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Climate Change Impacts on Dissolved Oxygen Concentration in Marine and

Coastal Waters around the UK and Ireland

Chapter · February 2023

DOI: 10.14465/2023.reu07.oxy

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 Dissolved Oxygen

Climate Change Impacts on Dissolved

Oxygen Concentration in Marine and
Coastal Waters around the UK and
Ireland
Mahaffey, C.1, Hull, T.2, Hunter, W.3, Greenwood, N.2,4, Palmer, M.5
Sharples, J.1, Wakelin, S.6 and Williams, C.6
1. Department of Earth, Ocean and Ecological Sciences, School of Environmental Sciences, 4
Brownlow Street, University of Liverpool, L69 3GP, UK
2. Centre for Environment, Fisheries and Aquaculture, Pakefield Road, Lowestoft, NR33
0HT, U K
3. Agri-Food and Biosciences Institute, 18a Newforge Lane, Belfast, BT9 5BX
4. Centre for Ocean and Atmospheric Sciences, School of Environmental Sciences, University
of East Anglia, Norwich, NR4 7TJ, UK
5. Plymouth Marine Laboratory, Prospect Place, Plymouth, Devon PL1 3DH
6. National Oceanography Centre, Joseph Proudman Building, 6 Brownlow Street, Liverpool,
L3 5DA, UK

KEY FACTS
What is already happening

• Since the 1960s, the global oceanic oxygen content has declined by
more than 2%.
• Sustained observations in the North Sea reveal the recent onset of
oxygen deficiency in late summer, partly due to ocean warming. The
intensity and extent of oxygen deficiency has also increased over
time.
• Short-term measurements for the Celtic Sea also indicate the onset of
oxygen deficiency in late summer.

Citation: Mahaffey, C., Hull, T.,

Hunter, W., Greenwood, N., What could happen in the future
Palmer, M., Sharples, J.,
• Average Annual mean dissolved oxygen concentration in the global
Wakelin, S. and Williams, C.
Climate change impacts on ocean is projected to decline by 1.5 to 4% by 2090 for all RCPs. This
dissolved oxygen concentration decline will be most acute below the thermocline.
in marine and coastal waters • For UK shelf waters, models project that annual mean oxygen
around the UK and Ireland. concentration will decline most strongly in North Sea regions and
MCCIP Science Review 2023,
31pp.
the Celtic Sea (5.6 to 5.9% by 2100, RCP 8.5). Deeper regions
exposed to exchange with the open ocean (the Irish shelf and
doi: 10.14465/2023.reu07.oxy
Shetland shelf) are expected to be less affected, decreasing by 2.9 to
3.1%.
Submitted: 10 2022

Published online: 11 2022

 Dissolved Oxygen

• The predicted increase in temperature over this century for UK shelf

seas will lead to a decrease in dissolved oxygen through the whole
water column because of reduced solubility.
• The risk of oxygen deficiency in summer will increase because of
lower oxygen levels experienced during the preceding winter and
spring.
• Increased rainfall and runoff would increase the risk of
eutrophication and cause oxygen concentrations to locally decrease.
• Continued warming and reduced oxygen availability will affect the
metabolism, health, and reproduction of many marine organisms,
which could have major impacts on ecosystems and commercial
fisheries.

SUPPORTING EVIDENCE

Introduction
Oxygen is the most important gas in the marine environment because it is
essential for breathing or ‘respiration’ by almost all life in the sea. Dissolved
oxygen concentrations in the global ocean have declined by 2% since the
1960s (Rhein et al., 2013; Schmidtko et al., 2017; Oschlies et al., 2018).
While excessive nutrient loading or ‘eutrophication’ may be the driver for
the decline in dissolved oxygen concentrations in near coastal regions (Diaz
and Rosenberg, 2008), the observed global decline in dissolved oxygen has
been attributed to ocean warming (Schmidtko et al., 2017). Climate models
predict future rates of oxygen decline will increase globally in response to
warming (Bopp et al., 2013; Keeling et al., 2010; van der Molen et al.,
2013). Depletion of dissolved oxygen can lead to a region being defined as
either oxygen deficient, when dissolved oxygen concentrations are less than
6 mg/litre (equivalent to ~192 mol/kg), hypoxic when oxygen
concentrations are less than 2 mg/litre (equivalent to ~64 –mol/kg) or
anoxic when oxygen concentrations are less than 0.5 mg/litre (equivalent to
~30 mol/kg). The development of oxygen deficiency can have deleterious
effects on the marine ecosystem (Vaquer-Sunyer and Duarte, 2008;
Breitburg et al., 2018) with the onset of hypoxia or anoxia increasing the
severity on the ecosystem response. While the decline in dissolved oxygen
in the deep ocean is a global problem, the intensity and impact of dissolved
oxygen depletion or ‘deoxygenation’ is also apparent in the coastal and shelf
seas (Diaz and Rossenberg, 2008; Breitburg et al., 2009; Gilbert et al.,
2010; Townhill et al., 2017a; Breitburg et al., 2018). The challenge is to
better understand the multiple processes that currently control dissolved
oxygen to accurately assess the future risk of oxygen deficiency, hypoxia or
anoxia in response to climate change (Oschlies et al., 2017). Here, we focus
 Dissolved Oxygen

on the processes controlling oxygen concentrations in the North-west

European shelf seas, which encompasses UK coastal and shelf seas.

What controls oxygen in the marine environment?

Oxygen constitutes 20.95% of our atmosphere and its behaviour is strongly
linked to the processes that cycle carbon in the atmosphere and ocean. Over
the past 20 years, there has been a small but measurable decrease in the
levels of oxygen in the atmosphere due to the burning of fossil fuels
(Keeling and Shertz, 1992; Manning and Keeling, 2006). Loss of
atmospheric oxygen poses no risk to ecosystems or humans for the
foreseeable future. Dissolved oxygen concentrations in the ocean are
controlled by a combination of physical and biological processes (Figure 1a
and b). Oxygen in the atmosphere readily exchanges with the surface ocean,
termed air-sea gas exchange (Figure 1a). Temperature affects the solubility
of dissolved oxygen in seawater, with oxygen being more soluble in colder
water and less soluble in warmer water (Figure 1a). For example, a 1°C
increase in ocean temperature will cause dissolved oxygen concentrations to
decrease by approximately 5 μmol/kg or 0.16 mg/litre over typical ranges of
salinity (30 to 35) and temperature (10 to 20°C) observed in UK marine
waters. Oxygen saturation is a common parameter used to describe the ratio
of dissolved oxygen in seawater relative to the maximum amount of oxygen
that will dissolve in seawater at a given temperature, salinity, and pressure if
it were in equilibrium with the atmosphere. Biological activity produces
dissolved oxygen via photosynthesis and tends to cause oxygen saturation to
be greater than 100%, termed oversaturation (Figure 1b). Dissolved oxygen
is removed from water via breathing or respiration by marine plants,
animals and microorganisms, and by the decay of organic matter back to
nutrients; the combination of removal processes is referred to collectively as
biological oxygen consumption in the present document. These removal
processes tend to cause oxygen saturation to be less than 100%, termed
undersaturation (Figure 1b). Note that in the sunlit surface layer of the
ocean, biological oxygen production is typically greater than oxygen
consumption resulting in net biological oxygen production, increasing the
oxygen concentration above the saturated state. Conversely, in the deeper,
dark layers of the ocean, oxygen consumption is typically greater than
oxygen production leading to net biological oxygen consumption,
decreasing the oxygen concentration below the saturated state (Figure 1b).

There is an important link between nutrients and oxygen in the marine

environment. An increase in nutrients will generally increase production of
organic matter via photosynthesis, causing an increase in dissolved oxygen
production in the surface ocean. However, decay of accumulated organic
matter by microbial respiration leads to a decrease in dissolved oxygen.
Enhanced nutrient inputs into coastal regions have been shown to cause
 Dissolved Oxygen

eutrophication (Painting et al., 2013), one important side effect being a

decline in dissolved oxygen. Hence, dissolved oxygen is used as an
indicator of the status of ecosystem health within the legislation designed to
protect the UK marine environment through the Water Environment
Regulations 2017 and the Marine Strategy Regulations 2010 and the
Programme of Measures 2015, which are the UK implementation of the EU
Water Framework Directive (WFD; Best et al., 2007) and the Marine
Strategy Framework Directive (MSFD, Ferreira et al., 2011) respectively.
However, in assessments made under this legislation, dissolved oxygen
concentrations are used only to identify undesirable disturbance from the
indirect effect of nutrient enrichment (OSPAR Common Procedure; Foden
et al., 2010) as an indicator within the eutrophication quality descriptor
(MSFD Descriptor 5) and an indirect effect of nutrient enrichment (MSFD
Criterion 5.3; Ferreira et al., 2011). There is no additional requirement to
monitor dissolved oxygen concentration under the legislation despite factors
other than eutrophication being responsible for development of oxygen
deficiency in coastal and shelf sea waters. Although dissolved oxygen was
proposed as an indicator for Seafloor Integrity within the MSFD (Rice et al.,
2012), it was not included in the final selection of indicators. As a critical
component of several indicators of good environmental status (GES), it is
important to develop a thorough understanding of the controls and
consequences of oxygen dynamics in UK coastal and shelf water and the
potential response of oxygen dynamics to climate change.
 Dissolved Oxygen

Warmer Temperature Cooler Higher oxygen Dissolved oxygen Lower oxygen

Processes Effect on dissolved Processes Effect on dissolved

in (a) oxygen in (b) oxygen
1 Equilibrates oxygen 1 Net oxygen production
between the as photosynthesis is
atmosphere and ocean greater than respiration
in surface waters
2 Oxygen is less soluble 2 Net oxygen
in warm water and consumption during
more soluble in cold degradation of organic
water matter in the water
column
3 Mixes oxygenated 3 Oxygen consumption
surface water with as more organic matter
less oxygenated is made available from
bottom waters sediment resuspension
4 Transports water with 4 Net oxygen
different oxygen consumption during
properties degradation of organic
matter in the sediments
Figure 1: Schematic representation of the vertical structure of (a) temperature and the physical
processes that control the distribution of dissolved oxygen and (b) dissolved oxygen and the
biological processes that control the distribution of dissolved oxygen in coastal and shelf seas. The
underlying tables highlight the principal processes and their effect on dissolved oxygen dynamics.
Surface and bottom waters are separated by a thermocline and thus this schematic represents
processes in a stratified water column.
 Dissolved Oxygen

Physical processes, including vertical mixing and horizontal transport,

redistribute oxygen vertically and horizontally (Figure 1a). The strength of
vertical mixing and the degree of density stratification are key factors
controlling the oxygen distribution in coastal and shelf seas (Figure 2a).
Stratification occurs when water becomes separated into layers of different
density with less-dense water overlaying a layer of denser water. Layers
close to the surface may be less dense due to the addition of lower salinity
water, for example from freshwater run-off from rivers, or from warming,
which typically occurs due to the seasonal solar heating cycle. While
salinity-driven stratification is generally restricted to near coastal regions
around the UK, there are large areas of UK shelf seas that undergo seasonal
thermal stratification when near surface waters and bottom waters are
separated by a temperature gradient, or thermocline (Figure 2b, Sharples et
al., 2020). Such regions include, but are not limited to, the central and
northern North Sea, the Celtic Sea, the western Irish Sea, the Malin Sea and
Outer Hebrides region (Sharples et al., 2020). During winter, the shelf seas
are mixed by convection (driven by surface water cooling) and wind (Figure
2b) and become well oxygenated as waters mix and are ventilated by air-sea
gas and heat exchange (Figure 2c). Winter mixing acts to homogenise
surface and bottom water and their associated oxygen concentrations
(Figure 2c). Critically, the water temperature in winter places a first-order
control on the dissolved oxygen concentration in coastal and shelf seas
because of the temperature dependence of oxygen solubility, with warmer
winters leading to lower dissolved oxygen concentrations. Seasonal
stratification occurs over much of the UK shelf seas when there is sufficient
solar heating of surface waters to overcome the combined effects of tidal
and wind mixing. This coincides with spring, typically in March or April in
UK waters (Figure 2d and 2e) and persists until winter storms and reduced
solar heating in autumn months return the water column to winter mixed
conditions, typically between October and December, although there is
significant spatial variability in both the onset and breakdown of
stratification. During this period of seasonal stratification, a thermocline
separates the well-mixed near-surface waters and bottom boundary layers.
This thermocline significantly reduces mixing between the two layers and
results in different oxygen dynamics in each layer (Figure 2c, 2d and 2e). In
the surface waters, net biological oxygen production and air-sea gas
exchange act to maintain dissolved oxygen concentrations at close to or
above 100% saturation (Figure 2c). In contrast, the thermocline restricts
mixing of bottom waters with surface waters and prevents direct air-sea gas
exchange. There is also net biological oxygen consumption in the bottom
waters (Figure 2c, 2d and 2e). If oxygen consumed in the bottom waters is
not replenished by episodic mixing events across the thermocline, such as
by enhanced mixing from strong tides and storms or by horizontal exchange
with oxygenated waters (Hull et al., 2020), then the bottom layer oxygen
 Dissolved Oxygen

concentration may decline to levels at risk of oxygen deficiency, hypoxia or

anoxia. However, the strong seasonal cycle in the UK temperate coastal and
shelf seas means that no regions experience year-round stratification and so
any depletion of oxygen during the summer stratified period is temporary,
as dissolved oxygen will be replenished during autumn and winter mixing
(Figure 2d and 2e). This seasonal pattern is in contrast to many oxygen
minimum zones (OMZs) found elsewhere in the marine environment where
oxygen deficiency, hypoxia or anoxia can be persistent features over many
years (e.g. the Baltic Sea).

In regions of UK coastal and shelf sea waters where there is sufficient

energy from tides and currents to permanently mix the water column (left
region of Figure 2a), the dissolved oxygen concentration in the entire water
column is relatively homogenous, with the absolute concentration changing
primarily due to seasonal changes in temperature and therefore solubility.
Regions that are classed as permanently mixed include the shallow regions
of the central and eastern English Channel, southern North Sea and central
Irish Sea. There are also areas that undergo periodic stratification due to the
variable nature of the tide, for instance in the eastern Irish Sea influenced by
river inputs from North-west England and the far south of the North Sea
influenced by the River Rhine. Bottom layers in such regions are typically
not at risk of oxygen depletion since stratification is short-lived.

(a)
Dissolved Oxygen
 Dissolved Oxygen

Figure 2: Schematic representation of: (a) the gradient in physical water column structure and
dissolved oxygen concentrations from the shallow, permanently mixed regions to deeper, seasonally
stratifying regions. Exchange between the two regions is indicated by horizontal transfer (yellow
arrows). Seasonal change in (b) water column temperature (°C) and (c) dissolved oxygen saturation
(%) in the seasonally stratifying central Celtic Sea at the end of winter (March, red), spring (April,
green), summer (June, blue) and autumn (November, purple, data from Hull et al., 2020). (d)
seasonal change in dissolved oxygen concentrations (mg/litre) at North Dogger in the North Sea from
the Cefas SmartBuoy in surface waters (green) and bottom waters (black) (data from Greenwood et
al., 2010). (e) Seasonal change in dissolved oxygen concentrations (mg/litre) in the Celtic Sea from
the Celtic Deep SmartBuoy (red), Celtic Deep benthic lander (blue) and Haig Fras lander (dark
blue), with discrete bottle oxygen concentrations indicated (yellow). (Data is available from
https://doi.org/10.14466/CefasDataHub.38 to https://doi.org/10.14466/CefasDataHub.41. Note that
the lowest oxygen concentrations in (a) are associated with intermediate rather than the deepest
water column in shelf seas due to the rapid depletion of oxygen in a thinner layer which has less total
oxygen available.)

Climate change is likely to affect physical and biological processes that lead
to oxygen depletion. As well as reducing the solubility of oxygen, ocean
warming is predicted to increase the strength and duration of seasonal
stratification over the next 50 years (Lowe et al., 2009; Holt et al., 2010;
Sharples et al., 2020). Enhanced stratification increases the isolation of
bottom waters from the sea surface thus increasing the risk of oxygen
deficiency and hypoxia in seasonally stratifying regions on the European
Shelf. Enhanced stratification will also reduce the nutrient supply to the
surface ocean, thus reducing biological productivity and the amount of
organic material available for microbial respiration. In contrast, ocean
warming will increase metabolic processes including biological oxygen
consumption (Brewer and Peltzer, 2017). The net effect of a decline in
nutrients stunting biological productivity, alongside the direct thermal
enhancement of metabolic processes, is currently unknown. Particularly
 Dissolved Oxygen

relevant to coastal and shelf seas systems is the change in nutrient inputs
from land, which may enhance or reduce the risk of eutrophication with
subsequent implications for oxygen dynamics. Changes to shelf sea and
deep ocean exchange may also have major impacts on both physical and
biogeochemical controls (Holt et al, 2018; Wakelin et al, 2020). Thus,
understanding potential causes of oxygen depletion, and the responses of
these drivers to climate change, land-use and wastewater inputs is key to
predicting the likelihood of oxygen deficiency or hypoxia, and subsequent
ecosystem harm, in UK marine waters.

What is already happening?

Detecting climate-driven changes in dissolved oxygen requires observations

and understanding of the natural seasonal and interannual variability in
surface and bottom water oxygen concentrations so that any long-term
deviation from baseline conditions can be accurately assessed. Dissolved
oxygen measurements in bottom waters from 1920s to 2017 in the North-
west European shelf region are available from the International Council for
the Exploration of the Sea (ICES) and the British Oceanographic Data
Centre (BODC) databases. Synthesis of these databases reveals that the
North Sea is the most intensely studied of UK marine waters for dissolved
oxygen, in both space (Figure 3a) and time (Figures 3g and 3h). This level
of coverage has not been replicated in other UK shelf sea regions, with a
stark disparity between North Sea data coverage and the Celtic Sea (Figure
3b), Malin Shelf (Figure 3c), Outer Hebrides (Figure 3d), Irish Sea (Figure
3e) and English Channel (Figure 3f). Confidence in any results drawn from
these poorly resolved regions is therefore inherently low and makes
quantitative comparisons between regions, or between observations and
models, extremely difficult.

Regional dynamics in dissolved oxygen concentrations

In the North Sea, oxygen concentrations typically vary from ~320 mol/kg
(equivalent to ~ 10 mg/litre or over 100% saturation) in winter to ~192
mol/kg in summer (equivalent to 6 mg/litre or 70% saturation). A decline
in oxygen saturation to 70% or less in bottom waters of the seasonally
stratified regions in the UK waters of the North Sea during late summer
conditions is well documented (Figure 2d, Weston et al., 2008; Greenwood
et al., 2010; Queste et al., 2013; Große et al., 2016; Queste et al., 2016;
Topcu and Brockmann, 2015). High temporal resolution data from the Cefas
SmartBuoy network has revealed the onset of oxygen deficiency (less than 6
mg/L) in bottom waters during the late summer period in the central and
northern North Sea around the Oyster Grounds (5.2 mg/litre, 60% saturation
or 167mol/kg; Greenwood et al., 2010). Re-analysis of dissolved oxygen
 Dissolved Oxygen

data over the past 100 years reveals that there has been an increase in the
intensity and spatial extent of oxygen deficiency in the North Sea (Queste et
al., 2013). This historical period of oxygen depletion coincided with a
period of ocean warming observed over the past two decades. Queste et al.,
(2013) postulate that ocean warming alone explains one third of the change
in oxygen, with the remaining two thirds of oxygen depletion being
attributed to an increase in biological oxygen consumption.
Dissolved Oxygen
 Dissolved Oxygen

Figure 3: Synthesis of data from ICES and BODC indicating (a) the spatial distribution of
measurements of dissolved oxygen below the thermocline or near the seabed from 1920s to 2017, with
red lines indicating approximate boundaries between regions, and the temporal distribution of
observations per year in the bottom waters from 1920s to 2017 for (b) Celtic Sea, (c) Malin Shelf, (d)
Outer Hebrides, (e) Irish Sea, (f) English Channel, (g) Northern North Sea and (h) Southern North
Sea. Note that the y-axis scale varies from (b) to (h) with (b) and (c) ranging from 0 to 800, (d), (e)
and (f) ranging from 0 to 500 and (g) and (h) ranging from 0 to 1600.

Unlike the well-studied North Sea, the Celtic Sea is poorly sampled in both
time and space (Figure 3a and 3d). Data from the recent UK-Shelf Sea
Biogeochemistry programme (2011-2017) has significantly improved our
understanding of this region. High-resolution data from two benthic landers
(Celtic Deep and Haig Fras) equipped with oxygen sensors collected
between spring 2014 and autumn 2015 reveals that towards the end of
seasonal stratification in late autumn 2014, oxygen concentrations in bottom
waters at Haig Fras (Figure 2e, dark blue) decreased below the 6 mg/litre
threshold defining oxygen deficiency (5.8 mg/litre or 62% saturation or 186
mol/kg). Note that oxygen concentrations at the Celtic Deep site remained
above 6 mg/litre (Figure 2e, blue), indicating the spatial heterogeneity in
oxygen dynamics. These are the first observations indicating the
development of oxygen deficiency in the Celtic Sea, albeit with a 10% error
of the oxygen sensor data due to lack of geo-located discrete samples for
calibration. The lack of observational data in Celtic Sea bottom waters
means this important result cannot be set in a historical context and so it is
not possible to verify using observational evidence whether this is a recent
development or a regularly occurring phenomenon.

There is currently no evidence of oxygen deficiency in the Irish Sea or

Malin Sea (O’Boyle and Nolan, 2010), but data is sparse (Figure 3c and e,
respectively) and large areas known to undergo seasonal stratification have
few or no observations of bottom-layer dissolved oxygen concentration in
available databases, and fewer still have data available during late summer
or autumn conditions when a seasonal oxygen minimum is most likely to
occur.

Looking to the future, a major challenge in detecting the onset of oxygen

deficiency in UK coastal and shelf sea regions is making measurements of
dissolved oxygen concentrations at the appropriate scales in time and space.
Benthic landers (Hull et al., 2020) and autonomous underwater vehicles
(Williams et al., 2022, Hull et al., 2021) can provide high-resolution data on
dissolved oxygen in shelf sea bottom waters. Coupled physical and
biogeochemical models specifically designed to represent the functioning of
coastal and shelf seas have been used to predict oxygen dynamics over
regional and whole shelf scales (Madec et al., 2012; Butenschön et al.,
2016). Model re-analysis by Ciavatta et al. (2016) has suggested that large
 Dissolved Oxygen

areas (~325,000 km2) of the North-west European Shelf region are

vulnerable to oxygen deficiency. UK regions designated as at risk of
deficiency in this study include large areas of the Celtic Sea, Irish Sea and
English Channel and small coastal regions around Scotland (Figure 4). A
small area of the North Sea in UK waters is also identified, which forms part
of a much larger area including Dutch and German waters. There is still
some debate on the ability of ocean and climate models to accurately
capture the myriad of processes that control seasonal oxygen dynamics. At
the global scale, climate models successfully estimate the change in oxygen
concentrations within 10% of observed values (IPCC, 2013; Oschlies et al.,
2018), but tend to underestimate the variability and decline in oxygen (Bopp
et al., 201; Ito et al., 2017). Assessment of model skill of a coupled
hydrodynamics-ecosystem model (NEMO-ERSEM) designed to represent
shelf sea processes showed that simulations underestimated oxygen by 0.6
mg/litre on average compared to observations (Wakelin et al., 2020).

Figure 4: Model output from ERSEM indicating areas vulnerable to oxygen deficiency in bottom
waters, defined as at least one daily value in 1998 to 2009 below the threshold of 6 mg/litre. Areas of
the shelf where oxygen concentrations are found to be higher than 6 mg/litre at 100% confidence are
highlighted in blue. Areas of the shelf where oxygen concentrations are found to be lower than 6
mg/litre at 1% confidence are highlighted in yellow. Areas of the shelf where oxygen concentrations
are found to be lower than 6 mg/litre at 100% confidence are highlighted in red. (Reproduced with
permission from Ciavatta et al., 2016.)

Causes of oxygen depletion

While the causes for the decline in dissolved oxygen concentrations have
been identified, their relative magnitude and net effect on oxygen dynamics
are still poorly constrained. For example, the net effect of temperature on
oxygen is complex; while warming will unambiguously reduce the
 Dissolved Oxygen

solubility of oxygen, the effects of warming on the strength and duration of

stratification and on biological oxygen consumption are less well known.
Understanding potential drivers of oxygen depletion is key to predicting the
likelihood of oxygen deficiency or hypoxia in UK marine waters. In near
coastal waters, nutrient enrichment can lead to eutrophication, causing
acceleration of phytoplankton growth with undesirable disturbance and
potentially harmful effects, including the growth of nuisance or toxic
phytoplankton, red tides and dissolved oxygen depletion (Painting et al.,
2013). Under EU directives, 21 UK coastal water bodies were assessed as
‘Problem Areas’ with respect to eutrophication status based on nutrient
concentrations (OSPAR 2017; Defra, 2010). In coastal systems, the
relatively slow process of dissolved oxygen depletion is frequently
interrupted by ventilation of water via physical mixing driven by wind,
waves or tides, allowing replenishment of dissolved oxygen by rapid mixing
with surface oxygenated waters and equilibration with the atmosphere.
However, the nutrient-enhanced organic matter present in these coastal
waters does have the potential to be transported away from coastal areas and
contribute to biological oxygen consumption elsewhere (Topcu and
Brockmann, 2015; Große et al., 2017) and so still may be a problem.

UK marine waters (defined here as areas where local salinity is greater than
30) are considered to be ‘Non-Problem Areas’ with respect to the risk and
impact of nutrient enrichment (OSPAR, 2017). However, eutrophication is
not the only driver or precursor of dissolved oxygen depletion. The timing,
duration and strength of stratification play critical roles in the seasonal
depletion of dissolved oxygen because they dictate the degree of isolation of
bottom waters and the potential for mixing. However, there is significant
regional disparity between waters that are stratified for long periods of time
and the magnitude of oxygen depletion they experience. In the North Sea,
areas of prolonged stratification in the central and northern regions
generally have a higher bottom-water dissolved oxygen concentration than
areas that are stratified for a shorter period of time in the south-central North
Sea region (Große et al., 2016; Queste et al., 2016; Topcu and Brockmann,
2015). This disparity indicates that factors other than the strength and
duration of stratification play an important role in controlling oxygen
depletion. The magnitude of photosynthesis in the sunlit surface layer
dictates the amount of organic matter that will eventually sink below the
thermocline, with more organic matter leading to greater biological oxygen
consumption in deeper, dark waters. In addition, organic matter generated
via photosynthesis is a food source for higher trophic levels, such as
zooplankton, which graze on phytoplankton and generate sinking faecal
material, which contributes to an enhanced downward flux of organic matter
which could intensify biological oxygen consumption. The volume of the
bottom layer, represented by both the thickness and areal extent of bottom
 Dissolved Oxygen

waters, is also a factor controlling the magnitude of oxygen depletion

(Große et al., 2016), with dissolved oxygen being more rapidly depleted by
both water column and sedimentary processes in areas with a thinner bottom
layer than a thicker bottom layer due to the lower total amount of oxygen
per unit volume available in a thinner layer. As such, differences in
productivity are thought to control the interannual variability in dissolved
oxygen conditions in the North Sea, while spatial differences in dissolved
oxygen dynamics have been attributed to variations in stratification and
water depth or volume alongside tidal energy for mixing (Große et al.,
2016).

While stratification is an important prerequisite for bottom water oxygen

depletion, other physical processes can contribute to dissolved oxygen
dynamics (Queste et al, 2016; Rovelli et al., 2016). Horizontal and vertical
advection may transport water into different depths or regions and can lead
to the exchange of water with different oxygen properties. In the central
North Sea, estimates of horizontal transport or advection are low (Weston et
al., 2004; Greenwood et al., 2010) and typically water masses are thought to
be transported into areas with similar properties and so have little net effect.
In the Celtic Sea, horizontal advection, tidal forcing and vertical mixing
control oxygen dynamics in bottom waters at various times over the
stratified period (Hull et al 2020). Vertical mixing across the thermocline
has the potential to mix well-oxygenated surface waters with oxygen-
deplete bottom waters (Rovelli et al., 2016; Queste et al., 2016; Williams et
al., 2022). The rate of mixing across the thermocline is highly variable,
depending on tides, meteorology and the proximity to banks and slopes, and
the contribution from each factor is modified by the strength of local
stratification. The combined effect of these processes results in thermocline
mixing in shelf seas spanning several orders of magnitude (e.g. Sharples et
al., 2009; Rippeth et al., 2014). Extended deployments of autonomous
underwater vehicles (AUVs) or ‘gliders’ have recently provided new insight
into the importance of mixing of dissolved oxygen across the thermocline in
the North Sea (Queste et al., 2016) and the Celtic Sea (Williams et al.,
2022). Thermocline mixing is weak when compared to tidal and wind-
driven mixing (Williams et al., 2013; Williams et al., 2022). However, these
physical processes can drive a steady, albeit small, vertical flux of oxygen
into the bottom mixed layer (Williams et al., 2022) or drive cyclic spring-
neap changes in oxygen consumption driven by sediment resuspension or
ventilation of the seabed (Hull et al 2020). Collectively, physical processes
add to the complexity of understanding dissolved oxygen dynamics in shelf
seas because they can both enhance oxygen depletion through stratification
or act to reduce the potential for oxygen depletion via mixing between
layers of the water column. Thus, understanding the role of physical
 Dissolved Oxygen

processes now and in the future ocean is vital towards understanding climate
change impacts on coastal and shelf sea oxygen dynamics (see Section 2.2).

Below the thermocline, biological processes continuously consume oxygen

both in the water column and in the sediment (Figure 1b; Große et al., 2016;
Queste et al., 2016; Hicks et al., 2017; Hull et al., 2020). Oxygen
consumption in sediments has been found to be dependent on sediment type
and season. The highest rates of oxygen consumption occur in cohesive
sediments (such as mud) rather than permeable sediments (such as sand and
gravel) (Hicks et al., 2017). Increased sedimentary oxygen consumption has
been observed during the spring bloom period when more organic matter is
immediately available (Hicks et al., 2017). Physical processes continue to
play a role as organic matter can aggregate to create ‘depocentres’ or hot
spots of benthic oxygen consumption. In addition, organic matter that is on
top of or within surficial sediments can potentially be disturbed by natural
mixing (e.g. tides and storms) or by human activities (e.g. trawling), thus
making benthic organic matter available for remineralisation in the water
column via resuspension, potentially contributing to event-driven oxygen
decline (van der Molen et al., 2013; Hull et al., 2020). In the Celtic Sea, the
spring-neap tidal cycle caused a cyclic change in oxygen consumption due
to sediment resuspension enhancing oxygen consumption alongside
ventilation introducing oxygen at a muddy-sandy site (Hull et al., 2020).
Estimates of the role of sediments in driving oxygen consumption on bottom
waters span from less than 20% (Rovelli et al., 2016; Hull et al., 2020) to
over 50% (Große et al., 2016). The range likely reflects region specific
productivity, remineralisation rates, sediment types or choice of parameters
in models.

What could happen in the future?

Results from a regional shelf seas model, the Proudman Oceanographic

Laboratory Coastal Ocean Modelling System (POLCOMS), predict an
average rise in temperature over the century (in 2069-2089 relative to 1960-
1989) of over 3°C for most of the North Sea, English Channel, Irish and
Celtic Seas using a medium emissions scenario (Tinker et al., 2016; Hughes
et al., 2017). This predicted increase in temperature will lead to a decrease
in dissolved oxygen in the whole water column due to a reduction in
solubility (Table 1), a decrease in dissolved oxygen in bottom waters due to
an increase in the strength and duration of stratification (Table 1: Conley et
al., 2007; Keeling et al., 2010; Rabalais et al., 2010; Hofmann et al., 2011;
Queste et al., 2013) and impose a greater risk of oxygen deficiency in
summer due to lower oxygen in the preceding winter/spring due to warmer
waters. Further model projections for the period 2070 to 2098 relative to
1961 to 1990 predict the period of stratification will increase by 10 to 15
 Dissolved Oxygen

days over the entire North-west European shelf region (Lowe et al., 2009;
Holt et al., 2010; Sharples et al., 2020).

Table 1. Potential implications of climate change related processes on oxygen dynamics in the
coastal and shelf sea environment, including an indication of the timescale over which the process
will act and the level of confidence.

Process Direction Timescale Confidence

Decrease in Decrease in oxygen Decadal High
solubility due to concentration in surface and
ocean warming bottom waters
Increase in Decrease in oxygen Decadal Medium
stratification concentration in bottom waters
due to ocean due to reduced mixing
warming
Decrease in Decrease in phytoplankton Seasonal Low
nutrient supply growth and amount of organic
to the surface matter that reaches bottom
mixed layer due waters, decrease in oxygen
to increased consumption in bottom waters
stratification causing a relative increase in
oxygen concentration
Increase in Net effect unknown due to Decadal Low
biological increase in both oxygen
processes due to production via photosynthesis
ocean warming and oxygen consumption via
respiration and other processes.
Nutrient availability not
considered
Increase in Localised change in water Seasonal Low
frequency and column stratification and
intensity of increase in oxygen
storms concentration due to water
column mixing
Resuspension of Increase in organic matter Seasonal Low
sediments due available for oxygen
to storms consumption causing a decrease
in oxygen concentration
Increased Increase in nutrients will increase Annual Low
precipitation the risk of eutrophication and
and river runoff associated decrease in oxygen
concentration

There are additional consequences of climate change that may indirectly

impact on the magnitude and even the direction of oxygen dynamics in
coastal and shelf seas. For example, while enhanced stratification may
reduce mixing between bottom waters and the sea surface, it may also
 Dissolved Oxygen

reduce the supply of nutrients to the surface ocean due to reduced mixing
across the thermocline. Any reduction in nutrient supply will decrease
phytoplankton growth and the amount of organic matter that reaches bottom
waters, which will likely decrease the demand for oxygen in the bottom
waters. The net effect of these two competing mechanisms is uncertain
(Table 1). In contrast, biological processes are thermally sensitive and will
likely increase in response to an increase in temperature but their net impact
on oxygen dynamics is unknown (Table 1). Increased storm activity driven
by a warming atmosphere will enhance ocean mixing. While this will
further be modified by a change in stratification from a warmer atmosphere,
a likely scenario is an increase in surface layer depth, an increase in energy
available to mix bottom waters and a reduction in areal extent of seasonal
stratification, and thus a reduction in the risk or extent of oxygen depletion
(Table 1). Other contributing processes from this enhanced mixing however
may include increased resuspension of sediments, which may increase
biological oxygen consumption in bottom waters or reduced light and
therefore reduced primary production in upper layers (Capuzzo et al., 2017).
Finally, winter precipitation and river flows are expected to increase across
northern Europe (EEA, 2015), potentially increasing the input of nutrients to
coastal systems and thus enhancing the risk of eutrophication and associated
depletion of oxygen (Table 1, Rabalais et al., 2010; Zhang et al., 2010;
Ockenden et al., 2017). Alternatively, a reduction in nutrient input via rivers
through improved water and land management has the potential to reduce
eutrophication and thus reduce the risk of oxygen deficiency (Lenhart et al.,
2010; Bussi et al., 2017).

The interaction between these complex processes and their combined effect
is difficult to predict, however a suite of global models of varying degrees
of complexity agree that dissolved oxygen concentrations in the global
ocean will decline by 1.5 to 4% by 2090 or by 6 to 12 mol/kg by 2100
(Ciais et al., 2013). Model simulations specifically focused on coastal and
shelf sea waters estimate that dissolved oxygen concentrations in the North
Sea will decline by 5.3 to 9.5 % by 2098 (van der Molen et al., 2013) or as
much as 11.5% by 2100 (Meire et al., 2013). Using a coupled
hydrodynamics-ecosystem model (NEMO-ERSEM), Wakelin et al (2020)
investigated the potential changes in and drivers of oxygen depletion across
the North-west European continental shelf seas under a ‘business-as-usual’
greenhouse gas emissions scenario (RCP 8.5). Model simulations estimate
that as the shelf sea waters become warmer and less saline under RCP8.5,
the mean near bed oxygen concentration will decrease by 6.3% by 2100,
with monthly minimum oxygen decreasing by 7.7%. In regions that
currently experience oxygen depletion, such as the central North Sea and
Celtic Sea, the model predicts that oxygen depletion will intensify and last
longer (see Figure 9 in Wakelin et al., 2020). For UK shelf waters (Figure
 Dissolved Oxygen

5), the mean oxygen concentration is projected to decline most strongly (5.6
to 5.9% by 2100) in North Sea regions and the Celtic Sea while the
reduction in the minimum of monthly-mean concentrations is highest (8.8 to
8.9%) in the English Channel, southern North Sea and Irish Sea. Deeper
regions exposed to exchange with the open ocean (the Irish shelf and
Shetland shelf) are less affected, with annual mean concentrations
decreasing by 2.9 to 3.1% and minimum monthly concentrations by ~4%.
However, in the western Irish Sea, warming of the sea surface may alter the
structure of the gyre system, strengthening stratification and thus increasing
the risk of oxygen depletion in bottom waters (Olbert et al., 2012). These
model outputs imply that the decline in dissolved oxygen in coastal and
shelf seas resulting from climate change would be amplified compared to
the effects in the open ocean.

In the open ocean, 15 to 32% of the decline in oxygen is attributed to a

decrease in solubility in a warming ocean, with the remaining 58 to 85% of
the decline attributed to reduced ventilation due to increased stratification
(Helm et al., 2011; Meire et al., 2013) or biological oxygen consumption
(Brewer and Peltzer 2017). Observations from the North Sea suggest that
one third of the historical oxygen decline is due to warming, whereas two
thirds is attributed to increased oxygen consumption (Queste et al., 2013).
More recently, model simulations focused on the North-west European
Shelf Sea suggest that 73% of the projected decline in oxygen up to 2100 at
RCP8.5 is due to solubility changes with the remainder attributed to changes
in the ecosystem processes (Wakelin et al., 2020). Thus, it appears that the
contribution of reduced solubility in reducing oxygen concentrations will
become more important as the ocean warms.
 Dissolved Oxygen

(a)

(b)

Figure 5: (a) NEMO-ERSEM projections of near

bed oxygen concentrations under a high greenhouse
gas emissions scenario (RCP8.5), averaged over (b)
the UK shelf regions. The black lines are annual
means, the grey lines are the minimum monthly-
mean concentrations per year and the dashed lines
indicate the 6 mg/litre oxygen deficiency threshold.
(Adapted from Wakelin et al., 2020.)

Consequences of oxygen depletion

Oxygen is required to sustain vital metabolic processes of marine organisms

and is essential for activities such as muscular activity, growth and
reproduction (Pörtner and Knust, 2007). Thus, depletion of dissolved
oxygen poses a serious threat to marine organisms, with the most-severe
responses occurring under hypoxic or anoxic conditions. Most marine
 Dissolved Oxygen

organisms have limits or thresholds to the severity of oxygen depletion they

can tolerate (Vaquer-Sunyer and Duarte, 2008; Pörtner, 2010). Even a small
decrease in oxygen below a threshold can affect oxygen-demanding
functions, such as movement and reproduction (Claireaux et al., 2000;
Claireaux and Chabot, 2016). Other key factors include exposure time,
species, type of organism, respiration mode and physiological requirements.
For example, highly active species are generally less tolerant of low oxygen
conditions (Stramma et al., 2011). Oxygen depletion can cause a reduction
in survival, growth and reproduction, alter behaviour of individual
organisms (Baden et al., 1990; Eriksson and Baden 1997; Chabot and
Claireaux, 2008; Long et al., 2008; Ludsin et al., 2009), affect predator-prey
relationships and in the most severe case, cause death (Shurmann and
Steffensen, 1992; Stramma et al., 2010; Urbina et al. 2011; Townhill et al.,
2017b). Even brief repeated exposure to oxygen depletion can alter the
immune system of macrofauna and thus increase disease and reduce growth
(Stierhoff et al., 2009; Keppel et al., 2015). Oxygen depletion can be
devastating for commercial fisheries. For example, during a period of
oxygen deficiency (oxygen concentration of 3.7 mg/litre, equivalent to a
saturation lower than 40% or 116 mol/kg) in 1982 in German and Danish
coastal waters of the North Sea, fish abundance decreased from ~400 kg per
30-minute trawl to less than 5 kg per 30 minute trawl (Westernhagen and
Dethlefsen, 1983). While the focus here is on dissolved oxygen, ocean
warming will also increase metabolic rates of organisms and thus it is the
response to multiple stressors, not just a decline in oxygen, that needs to be
understood. To delineate the role of temperature versus oxygen availability
on metabolism, Deutsch et al. (2015) defined a metabolic index, which
compares the ratio of the oxygen supply to an organism’s resting oxygen
demand. Using this physiological framework, projected ocean warming
alongside deoxygenation would reduce the metabolic index as the increase
in metabolism increases oxygen demand relative to oxygen supply. This
implies that future marine hypoxia will be driven primarily by warming and
not a decline in oxygen (Deutsch et al., 2015).

In addition to the ecological risks, oxygen depletion affects ecosystem

structure (Levin et al., 2009; Hughes et al., 2009) and the activity of the
constituent organisms (Woulds et al., 2007; Hunter et al., 2012). This has
consequences for the transfer and storage of organic matter to the sediments
(Keil et al., 2016; Cavan et al., 2017), biological removal of nitrate
(Neubacher et al., 2011; Neubacher et al., 2013; Kitidis et al., 2017),
production of greenhouse gases such as nitrous oxide (Naqvi et al. 2010;
Freing et al., 2012; Bianchi et al., 2012), and the release of phosphorus and
iron from sediments (Scholz et al., 2014; Watson et al., 2018). These
biogeochemical responses to oxygen depletion have the potential to affect
primary production locally in coastal and shelf seas and lead to feedback
 Dissolved Oxygen

loops, which may have both positive and negative effects on ecosystem
functioning (Niemeyer et al., 2017).

Although hypoxia has not been detected in UK marine waters, it has been
detected in the wider North-west European shelf seas including the North
Sea (Topcu and Brockmann, 2015) and Baltic Sea (Meier et al., 2019) and
thus will affect species that contribute to the ecosystem and perhaps
economy of the UK due to connectivity of the marine environment and
transfer of migratory species between regions. Periods of oxygen deficiency
have been detected in the UK waters of the North Sea and now the Celtic
Sea. While the impact of deoxygenation has been documented in non-UK
waters of the North Sea (Westernhagen and Dethlefsen, 1983) and globally
(Rose et al., 2019) the impacts on the marine ecosystem, specifically
commercial fisheries has not yet been documented in UK marine waters.
 Dissolved Oxygen

CONFIDENCE ASSESSMENT
What is already happening?

Level of
agreement or H
consensus

(inc. dataset M X
agreement
and model
confidence)
L

L M H
Amount of evidence (theory /observations /models)
L

What could happen in the future?

Level of
agreement or H
consensus

(inc. dataset M x
agreement
and model
confidence)
L

L M H
Amount of evidence (theory /observations /models)
L

On a global scale, there is a high level of confidence that the oceans are
losing oxygen due to ocean warming. In UK coastal waters, there is a high
level of understanding of the seasonal and interannual variability in oxygen
dynamics in the permanently mixed and seasonally stratifying waters in the
North Sea due to the relatively extensive sampling regime for dissolved
oxygen concentrations in this region over the past four to five decades.
Repeat sampling at specific sites has provided insight into the occurrence
and onset of oxygen deficiency but the spatial extent of oxygen deficiency
outside of these specific regions within the North Sea is uncertain due to the
paucity of direct observations. Approximately one third of the historical
seasonally focused depletion of oxygen in the North Sea has been attributed
to warming but the remaining two thirds are thought to be due to enhanced
oxygen consumption. The relative importance of the processes that drive
enhanced oxygen consumption (e.g. more organic matter, decreased
ventilation of bottom waters), however, remain poorly understood. Finally,
whereas the North Sea is well sampled in time and space, the rest of the
 Dissolved Oxygen

Northwest European Shelf waters, especially the Celtic and Irish seas, are
relatively poorly sampled and offer low levels of confidence in the
occurrence or risk of oxygen depletion. Nevertheless, both observations and
models agree that the UK coastal and shelf seas are losing oxygen and thus
there is a medium level of confidence on the direction of change.
At the global scale, there is a high level of confidence that an increase in
temperature will continue to reduce the solubility of oxygen and enhance
stratification and thus lead to the ongoing decline in dissolved oxygen
concentrations, especially below the thermocline. On a regional scale
appropriate for coastal and shelf seas, there is a consensus that the ocean
will lose oxygen. Model simulations can provide estimates of the magnitude
and causes of the decline in dissolved oxygen but there is still uncertainty in
how well they represent the coupling between physical and biogeochemical
processes, biological processes specifically, strong seasonality in nutrient
supply in a shallow water column and interaction with the sediment.
Therefore, there is a medium level of confidence on the future of dissolved
oxygen dynamics on a regional scale relevant to UK marine waters.

KEY CHALLENGES AND EMERGING ISSUES

1. We need to be able to determine the mechanisms driving spatial and

temporal trends in dissolved oxygen and confidently identify when
and where changes in dissolved oxygen are being driven by human-
induced activity such as ocean warming or nutrient enrichment
relative to background natural variability.

2. Assessing the occurrence, frequency and spatial extent of oxygen

deficiency in UK coastal and shelf waters is hampered by the lack of
long-term data in regions outside of the North Sea. The poor
resolution of dissolved oxygen data also hampers the ability to
confidently test coastal and shelf-sea models. An integrated
observing system providing high resolution, continuous time-series
data using new technologies such as autonomous ocean gliders or
instrumented moorings would provide the means to improve
detection of oxygen depletion in the future. Recent and current
programmes such as the NERC-Defra Shelf Sea Biogeochemistry
programme (Kröger et al., 2018) NERC-Defra WWF AlterEco
project (https://altereco.ac.uk) and EU H2020 AtlantOS programme
(https://atlantos-h2020.eu) are providing emerging insight into best
practices on how to operate autonomous ocean gliders to study
dissolved oxygen dynamics in UK marine waters.
 Dissolved Oxygen

3. There is still uncertainty surrounding the ability of models to

simulate the individual processes and coupling between processes
that control dissolved oxygen dynamics. To accurately predict
dissolved oxygen, models need to simulate each contributing process
correctly, in isolation but also coupled to other processes. This is an
enormous challenge for ocean models since it is not possible to
include all physical, chemical and biological processes in any model.
Instead, complex processes must be parameterised to produce net
effects that are close to that observed, but that may have differing
levels of success dependent on local conditions. We do not yet fully
understand all processes contributing to the decline in oxygen in the
marine environment and thus representing these processes in models
is challenging. The lack of understanding is particularly acute within
coastal and shelf sea sediments. The lack of long-term time series
data for testing coupled physical-ecosystem models, or the
variability in functioning between sites with different conditions is
also problematic.
 Dissolved Oxygen

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