Abstract. Seasonal depletion of dissolved oxygen (DO) in stratified shelf seas has important consequences for benthic and pelagic ecosystems and biogeochemical cycling. There is a need to understand the importance of different processes contributing to the bottom water oxygen budget, and how those processes might change in a warmer ocean. Using CTD observations from the UK Shelf Sea Biogeochemistry programme, we construct a summer DO budget for the bottom waters of the Celtic Sea and use the budget to assess how bottom water DO might change in a warmer climate. Across the shelf, bottom water DO concentration declined during summer stratification, with greater losses in shallow northern waters and slower depletion in deeper southern regions. At a well-sampled site in the central Celtic Sea, the bottom water shows a consistent net DO loss of –44 ± 4 mmol m⁻² d⁻¹. Respiration and remineralization dominate this decline (–54 ± 19 mmol m⁻² d⁻¹), while vertical turbulent fluxes from the subsurface chlorophyll maximum (SCM) form an important DO source (30 ± 17 mmol m-2 d-1). Episodic wind events enhance DO supply from the SCM, helping to offset some of the DO consumption in the bottom water. Benthic oxygen demand and horizontal transports make minor contributions to the DO budget. A +2 °C “business as usual” climate warming will reduce oxygen solubility and lead to a 12 mmol m-3 drop in DO concentration in the bottom water over the summer stratified period. However, we find that increased microbial metabolic rates in the warmer bottom water are more important for changes in bottom water DO concentrations, potentially driving a decrease in DO concentration of about 35 – 66 mmol m-3. Combined, these effects will lead to increased oxygen deficiency in the central and northern Celtic Sea. Our results demonstrate the importance of metabolic responses to a warmer ocean, but also the need to better understand changes in winds and wind-driven mixing across the seasonal thermocline.

Seasonally stratified shelf seas are amongst the most biologically productive on the planet. A consequence is that the deeper waters can become oxygen deficient in late summer in response to deep water remineralisation of sunken organic matter. Model simulations suggest that a warming climate will accelerate this deficiency due to strengthening stratification and the increased duration of stratification. In the next decades the seasonally stratified seas will host the massive expansion of off-shore wind farms, and in particular the new generation of floating wind turbines. Here we integrate novel turbulence timeseries measurements with vertical profiles of water column properties from a seasonally stratified shelf sea to estimate oxygen and biogeochemical fluxes. These new results are used to investigate the role of vertical mixing in the development of the seasonal O2 deficit with particular reference to the potential impact of enhanced mixing associated with the wake of the floating wind turbines.

Seasonally stratified shelf seas are amongst the most biologically productive on the planet. A consequence is that the deeper waters can become oxygen deficient in late summer. Predictions suggest global warming will accelerate this deficiency. Here we integrate turbulence timeseries with vertical profiles of water column properties from a seasonal stratified shelf sea to estimate oxygen and biogeochemical fluxes. The profiles reveal a significant subsurface chlorophyll maximum and associated mid-water oxygen maximum. We show that the oxygen maximum supports both upward and downwards O2 fluxes. The upward flux is into the surface mixed layer, whilst the downward flux into the deep water will partially off-set the seasonal O2 deficit. The results indicate the fluxes are sensitive to both the water column structure and mixing rates implying the development of the seasonal O2 deficit is mediated by diapcynal mixing. Analysis of current shear indicate that the downward flux is supported by tidal mixing, whilst the upwards flux is dominated by wind driven near-inertial shear. Summer storminess therefore plays an important role in the development of the seasonal deep water O2 deficit. Oxygen deprivation in the marine environment is likely to be exacerbated by climate change. We present data to show the potential impact of changing weather patterns in the development of a seasonal oxygen deficit in seasonally stratifying shelf seas.

The North Atlantic Storm Track acts as a conveyor belt for extratropical cyclones that frequently deliver high winds and rainfall to northwest European shelf seas. Storms are primarily considered detrimental to shelf sea stratification due to wind-driven mixing countering thermal buoyancy, but their impact on shelf scale stratification cycles remains poorly understood. Here, we show that storms trigger stratification through enhanced surface buoyancy from rainfall. A multidecadal model confirms that rainfall contributed to triggering seasonal stratification 88% of the time from 1982 to 2015. Stratification could be further modulated by large-scale climate oscillations, such as the Atlantic Multidecadal Variability (AMV), with stratification onset dates being twice as variable during a positive AMV phase than a negative one. Further insights into how changing storm activity will impact shelf seas are discussed beyond the current view of increasing wind-driven mixing, with significant implications for marine productivity and ecosystem function.

<p>Oxygen is a vital resource in the ocean, particularly for the high oxygen demand consumers such as fish. In temperature shelf seas, the bottom water oxygen is frequently seen to decrease during the stratified period as a natural consequence of organic matter being remineralised and the seasonal thermocline preventing the replenishment of oxygen from the atmosphere. However, the subsurface chlorophyll maximum (SCM) is a generator of oxygen in the base of the thermocline. Mixing across the thermocline by episodic strong wind events could supply oxygen from the SCM into the bottom water and so offset some of the oxygen reduction arising from organic matter degradation. To explore this possibility, we set up a simple 1-D numerical model to simulate the seasonal cycle of stratification, phytoplankton, nutrients and oxygen in a temperate shelf sea. By adding strong wind mixing, the oxygen concentration in the bottom water becomes lower by the end of autumn than in the case with no wind events. This paradoxical result occurs because the wind mixing also brings organic matter from the SCM into the bottom water, which increases respiration and degradation. A warmer climate will lead to lower oxygen concentrations simply as a result of the reduction in oxygen solubility in seawater; our results also suggest that any climate-driven increases in wind mixing could further worsen bottom water oxygen conditions in temperate shelf seas.</p>

Ocean deoxygenation threatens ocean productivity, carbon cycling and marine ecosystems. Shelf seas are highly dynamic regions, which contributes to their high productivity and also makes monitoring and constraining their oxygen status a challenge. Here, using the temperate Celtic shelf sea (April and July 2015) as a case study, we present high‐resolution ocean glider observations of turbulence and biogeochemical parameters, demonstrating the potential of these autonomous platforms for environmental monitoring. We estimate vertical turbulent oxygen fluxes be 25% higher in summer than in spring, due to the presence of subsurface chlorophyll and associated oxygen maxima at the seasonal thermocline. We demonstrate that glider‐based estimates were able to constrain similar bottom layer respiration rates as those derived from traditional ship‐based measurements. We suggest ocean gliders are useful monitoring tools that can aid sustainable management of shelf sea ecosystems.

The offshore wind energy sector has rapidly expanded over the past two decades, providing a renewable energy solution for coastal nations. Sector development has been led in Europe, but is growing globally. Most developments to date have been in well-mixed, i.e., unstratified, shallow-waters near to shore. Sector growth is, for the first time, pushing developments to deep water, into a brand new environment: seasonally stratified shelf seas. Seasonally stratified shelf seas, where water density varies with depth, have a disproportionately key role in primary production, marine ecosystem and biogeochemical cycling. Infrastructure will directly mix stratified shelf seas. The magnitude of this mixing, additional to natural background processes, has yet to be fully quantified. If large enough it may erode shelf sea stratification. Therefore, offshore wind growth may destabilize and fundamentally change shelf sea systems. However, enhanced mixing may also positively impact some marine ecosystems. This paper sets the scene for sector development into this new environment, reviews the potential physical and environmental benefits and impacts of large scale industrialization of seasonally stratified shelf seas and identifies areas where research is required to best utilize, manage, and mitigate environmental change.

Withdrawal:“Observations of nutrient supply by mesoscale eddy stirring and diapycnal mixing in the oligotrophic North Atlantic” from the Authors: Carl P. Spingys, Richard G. Williams, Robyn E. Tuerena, Alberto Naveira Garabato, Clément Vic, Alexander Forryan, and Jonathan Sharples, published online on 17 June, 2021 on Wiley Online Library (http://wileyonlinelibrary.com), has been withdrawn by agreement between the authors and Wiley Periodicals LLC on behalf of the American Geophysical Union due to major flaws in the paper uncovered by the authors during further research.

A 4-month time series of water column temperature structure on the shelf of North Island New Zealand is used to calculate the energy in the internal tide as stratification evolved between spring and summer. Average total energy in the internal tidal wave was 200 J m(-2), with peaks reaching 600 J m(-2). Wave energy was weakly correlated with stratification (r = 0.2) and with the spring-neap cycle of tidal currents (r = 0.17). Overall there was little predictability in internal tide behaviour in response to the physical environment. Reduction in wave energy was associated with a downwelling-favourable wind event which reduced stratification by mixing and by removing deeper water off the shelf. Vertical eddy diffusivity driven by internal wave dissipation ranged between 5.6 x 10(-5) and 3.2 x 10(-4) m(2) s(-1). Combined with nitrate data this diffusivity resulted in diapycnal nitrate fluxes towards the sea surface of 1.6-2.2 mmol m(-2) day(-1). Strong stratification in summer reduced the eddy diffusivity but had little effect on nitrate flux as a strengthened vertical nitrate gradient compensated for the reduced diffusivity. This compensation will be important when predicting how stronger stratification in a warmer climate might alter diapycnal nutrient supplies to the upper ocean.

The balance of physical and biological processes governing phytoplankton growth rates and the accumulation of biomass is widely debated in the literature, notably during the winter–spring transition. Here we show, in a temperate shelf sea that variability in the depth of the actively mixing surface layer is the leading order control. During a 2‐week period preceding the peak of the spring bloom we observe two distinct regimes; first, growth within the euphotic zone during the day and re‐distribution of new biomass to the seasonal pycnocline at night by convective mixing; then, more rapid biomass accumulation trapped within a shallower, wind‐driven actively mixing layer that was decoupled from the pycnocline below. Our observations of the bloom in the Celtic Sea, Northwest European Shelf, were made using ocean gliders and include measurements of the dissipation of turbulent kinetic energy. A 1‐D phytoplankton growth model driven by our measurements of dissipation and incident irradiance replicates the observed bloom and reinforces the conclusion that physical processes that mediate light availability were key. Day‐to‐day variability in cloud cover and the ability of phytoplankton to acclimate to their light environment were also important factors in determining growth rates, and the timing of the biomass peak. Our results emphasize the need for accurate turbulent mixing parameterizations in coupled hydrodynamic‐ecosystem models. Our findings are applicable to any region where wind‐driven mixing can modify nutrient and light availability, especially across subpolar shelves in the northern hemisphere where light rather than nutrients is typically the limiting factor on phytoplankton growth.

Individual specialisations in behaviour are predicted to arise where divergence benefits fitness. Such specialisations are more likely in heterogeneous environments where there is both greater ecological opportunity and competition-driven frequency dependent selection. Such an effect could explain observed differences in rates of individual specialisation in habitat selection, as it offers individuals an opportunity to select for habitat types that maximise resource gain while minimising competition; however, this mechanism has not been tested before. Here, we use habitat selection functions to quantify individual specialisations while foraging by black-legged kittiwakes Rissa tridactyla, a marine top predator, at 15 colonies around the United Kingdom and Ireland, along a gradient of environmental heterogeneity. We find support for the hypothesis that individual specialisations in habitat selection while foraging are more prevalent in heterogeneous environments. This trend was significant across multiple dynamic habitat variables that change over short time-scales and did not arise through site fidelity, which highlights the importance of environmental processes in facilitating behavioural adaptation by predators. Individual differences may drive evolutionary processes, and therefore these results suggest that there is broad scope for the degree of environmental heterogeneity to determine current and future population, species and community dynamics.

Sustaining biological export over the open ocean requires a physical supply of nutrients to the mixed layer and thermocline. The relative importance of diapycnal mixing, diapycnal advection, and isopycnal stirring by mesoscale eddies in providing this nutrient supply is explored using a field campaign in oligotrophic waters in the subtropical North Atlantic, consisting of transects over and off the mid‐Atlantic ridge. Eddy stirring rates are estimated from the excess temperature variance dissipation relative to the turbulent kinetic energy dissipation, and using eddy statistics from satellite observations combined with 9‐month‐long mooring data. The vertical nutrient fluxes by diapycnal mixing, diapycnal advection, and isopycnal mesoscale eddy stirring are assessed using nitrate measurements from observations or a climatology. Diapycnal mixing and advection provide a nutrient supply within the euphotic zone, but a loss of nutrients within the upper thermocline. Eddy stirring augments, and is comparable to, the diapycnal transfer of nutrients within the summertime upper thermocline, while also acting to replenish nutrients within the deeper parts of the thermocline. The eddy supply of nitrate is relatively small in the center of the subtropical gyre, reaching up to 0.06 mol N m −2 yr −1 , but is likely to be enhanced on the flanks of the gyre due to larger isopycnal slopes and lateral nitrate gradients. The nutrient supply to the euphotic zone is achieved via a multistage mechanism: a diapycnal transfer of nutrients by small‐scale turbulence to the euphotic zone, and an isopycnal stirring of nutrients by mesoscale eddies replenishing nutrients in the upper thermocline.

The role of the internal tide in driving tracer transport across the continental slope is examined using simplified layered theory, channel model experiments and observational diagnostics of near shelf‐edge moorings. The effect of the internal tide is interpreted in terms of its Stokes' drift, which is separated into two distinct components: a bolus component, driven by the co‐variance of layer thickness and the velocity; and a shear component, driven by the velocity following the movement of an interface. For a three layer ocean, in the model experiments and observations, the onshore propagation of an internal tide drives a Stokes' transport directed onshore in the surface and the bottom layers, and directed offshore in the pycnocline. This reversing structure is due to the bolus component dominating near the boundaries, while the shear component dominates at the pycnocline. In the observational diagnostics, the Stokes' transport is not cancelled by the Eulerian transport, which is mainly directed along bathymetric contours. The Stokes' drift of the internal tide then provides a systematic on shelf tracer transport if there is a tracer sink on the shelf, carried in the surface or bottom layers. Conversely, the tracer transport is directed offshore if there is a tracer source on the shelf with plumes of shelf tracer expected to be carried offshore along the pycnocline. This tracer transport as a result of the internal tide is diagnosed for heat, salt and nitrate. The depth‐integrated nitrate flux is directed onto the shelf supplying nutrients to the productive shelf seas.

Long-term observations (March’14 − July’15) of ocean density and velocity from the North West European shelf reveal a seasonality in internal wave energy linked to the seasonal cycle of stratification. Further, this seasonality extends to internal mixing associated with internal waves that can be effectively described by the buoyancy frequency (N2), with the strongest mixing associated with strongly stratified summer conditions. To better understand these results a model was used that employed three different, commonly used parameterisations of internal mixing. Each parameterisation produced some degree of seasonality in internal mixing. Contrary to observed results however, all three model scenarios produced a minimum in internal mixing during summer, with enhanced mixing observed during spring and autumn. This failure in each model was attributed to the lack of realistic levels of enhanced baroclinic energy and shear (S2) that is identified in observations to be attributable to internal waves. These observations reveal a close relationship between N2and S2, resulting in a near continuous state of marginal stability; where the gradient Richardson number is maintained at a near critical level. Due to the observed strong dependence of internal wave energy and internal mixing on stratification, a modified version of the MacKinnon and Gregg (2003a) turbulence scaling was employed. This modified parameterisation successfully replicated the observed seasonality in internal mixing. This important result implies that future parameterisations should aim to scale internal mixing on enhanced levels of S2 from internal waves, which are shown here to be suitably predicted by the seasonal cycle of stratification (N2).

This work examines the seasonal cycle of vertical density structure and its influence on primary production in a temperate shelf sea, with a particular focus on the breakdown of stratification in autumn. We do this by combining new, high resolution observations of water column structure, meteorological forcing, nitrate and chlorophyll fluorescence collected between March 2014 and July 2015 on the North West European Shelf. Our results challenge the generally accepted assumption that convection dominates over wind driven mixing resulting in seasonal breakdown of stratification. Furthermore we found, that vertical mixing in autumn not only transformed the vertical density structure but also the vertical structure of chlorophyll biomass and surface nutrients. The subsurface chlorophyll maximum was eroded and a vertically homogeneous profile of chlorophyll biomass established itself above the pycnocline. This increased mixing also led to replenishment of surface nitrate concentrations, which supported an autumn phytoplankton bloom. While the significance of phytoplankton blooms in autumn has previously not been well quantified, we argue that these can act as a significant contributor to the seasonal drawdown of carbon.

Diapycnal mixing of nutrients from the thermocline to the surface sunlit ocean is thought to be relatively weak in the world's subtropical gyres as energy inputs from winds are generally low. The interaction of internal tides with rough topography enhances diapycnal mixing, yet the role of tidally induced diapycnal mixing in sustaining nutrient supply to the surface subtropical ocean remains relatively unexplored. During a field campaign in the North Atlantic subtropical gyre, we tested whether tidal interactions with topography enhance diapycnal nitrate fluxes in the upper ocean. We measured an order of magnitude increase in diapycnal nitrate fluxes to the deep chlorophyll maximum (DCM) over the Mid‐Atlantic Ridge compared to the adjacent deep ocean. Internal tides drive this enhancement, with diapycnal nitrate supply to the DCM increasing by a factor of 8 between neap and spring tides. Using a global tidal dissipation database, we find that this spring‐neap enhancement in diapycnal nitrate fluxes is widespread over ridges and seamounts. Mid‐ocean ridges therefore play an important role in sustaining the nutrient supply to the DCM, and these findings may have important implications in a warming global ocean.

The onset and breakdown of stratification are key physical drivers of phytoplankton growth in shelf seas and the open ocean. We show how in the Celtic Sea, where seasonality in stratification is generally viewed as controlled by heat input, a cross‐shelf salinity gradient horizontally strained by the wind prolonged the stratified period by 5–6 days in autumn prior to full winter mixing, while in spring caused seasonal stratification to begin 7 days early. Salinity straining has important implications for setting light conditions during the start of the spring bloom and for the timing of bottom‐water ventilation in winter. Analysis of winds around the time of likely onset of spring stratification between 1979 and 2016 showed that in 60% of the years' wind conditions were favorable for salinity straining. Accurate knowledge of the horizontal salinity field and wind stress are required to correctly determine the onset and breakdown of stratification.