, and results are based on time-domain simulations performed using an aero-hydro-servo-elastic simulation tool. The results show that, when all the investigated control strategies are utilized, a fatigue damage reduction over the 20-year lifetime of approximately 50% is possible. Furthermore, it is shown that adverse side-effects such as wear of pitch actuators and fluctuations in the power output can be significantly reduced by limiting the use of control strategies to some predefined situations. With only moderate cost to other system components, the control system is able to compensate for 20% variation in soil stiffness, and 5% (1.5 m) variation in water depth.

Optimal Design of Galvanic Corrosion Protection Systems for Offshore Wind Turbine Support Structures

The current work addresses a mass/cost-optimization procedure for galvanic anode cathodic protection systems based on both cathodic protection (CP) standards and numerical simulation. An approach is developed for optimizing the number and dimensions of the galvanic anodes, distributing the optimized anodes on the support structure, and finally evaluating the protective potential on the structure during the lifetime by using finite element (FE) software. An algorithm based on sequential quadratic programming is used for optimizing the number and dimensions of the anodes. Both simplified and detailed models are suggested for calculating the protective potential on the structure. The simplified model is selected based on its advantages in terms of calculation time and compatibility with DNV standard data. A time-dependent FE model is used to take into account the electrical isolation degradation of the structure coating as well as the mass reduction of the anodes during the CP lifetime. The performance of the proposed optimization process is examined on a mono bucket inspired (with some simplifications) by the Dogger Bank metrological mast in England. The optimized designs for different coating and anode types are compared and the best designs in terms of both cost and protective potential during the lifetime are suggested. The achieved results show that the proposed optimization procedure can reduce the cost of the CP system around 70% compared to the original non-optimized CP design of the Dogger Bank metrological mast. Furthermore, evaluating the time-evolution performance of the CP systems can reduce their lifetime uncertainty.

Integrating Natural and Social Marine Sciences to Sustainably Manage Vectors of Change and Inform Marine Policy: Dogger Bank Transnational Case Study

The management of marine resources is a complex process driven by the dynamics of the natural system and the influence of stakeholders including policy-makers. An integration of natural and social sciences research is required by policy-makers to better understand, and manage sustainably, natural changes and anthropogenic activities within particular marine systems. Given the uncertain development of activities in the marine environment, future scenarios assessments can be used to investigate whether marine policy measures are robust and sustainable. This paper develops an interdisciplinary framework, which incorporates future scenarios assessments, and identifies four main types of evaluation needed to integrate natural and social sciences research to support the integrated management of the marine environment: environmental policy and governance assessments; ecosystem services, indicators and valuation; modelling tools for management evaluations, and risk assessment and risk management. The importance of stakeholder engagement within each evaluation method is highlighted. The paper focuses on the transnational spatial marine management of the Dogger Bank, in the central North Sea, a site which is very important ecologically, economically and politically. Current management practices are reviewed, and research tools to support future management decisions are applied and discussed in relation to two main vectors of change affecting the Dogger Bank, namely commercial fisheries and offshore wind farm developments, and in relation to the need for nature conservation. The input of local knowledge through stakeholder engagement is highlighted as a necessary requirement to produce site-specific policy recommendations for the future management of the Dogger Bank. We present wider policy recommendations to integrate natural and social sciences in a global marine context.

Productivity and recovery of forage fish under climate change and fishing: North Sea sandeel as a case study

AbstractForage fish occupy a central position in marine food‐webs worldwide by mediating the transfer of energy and organic matter from lower to higher trophic levels. The lesser sandeel (Ammodytes marinus) is one of the ecologically and economically most important forage fish species in the North‐east Atlantic, acting as a key prey for predatory fish and sea birds, as well as supporting a large commercial fishery. In this case study, we investigate the underlying factors affecting recruitment and how these in turn affect productivity of the North Sea sandeel using long‐term data and modelling. Our results demonstrate how sandeel productivity in the central North Sea (Dogger Bank) depends on a combination of external and internal regulatory factors, including fishing and climate effects, as well as density dependence and food availability of the preferred zooplankton prey (Calanus finmarchicus and Temora longicornis). Furthermore, our model scenarios suggest that while fishing largely contributed to the abrupt stock decline during the late 1990s and the following period of low biomass, a complete recovery of the stock to the highly productive levels of the early 1980s would only be possible through changes in the surrounding ecosystem, involving lower temperatures and improved feeding conditions. To that end, we stress the need for ecosystem‐based management accounting for multiple internal and external factors occurring within the broader context of the ecosystem in which forage fish species, such as sandeel, play an important and integral part.