NW European Shelf Seas Research Articles

Twenty-first century marine climate projections for the NW European shelf seas based on a perturbed parameter ensemble

Twenty-first century marine climate projections for the NW European shelf seas based on a perturbed parameter ensemble

Ocean shelf exchange, NW European shelf seas: Measurements, estimates and comparisons

Transports across the continental shelf edge enhance shelf-sea production, remove atmospheric carbon and imply an active boundary to ocean circulation. We estimate relatively large overall transport across three contrasted sectors of north-west European shelf edge: the Celtic Sea south-west of Britain, the Malin-Hebrides shelf west of Scotland, the West Shetland shelf north of Scotland. The estimates derive from measurements in the project FASTNEt (Fluxes across sloping topography of the North East Atlantic): drifters, moored current meters, effective “diffusivity” from drifter dispersion and salinity surveys, other estimates of velocity variance contributing to exchange. Process contributions include transport by along-slope flow, internal waves and their Stokes drift, tidal pumping, eddies, Ekman transports in the wind-driven surface layer and bottom boundary layer.Overall exchange across the shelf edge is estimated as several m2s−1: if extrapolated globally even 1 m2s−1 is large compared with oceanic transports and potentially important to shelf-sea and adjacent oceanic budgets. In our context, most exchange is in tides, and other motion with periods ∼ one day or less, and so effective only for water properties that evolve on such short time-scales. Nevertheless, cross-slope fluxes, and exchange by low-frequency motion (periods > two days), are large by global standards and also very variable. Deployment-mean fluxes nearest the shelf break were in the range 0.3–4 m2s−1; mean exchanges from low-frequency motion were 0.8–3 m2s−1. Deeper longer-term moorings and drifters crossing 500 m depth gave much larger fluxes and exchanges up to 20 m2s−1. These transports’ significance depends on distinctive properties of the water, or its contents, and on internal shelf-sea circulation affecting further transport. For the NW European shelf, transports across the shelf edge enable its disproportionately strong CO2 “pump”.The complex context, and small scales of numerous processes enabling cross-slope transports, imply a need for models. Measurements remain limited in extent and duration, but widely varied contexts, particular conditions, events, processes and behaviours are now available to support model validation, especially around the north-west European continental shelf edge. Variability still renders observations insufficient for stable estimates of transports and exchanges, especially if partitioned by sector and season; indeed, there may be significant inter-annual differences. Validated fine-resolution models give the best prospect of spatial and temporal coverage and of estimating present-day and potential future shelf-sea sensitivities to the adjacent ocean.

Total Water Column Analysis Shows the Importance of a Single Species in Subsurface Chlorophyll Maximum Thin Layers in Stratified Waters

Marine phytoplankton form the base of marine food webs and are the driving force of the marine carbon cycle, so understanding the dynamics of their blooms is critical. While near-surface marine productivity (<10 m water depths) is extensively documented, that of the subsurface is less well characterised. Increasing evidence of the importance of subsurface chlorophyll maxima (SCM) and climatically driven increases in stratification of surface waters that promote SCM development call for improved sampling of the subsurface. To address this, we targeted the summer stratified waters of the Western English Channel, part of the NW European shelf seas, where SCM are commonly developed. In situ holography was applied to undertake the highest ever resolution, total water column, quantitative analysis of microplankton distribution, and demonstrated the importance of a SCM, co-located with the thermocline, dominated by a single species, the dinoflagellate Ceratium fusus. This species was dominant in the SCM over a wide area of the NW European shelf in the June/July 2015 study period and comprised up to 85% of the SCM biomass. Analysis of similarity and multivariate non-metric multidimensional scaling showed the phytoplankton community of the SCM to be statistically distinct from those of the surface and deep waters. Holography also revealed a fine scale layering of taxa at different levels within the SCM, likely reflecting ecological differences. Some taxa followed the peak abundance of C. fusus, while others reached maximum abundances immediately below or above the C. fusus maximum, suggesting the possible operation of exclusion mechanisms. Additionally, the detection of abundant aggregates located only within and beneath the SCM demonstrates the potential importance of this deep production for the export of carbon to the sea floor. Some predictions of phytoplankton productivity propose a shift to smaller cells in the more stratified oceans of the future resulting in declining production and export. Results presented here, however, contribute to a growing body of evidence that suggests, on the contrary, that key species among the larger celled/colonial, SCM-adapted diatoms and dinoflagellates may instead be selected in stratified conditions, driving increased production and export.

Tidal energy leasing and tidal phasing

In addition to technical and economic constraints, tidal energy leasing is generally governed by demand for sites which contain the highest tidal streams, and does not take into account the phase relationship (i.e. the time lag) between sites. Here, the outputs of a three-dimensional tidal model are analysed to demonstrate that there is minimal phase diversity among the high tidal stream regions of the NW European shelf seas. It is therefore possible, under the current leasing system, that the electricity produced by the first generation of tidal stream arrays will similarly be in phase. Extending the analysis to lower tidal stream regions, we demonstrate that these lower energy sites offer more potential for phase diversity, with a mean phase difference of 1.25 h, compared to the phase of high energy sites, and hence more scope for supplying firm power to the electricity grid. We therefore suggest that a state-led leasing strategy, favouring the development of sites which are complementary in phase, and not simply sites which experience the highest current speeds, would encourage a sustainable tidal energy industry.

The influence of ocean acidification on nitrogen regeneration and nitrous oxide production in the northwest European shelf sea

Abstract. The assimilation and regeneration of dissolved inorganic nitrogen, and the concentration of N2O, was investigated at stations located in the NW European shelf sea during June/July 2011. These observational measurements within the photic zone demonstrated the simultaneous regeneration and assimilation of NH4+, NO2− and NO3−. NH4+ was assimilated at 1.82–49.12 nmol N L−1 h−1 and regenerated at 3.46–14.60 nmol N L−1 h−1; NO2- was assimilated at 0–2.08 nmol N L−1 h−1 and regenerated at 0.01–1.85 nmol N L−1 h−1; NO3− was assimilated at 0.67–18.75 nmol N L−1 h−1 and regenerated at 0.05–28.97 nmol N L−1 h−1. Observations implied that these processes were closely coupled at the regional scale and that nitrogen recycling played an important role in sustaining phytoplankton growth during the summer. The [N2O], measured in water column profiles, was 10.13 ± 1.11 nmol L−1 and did not strongly diverge from atmospheric equilibrium indicating that sampled marine regions were neither a strong source nor sink of N2O to the atmosphere. Multivariate analysis of data describing water column biogeochemistry and its links to N-cycling activity failed to explain the observed variance in rates of N-regeneration and N-assimilation, possibly due to the limited number of process rate observations. In the surface waters of five further stations, ocean acidification (OA) bioassay experiments were conducted to investigate the response of NH4+ oxidising and regenerating organisms to simulated OA conditions, including the implications for [N2O]. Multivariate analysis was undertaken which considered the complete bioassay data set of measured variables describing changes in N-regeneration rate, [N2O] and the biogeochemical composition of seawater. While anticipating biogeochemical differences between locations, we aimed to test the hypothesis that the underlying mechanism through which pelagic N-regeneration responded to simulated OA conditions was independent of location. Our objective was to develop a mechanistic understanding of how NH4+ regeneration, NH4+ oxidation and N2O production responded to OA. Results indicated that N-regeneration process responses to OA treatments were location specific; no mechanistic understanding of how N-regeneration processes respond to OA in the surface ocean of the NW European shelf sea could be developed.

A coupled tide-wave model for the NW European shelf seas

Understanding the interaction of tides and waves is essential in many studies, including marine renewable energy, sediment transport, long-term seabed morphodynamics, storm surges and the impacts of climate change. In the present research, a COAWST model of the NW European shelf seas has been developed and applied to a number of physical processes. Although many aspects of wave–current interaction can be investigated by this model, our focus is on the interaction of barotropic tides and waves at shelf scale. While the COWAST model was about five times more computationally expensive than running decoupled ROMS (ocean model) and SWAN (wave model), it provided an integrated modelling system which could incorporate many wave–tide interaction processes, and produce the tide and wave parameters in a unified file system with a convenient post-processing capacity. Some applications of the model such as the effect of tides on quantifying the wave energy resource, which exceeded 10% in parts of the region, and the effect of waves on the calculation of the bottom stress, which was dominant in parts of the North Sea and Scotland, during an energetic wave period are presented, and some challenges are discussed. It was also shown that the model performance in the prediction of the wave parameters can improve by 25% in some places where the wave-tide interaction is significant.

Changing ocean circulation and hydrothermal inputs during Ocean Anoxic Event 2 (Cenomanian–Turonian): Evidence from Nd-isotopes in the European shelf sea

Nd isotopes of fish debris collected from the English Chalk at Eastbourne (Sussex, UK) are used to reconstruct the history of ocean circulation in the NW European shelf sea during Oceanic Anoxic Event 2 (OAE 2, Cenomanian–Turonian). The Eastbourne εNd record exhibits a 1-unit negative excursion (decreasing from ∼−9 to ∼−10), immediately followed by a 3-unit positive excursion reaching ∼−7. The onset of the negative εNd excursion lags the global δ13C rise characteristic of OAE 2, suggesting stable patterns of ocean circulation in the NW European shelf sea at this time. Both negative and positive Nd-isotope excursions took place during a transient cooling episode within OAE 2. The negative εNd excursion is interpreted as due to a change in ocean circulation with northerly sourced water masses becoming the dominant bottom waters at Eastbourne. The positive excursion is best explained by the transport of radiogenic Nd derived from a volcanic source, possibly the High Arctic or Caribbean large igneous province (LIP). An input of volcanic Nd may reconcile the Eastbourne record with coeval εNd records on Demerara Rise in the western tropical Atlantic. The broad synchroneity of high εNd values (∼−7) registered at both sites suggests a possible period with efficient oceanic mixing between the tropical Atlantic and the NW European shelf sea during the cooling episode. The Eastbourne εNd record of OAE 2, together with coeval temperature reconstructions, provides evidence for the coincidence of changes in ocean circulation and transient climatic cooling, implying a tight coupling between the two phenomena during this interval.

Wave power variability over the northwest European shelf seas

Regional assessments of the wave energy resource tend to focus on averaged quantities, and so provide potential developers with no sense of temporal variability beyond seasonal means. In particular, such assessments give no indication of inter-annual variability – something that is critical for determining the potential of a region for wave energy convertor (WEC) technology. Here, we apply the third-generation wave model SWAN (Simulating Waves Nearshore) at high resolution to assess the wave resource of the northwest European shelf seas, an area where many wave energy test sites exist, and where many wave energy projects are under development. The model is applied to 7years of wind forcing (2005–2011), a time period which witnessed considerable extremes in the variability of the wind (and hence wave) climate, as evidenced by the variability of the North Atlantic Oscillation (NAO). Our simulations demonstrate that there is much greater uncertainty in the NW European shelf wave resource during October–March, in contrast to the period April–September. In the more energetic regions of the NW European shelf seas, e.g. to the northwest of Scotland, the uncertainty was considerably greater. The winter NW European shelf wave power resource correlated well with the NAO. Therefore, provided trends in the NAO can be identified over the coming decades, it may be possible to estimate how the European wave resource will similarly vary over this time period. Finally, the magnitude of wave power estimated by this study is around 10% lower than a resource which is used extensively by the wave energy sector – the Atlas of UK Marine Renewable Energy Resources. Although this can partly be explained by different time periods analysed for each study, our application of a third-generation wave model at high spatial and spectral resolution significantly improves the representation of the physical processes, particularly the non-linear wave-wave interactions.

Repeated bottom-water oxygenation during OAE 2: timing and duration of short-lived benthic foraminiferal repopulation events (Wunstorf, northern Germany)

Abstract. Published proxy data for Oceanic Anoxic Event 2 (OAE 2 or Cenomanian–Turonian Boundary Event) and other mid-Cretaceous OAEs indicate widespread anoxic bottom-water conditions. However, increasing evidence shows that anoxia was not permanent but subject to significant fluctuations. We have generated X-ray fluorescence elemental concentration and benthic foraminiferal assemblage records for a short section of OAE 2 black shales from Wunstorf, northern Germany. Two intervals of low sulphur elemental concentration are interpreted as periods of increased oxygenation of bottom waters. This is supported by benthic foraminiferal assemblage data showing repopulation events associated with these intervals. These repopulation events are characterized mainly by the occurrence of agglutinated taxa, with Lingulogavelinella globosa being the only abundant calcareous species. This observation is interpreted in terms of short-term interruptions of the otherwise anoxic bottom-water environment. Comparison with repopulation events during OAE 1b and Quaternary sapropels make it reasonable to speculate that short-term cooling and an associated increase in bottom-water ventilation at the NW European shelf sea are the main trigger mechanisms for the observed repopulation events at Wunstorf. As source area for benthic foraminifera, shallower parts of the Lower Saxony basin are proposed.

Climatology of surface and near-bed temperature and salinity on the north-west European continental shelf for 1971–2000

A 30-year (1971–2000) temperature and salinity climatology is presented for surface and near-bed regions of the NW European shelf seas, with a resolution of 1/6∘ longitude by 1/10∘ latitude. The data have been extracted from the International Council for the Exploration of the Sea (ICES) data centre and supplemented by additional records from the World Ocean Data Centre (WODC). From the original data, which are irregularly distributed in space and time, the mean monthly temperature and salinity are calculated, as well as the climatic mean annual cycle. The climatology presented here is an improvement upon all existing climatologies presented in the literature for the NW European shelf; covering a wider area on a finer scale and including the surface and near-bed distribution of both temperature and salinity. Comparison of our data with existing climatologies shows good agreement, with differences occurring where our climatology is an improvement. This climatology, which will prove to be valuable to many users in the marine community, will be regularly updated and made available to all users via the ICES data centre.

Post-glacial sediment dynamics in the Irish Sea and sediment wave morphology: Data–model comparisons

The irregular seafloor of the narrow Irish Sea on the NW European Shelf has been documented over several decades. From recently collected swath bathymetry data, very large trochoidal, nearly symmetrical sediment waves are observed in many parts of the Irish Sea and appear similar to those described from other continental shelf seas in North America that were covered by glacigenic sediments during the Last Glacial Maximum. Swath multibeam and single beam bathymetry data, backscatter intensity, shallow seismic imagery, video footage and sediment cores from the Irish Sea high sediment waves have been integrated to identify their genesis with reference to present and past hydrodynamic variability. From cross-sectional profiles over asymmetrical sediment waves in the Irish Sea the direction of asymmetry is used to map residual bed stress directions and associated bedload transport paths. Irish Sea peak bed stress vectors were generated using a two-dimensional palaeo-tidal model for the NW European shelf seas and compare well with the observations. Tidally induced bed stresses are modelled to have increased between 7–10 ka BP, to be nearly symmetrical in magnitude and to have reversed in dominant direction on a millennial scale. These environmental conditions during the post-glacial marine transgression are suggested here to help comprehend the construction of the very large sediment waves, with local variations due to differences in sediment grain size, sediment supply, water depth and intensified currents due to seafloor slopes. Model parameterisation using an open ocean boundary with time-dependent tidal changes and the implementation of high-resolution bathymetric information will improve future models of small-scale bed shear stress patterns and improve the predictive value of such modelling efforts.

Changes in wave climate over the northwest European shelf seas during the last 12,000 years

Because of the depth attenuation of wave orbital velocity, wave‐induced bed shear stress is much more sensitive to changes in total water depth than tidal‐induced bed shear stress. The ratio between wave‐ and tidal‐induced bed shear stress in many shelf sea regions has varied considerably over the recent geological past because of combined eustatic changes in sea level and isostatic adjustment. In order to capture the high‐frequency nature of wind events, a two‐dimensional spectral wave model is here applied at high temporal resolution to time slices from 12 ka BP to present using paleobathymetries of the NW European shelf seas. By contrasting paleowave climates and bed shear stress distributions with present‐day conditions, the model results demonstrate that, in regions of the shelf seas that remained wet continuously over the last 12,000 years, annual root‐mean‐square (rms) and peak wave heights increased from 12 ka BP to present. This increase in wave height was accompanied by a large reduction in the annual rms wave‐induced bed shear stress, primarily caused by a reduction in the magnitude of wave orbital velocity penetrating to the bed for increasing relative sea level. In regions of the shelf seas which remained wet over the last 12,000 years, the annual mean ratio of wave‐ to (M2) tidal‐induced bed shear stress decreased from 1 (at 12 ka BP) to its present‐day value of 0.5. Therefore compared to present‐day conditions, waves had a more important contribution to large‐scale sediment transport processes in the Celtic Sea and the northwestern North Sea at 12 ka BP.

Enhanced ocean carbon storage from anaerobic alkalinity generation in coastal sediments

Abstract. The coastal ocean is a crucial link between land, the open ocean and the atmosphere. The shallowness of the water column permits close interactions between the sedimentary, aquatic and atmospheric compartments, which otherwise are decoupled at long time scales (≅ 1000 yr) in the open oceans. Despite the prominent role of the coastal oceans in absorbing atmospheric CO2 and transferring it into the deep oceans via the continental shelf pump, the underlying mechanisms remain only partly understood. Evaluating observations from the North Sea, a NW European shelf sea, we provide evidence that anaerobic degradation of organic matter, fuelled from land and ocean, generates total alkalinity (AT) and increases the CO2 buffer capacity of seawater. At both the basin wide and annual scales anaerobic AT generation in the North Sea's tidal mud flat area irreversibly facilitates 7–10%, or taking into consideration benthic denitrification in the North Sea, 20–25% of the North Sea's overall CO2 uptake. At the global scale, anaerobic AT generation could be accountable for as much as 60% of the uptake of CO2 in shelf and marginal seas, making this process, the anaerobic pump, a key player in the biological carbon pump. Under future high CO2 conditions oceanic CO2 storage via the anaerobic pump may even gain further relevance because of stimulated ocean productivity.

New primary production in northwest European shelf seas, 1960–2003

Spatial and temporal patterns from 1960 to 2003 in annual potential new primary production (PNP) of the NW European shelf seas were derived from general additive models of nitrate concentrations and from data on riverine and atmospheric fluxes of oxidized nitrogen. Average PNP was highest in the seasonally stratified outer shelf regions (>70 gC m-2 yr-1), where the proportion of PNP accounted for by vertical fluxes from deep water (>65%) was correlated with the North Atlantic Oscillation (NAO) index. PNP was lowest in the central North Sea (~30 gC m-2 yr-1) and in the southern North Sea was correlated with river inputs that accounted for 24% of the annual total (average ~50 gC m-2 yr-1). Atmospheric deposition accounted for ~3% of annual PNP region-wide, but in the northern North Sea this was higher than the contribution from rivers. Tidal fronts are traditionally considered to be highly productive zones, but we find them to have characteristically low PNP and conclude that they must be loci of high recycled production. The results indicate an exceptional flux of nitrate-rich ocean water onto the shelf in the early 1990s, which resulted in a pulse of PNP coincident with a well-documented 'regime shift' in the pelagic food web. North Sea-wide, long-term average PNP was approximately equal to production by all higher trophic levels combined, though trophic propagation of inter-annual variations was weakly defined. Nevertheless, there is a case for proposing that harvesting in areas and periods of low PNP should be managed more conservatively to minimize the risk of detrimental effects on the food web.

Rapid decline of the CO2 buffering capacity in the North Sea and implications for the North Atlantic Ocean

New observations from the North Sea, a NW European shelf sea, show that between 2001 and 2005 the CO2 partial pressure (pCO2) in surface waters rose by 22 μatm, thus faster than atmospheric pCO2, which in the same period rose approximately 11 μatm. The surprisingly rapid decline in air‐sea partial pressure difference (ΔpCO2) is primarily a response to an elevated water column inventory of dissolved inorganic carbon (DIC), which, in turn, reflects mostly anthropogenic CO2 input rather than natural interannual variability. The resulting decline in the buffering capacity of the inorganic carbonate system (increasing Revelle factor) sets up a theoretically predicted feedback loop whereby the invasion of anthropogenic CO2 reduces the ocean's ability to uptake additional CO2. Model simulations for the North Atlantic Ocean and thermodynamic principles reveal that this feedback should be stronger, at present, in colder midlatitude and subpolar waters because of the lower present‐day buffer capacity and elevated DIC levels driven either by northward advected surface water and/or excess local air‐sea CO2 uptake. This buffer capacity feedback mechanism helps to explain at least part of the observed trend of decreasing air‐sea ΔpCO2 over time as reported in several other recent North Atlantic studies.

Tidal evolution of the northwest European shelf seas from the Last Glacial Maximum to the present

Two‐dimensional paleotidal simulations have been undertaken to investigate tidal and tide‐dependent changes (tidal amplitudes, tidal current velocities, seasonal stratification, peak bed stress vectors) that have occurred in the NW European shelf seas during the last 20 ka. The simulations test the effect of shelf‐wide isostatic changes of sea level by incorporating results from two different crustal rebound models, and the effect of the ocean‐tide variability by setting open boundary values either fixed to the present state or variable according to the results of a global paleotidal model. The use of the different crustal rebound models does not affect the overall changes in tidal patterns, but the timing of the changes is sensitive to the local isostatic effects that differ between the models. The incorporation of ocean‐tide changes greatly augments the amplitude of tides and tidal currents in the Celtic and Malin seas before 10 ka BP, and has a large impact on the distribution of seasonally stratified conditions, magnitude of peak bed stress vectors and tidal dissipation in the shelf seas. The predictions on seasonal stratification are supported by well‐dated evidence on tidal mixing front migration in the Celtic Sea. Additional experiments using the global model suggest that the variability of offshore tides has been caused mainly by changes of eustatic sea level and ice‐sheet extent. In particular, a large decrease observed at 10–8 ka BP is attributed to the opening of Hudson Strait accompanied by the retreat of the Laurentide Ice Sheet.

Thermocline mixing in summer stratified continental shelf seas

A key process in the shelf sea pumping of CO2 to the open ocean is vertical mixing in the summer stratified zones of continental shelf seas. Here, we present measurements of profiles of the rate of dissipation of turbulent kinetic energy (ɛ), from which vertical mixing is inferred, for several locations in the summer stratified zones of the NW European shelf seas. At both sites we find significant thermocline mixing, with ∼20% of the observed turbulent dissipation taking place within the thermocline. We then present evidence to demonstrate that the thermocline mixing is a result of shear instability within the thermocline. This is contrary to the conventional view that vertical mixing in shelf seas is largely driven by boundary shear stresses.

Palaeoecology of calcareous dinoflagellate cysts in the mid-Cenomanian Boreal Realm: implications for the reconstruction of palaeoceanography of the NW European shelf sea

A transect from the bathyal to proximal shelf facies of the Boreal Realm was investigated to compare spatial and temporal distribution changes of calcareous dinoflagellate cysts (c-dinocysts) throughout the mid-Cenomanian in order to gain information on the ecology of these organisms. Pithonelloideae dominated the cyst assemblages to more than 95% on the shelf, a prevalence that can be observed throughout most of the Upper Cretaceous. The affinity of this group with the dinoflagellates, which is still controversially discussed, can be confirmed, based on evidence from morphological features and distribution patterns. The consistent prevalence of Pithonella sphaerica and P. ovalis in c-dinocyst assemblages throughout the Upper Cretaceous indicates that they were produced more frequently than cysts of the other species and might, therefore, represent a vegetative dinoflagellate life stage. P. sphaerica and P. ovalis are interpreted as eutrophic species. P. sphaerica is the main species in a marginal-shelf upwelling area, offshore Fennoscandia. Here, sedimentary cyclicity appears to have been reduced to the strongest light/dark changes, while in the outer shelf sediments, light/dark cycles are well-developed and show pronounced temporal assemblage changes. Cyclic fluctuations in the P. sphaerica / P. ovalis ratio reflect shifts of the preferred facies zones and indicate changes in surface mixing patterns. During periods of enhanced surface mixing most parts of the shelf were well-ventilated, and nutrient-enriched surface waters led to high productivity and dominance of the Pithonelloideae. These conditions on the shelf contrasted with those in the open ocean, where more oligotrophic and probably stratified waters prevailed, and an assemblage with very few Pithonelloideae and dominance of Cubodinellum renei and Orthopithonella ? gustafsonii was characteristic. While orbitally-forced light/dark sedimentary cyclicity of the shelf sections was mainly related to surface-water carbonate productivity changes, no cyclic modulation of productivity was observed in the oceanic profile. Therefore, dark layer formation in the open ocean was predominantly controlled by the cyclic establishment of anoxic bottom water conditions. Orbitally-forced interruptions in mixing on the shelf resulted in cyclic periods of stratification and oligotrophy in the surface waters, an expansion of oceanic species to the outer shelf, and a shelfward shift of pithonelloid-facies zones, which were probably related to shelfward directed oceanic ingressions.