BGBiogeosciencesBGBiogeosciences1726-4189Copernicus PublicationsGöttingen, Germany10.5194/bg-13-1209-2016Drivers of summer oxygen depletion in the central North SeaQuesteBastien Y.b.queste@uea.ac.ukhttps://orcid.org/0000-0002-3786-2275FernandLiamJickellsTimothy D.HeywoodKaren J.https://orcid.org/0000-0001-9859-0026HindAndrew J.Centre for Ocean and Atmospheric Sciences (COAS),
School of Environmental Sciences, University of East Anglia, NR4 7TJ, Norwich, UKCentre for Environment, Fisheries & Aquaculture Science (CEFAS), Pakefield Road, NR33 0HT, Lowestoft, UKBastien Y. Queste (b.queste@uea.ac.uk)29February20161341209122221April201512June20153February20169February2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://bg.copernicus.org/articles/13/1209/2016/bg-13-1209-2016.htmlThe full text article is available as a PDF file from https://bg.copernicus.org/articles/13/1209/2016/bg-13-1209-2016.pdf
In stratified shelf seas, oxygen depletion beneath the thermocline is a
result of a greater rate of biological oxygen demand than the rate of supply
of oxygenated water. Suitably equipped gliders are uniquely placed to observe
both the supply through the thermocline and the consumption of oxygen in the
bottom layers. A Seaglider was deployed in the shallow (≈ 100 m)
stratified North Sea in a region of known low oxygen during August 2011 to
investigate the processes regulating supply and consumption of dissolved
oxygen below the pycnocline. The first deployment of such a device in this
area, it provided extremely high-resolution observations, 316 profiles (every
16 min, vertical resolution of 1 m) of conductivity, temperature, and depth
(CTD), dissolved oxygen concentrations, backscatter, and fluorescence during
a 3-day deployment.
The high temporal resolution observations revealed occasional small-scale
events (< 200 m or 6 h) that supply oxygenated water to the bottom
layer at a rate of 2 ± 1 µmol dm-3 day-1. Benthic
and pelagic oxygen sinks, quantified through glider observations and past
studies, indicate more gradual background consumption rates of
2.5 ± 1 µmol dm-3 day-1. This budget revealed
that the balance of oxygen supply and demand is in agreement with previous
studies of the North Sea. However, the glider data show a net oxygen
consumption rate of 2.8 ± 0.3 µmol dm-3 day-1,
indicating a localized or short-lived (< 200 m or 6 h) increase in
oxygen consumption rates. This high rate of oxygen consumption is indicative
of an unidentified oxygen sink. We propose that this elevated oxygen
consumption is linked to localized depocentres and rapid remineralization of
resuspended organic matter.
The glider proved to be an excellent tool for monitoring shelf sea processes
despite challenges to glider flight posed by high tidal velocities, shallow
bathymetry, and very strong density gradients. The direct observation of
these processes allows more up to date rates to be used in the development of
ecosystem models.
Introduction
Mooring observations from 2007 and 2008 , historical
data and a hydrographic survey of the North Sea in August 2010
revealed repeated incidents of seasonal oxygen depletion
in offshore waters of the central North Sea. In August 2011, a Seaglider was
deployed in the region with the lowest recorded bottom mixed layer (BML)
oxygen saturation from the 2010 survey to further investigate the mechanisms
causing this seasonal oxygen depletion.
Bathymetry of the study area in metres . Depth
contours have been added for the 20, 40, 80, and 160 m isobaths. The region
where the glider was deployed is indicated by the large white dot at
approximately 57∘ N, 2∘30′ E. Major landmarks are indicated
as follows: A and B – Shetland and Orkney isles; C – Humber estuary; D – the Channel; E – Dogger Bank; F – Norwegian Trench; G – German Bight; H – Skagerrak. The general
circulation of the North Sea (adapted from and
) is overlaid as arrows for FIC (Fair Isle Current), DC (Dooley Current), SCC (Scottish Coastal Current), CNSC (Central North Sea
Current), and SNSC (Southern North Sea Current). Hatch marks cover areas not
subject to thermal stratification in summer.
BML oxygen saturation (%) and with the 70 % saturation contour
during the August 2010 (a) and August 2011 surveys (b),
mean summer bottom oxygen saturation values (%) from 1900 to 2010 from the
ICES database (c), and mean summer bottom oxygen saturation values
(%) from 1958 to 2008 from the General Estuarine Transport Model–European
Regional Seas Ecosystem Model (GETM-ERSEM) output using European Centre for
Medium-Range Weather Forecasts (ECMWF) European Reanalysis (ERA)-interim data
(d).
The North Sea is a relatively shallow (15–200 m) shelf sea situated between
the islands of Great Britain, Orkney, and Shetland and northwestern
continental Europe. It gradually deepens from south to north with the
exception of the shallow Dogger Bank (Fig. ). Dogger Bank
effectively separates the North Sea into two regions of different physical,
chemical, and biological properties . Water
properties in the northern half of the North Sea are largely dominated by
North Atlantic inflow. For their study of carbon dynamics,
determine the waters of the North Sea to be 90 % sourced from the North
Atlantic (9000 km3 yr-1 via the Pentland Firth and Fair Isle,
42 000 km3 yr-1 via the Shetlands), 8 % from the English
Channel (4900 km3 yr-1), and 2 % from a combination of Baltic
(500 km3 yr-1) and riverine flow (200 km3 yr-1).
Water entering the North Sea from the northern boundary generally circulates
anticlockwise, following the Scottish coast southward before turning east and
crossing the North Sea north of Dogger Bank
. The strength of this anticlockwise
circulation is strongly correlated with the North Atlantic Oscillation Index
(NAOI); positive NAOI is associated with strong anticlockwise circulation,
while negative NAOI is associated with greatly reduced anticlockwise
circulation in the northern North Sea
. The northern half of the North
Sea is seasonally stratified through surface heating. This stratification
breaks down completely in winter
. Despite representing only
2 % of the inflow, Baltic waters can play a disproportionate role in
defining the biochemical properties of the central North Sea as under
specific conditions of circulation or surface wind stress, the fresh water
run-off issuing from the Baltic, Kattegat, and Skagerrak can be advected into
the central region . This not only affects stratification by
increasing the salinity difference between surface mixed layer (SML) and BML
but also provides supplementary nutrients to the SML.
The August 2010 North Sea survey recorded bottom waters
with oxygen saturations below 70 % in two regions; north of Dogger Bank (ND)
and the Oyster Grounds (OG) (Fig. ) exhibited dissolved
oxygen saturations of circa 65 % (200 µmol dm-3) and 68 % (180 µmol dm-3) respectively. Historical data originating
from the International Council for the Exploration of the Sea (ICES) database revealed a similar distribution of
low dissolved oxygen (DO) in the BML during summer in the ND and OG regions.
The historical data also showed a sharp decline in summer oxygen saturations
at these two sites since 1990. suggest that the same
mechanisms likely lead to the depletion of oxygen at both the OG and ND
sites. These mechanisms are thermal stratification preventing vertical
resupply of oxygen; reduced advection slow replenishment of local BML oxygen
and continuous remineralization of organic matter in the BML leads to gradual
decline in oxygen concentrations. At both sites, it is thought that the
replenishment of oxygenated waters through advective processes is limited by
local topography. ND is situated away from the fronts associated with the
north side of the shallow Dogger Bank; it is characterized by variable, weak
wind-driven currents . This slow circulation limits the
horizontal supply of DO while weak tidal currents also promote settling of
organic matter. Weak winds and strong surface heat fluxes promote
stratification and keep the BML isolated from air–sea exchange. There is also a weak inflow of oxygenated waters as tides and topography lead to the
formation of bottom fronts to the south and east of the OG site
. Organic matter produced at the
pycnocline along the deep chlorophyll maximum (DCM), particularly at ND, is
exported directly into the BML .
This organic matter, when remineralized, leads to the consumption of DO in
the BML. At both the OG and ND sites, it has been suggested that mixing
events may cause resuspension of bottom sediment. Transfer of this
organically rich sediment into an oxic water column could cause rapid
increases in pelagic DO consumption .
The intensity and duration of oxygen depletion depend on the relative
magnitudes of oxygen consumption and oxygen supply. These processes can
be both spatially and temporally variable. This study took place in 2011,
when we observed a different spatial pattern than in 2010. The oxygen-depleted area experienced an overall northward shift but with similar
intensities of oxygen depletion (Fig. ). To accurately
predict potential seasonal oxygen consumption under future climate scenarios,
it is necessary to identify the relative magnitude of sources of organic
matter that lead to the consumption of DO in the BML and the amount of DO
supply through the pycnocline on a finer scale. There is growing interest in
the issue of low oxygen in coastal waters around the world , and further research is required to effectively manage the impact of hypoxia
in coastal waters. This study aims to improve our understanding of the key
short-term mixing processes that regulate oxygen supply at high temporal
frequency by quantifying through Seaglider observations the major inputs and
sinks of dissolved oxygen.
Seaglider section of temperature (a; ∘C), salinity
(b; PSU), potential density (c; kg m-3),
chlorophyll a concentration (d; mg m-3), optical scattering
as a volume scattering function at 650 nm
(e; × 10-4β(θc) m-1 sr-1),
apparent oxygen utilization (f; µmol dm-3) and oxygen
concentration (g; µmol dm-3) sampled along the
transect. Bathymetry as detected by the onboard altimeter is indicated along
the bottom. Daytime chlorophyll a values showing signs of quenching are
blanked. The vertical black line indicates the transition to a different
water mass; data after this vertical black line are excluded from the oxygen
calculations. The solid black contour indicates the 7 ∘C isotherm.
The dotted black contour indicates the mixed layer depth (σ> 0.01 kg m-3).
Seaglider observations
A Seaglider from the University of East Anglia, SG510, was deployed at
56∘41.96′ N, 2∘26.37′ E on the 19 August
2011 (Fig. 1) at a water depth of ≈ 75 m. Seagliders are
buoyancy-driven autonomous underwater vehicles that move through the water in
a sawtooth pattern between the surface and the seabed (or 1000 m in deeper
water; ). Typically, the glider recorded data between 2 m
from the surface and 7 m off the seabed; sample points beyond this range
were discarded because of poor flow conditions due to the glider turning
around. Seagliders sample every 5 s using an on-board sensor suite
composed of a Seabird unpumped conductivity and temperature sensor, a Wetlabs
Triplet EcoPuck, and an Aanderaa 4330F optode. SG510 sampled along a 32 km
northward transect during a 3-day period (158 dives, 316 vertical profiles)
providing a full depth cast every 7.8 ± 0.8 min.
Mean profiles as observed by the Seaglider of temperature
(a; ∘C), salinity (b; PSU), potential density (c; kg m-3),
chlorophyll a concentration (d; mg m-3), optical scattering as a
volume scattering function at 650 nm (e; × 10-4β(θc) m-1 sr-1), and oxygen saturation
(f; %) as height above the seabed. Daytime profiles were omitted for
chlorophyll a.
Temperature and salinity were calibrated against an in situ ship-based
profile at launch; dissolved oxygen and chlorophyll a were calculated using
recent manufacturer calibrations. Chlorophyll a concentrations obtained
from the glider were validated against data obtained by an ESM2 logger
deployed during the glider launch; the ESM2 logger calibration is regularly
compared to in situ high-performance liquid chromatography (HPLC) and fluorometry samples. Data were corrected for
sensor lag and time offsets; further details are provided by
. Due to the sharp temperature gradient, significant
hysteresis remained in temperature-dependent variables (salinity, density,
oxygen) relating to the thermal inertia of the different sensors. As the
region contained two fairly uniform mixed layers, composite profiles were
created using upcasts between the seabed and the thermocline and downcasts
between the surface and the thermocline. This eliminated the bias due to the
thermal inertia of the conductivity cell and oxygen optode. The composite
profiles were then gridded over time and pressure by taking the mean value in
each 45 min by 1 dbar grid square to provide regularly spaced data.
Chlorophyll a measurements were impacted by daytime quenching. Data
prone to quenching were removed from the analysis and show up as blank in
Fig. . Daytime profiles were omitted when calculating
the mean chlorophyll a profile in Fig. .
Three-day composite of MODIS Aqua daytime sea surface
temperature at 11 µm (∘C, a and b) and
chlorophyll a concentration (mg m-3, (c) and d)
across the entire North Sea (left) and around the Seaglider deployment area
(right) from the 20 to 22 August 2011. The colour scale for the North Sea map
of chlorophyll a is logarithmic to highlight chlorophyll a distribution
in the central North Sea where concentrations are low. Northward Seaglider
track is indicated by the black line.
Figures and present the data
collected by SG510 during its 3-day mission. The glider is able to
resolve significant and systematic changes in all the measured parameters on
this timescale. The glider observed strong stratification (> 1.5 kgm-3) throughout the
3 days (Figs. and ), separating two well-mixed layers: the SML and the
BML. The SML showed warming throughout the survey with a temperature
difference of 1 ∘C between beginning and end and a decrease in
surface salinity (0.35) as the glider entered a region of fresher surface
water on the third day; this fresher surface water likely originated from Baltic outflow (Fig. ). The BML exhibits a small and opposite pattern, with
temperatures decreasing by less than 0.1 ∘C on the third day and
salinity increasing by 0.15. As the glider crossed a surface feature and
observed a significant change in temperature and salinity at the end of the
survey, data from the last 15 h of the survey are excluded when
calculating rates of oxygen depletion.
A conceptual representation of the processes affecting oxygen
supply and consumption to the bottom mixed layer during the glider survey.
The water column is separated into two layers: the SML and BML (red and blue).
The observed mean oxygen saturation profile (from Fig. )
is overlaid on the water column to illustrate the position of the oxycline
and deep chlorophyll maximum (indicated by the mid-water peak). ASE: air–sea
exchanges; VMP: vertical mixing processes; Remin.: remineralization of
organic matter.
The strong pycnocline maintains a subsurface phytoplankton community, as
evidenced by the DCM (Figs. d and d).
Variations in chlorophyll a concentration (as estimated by fluorescence
intensity) occur throughout the survey; peaks in DCM fluorescence coincide
with decreases at the surface. The surface variation of chlorophyll a
fluorescence is likely caused by quenching rather than by a loss of biomass or pigment as it occurs daily at noon. Fluctuations in chlorophyll a
concentration are visible throughout the BML but do not coincide with the
diurnal signal described above. These BML fluctuations likely relate to
sinking organic matter and are also observed by the optical backscatter
sensor; pulses in optical backscatter at 650 nm can be seen in the BML
repeatedly throughout the 3-day survey (Fig. d and e).
Apparent oxygen utilization (AOU; defined as the difference between the
solubility of dissolved oxygen and observed oxygen concentration; Fig. f) shows a pattern typical of oxygen change within the
central stratified North Sea . The SML is homogenously
saturated. At the pycnocline, supersaturation (negative AOU) is visible and
correlated with elevated chlorophyll a concentration (Fig. f); this supersaturation is evidence of strong primary
production occurring at the DCM . The BML exhibits
increasing AOU throughout the survey; this oxygen depletion is caused by the
remineralization of organic matter within the BML and isolation from air–sea
exchange by the strong density gradient.
Dissolved oxygen budget
To constrain the balance of DO consumption and supply, we quantify the
various sources and sinks of DO and compare these with the observed change in
DO over the survey period. review the processes that affect
DO. These are represented conceptually in Fig. . Advective
and dispersive transport (both horizontal and vertical), air–sea exchange,
and biological production and consumption are the three dominant processes.
Seaglider apparent oxygen utilization (µmol dm-3) along
isobars at 1 dbar intervals from 45 to 71 dbar, coloured by pressure.
Black vertical lines indicate the maximum difference in temperature at each
time step as shown in Fig. . Binned data from the BML
(45 dbar and below) are plotted against time and coloured by pressure to
highlight any vertical gradients. The dotted vertical lines indicate when
vertical mixing events were evident. The vertical black line indicates the
transition to a different water mass.
Seaglider temperature (∘C) along isobars at 1 dbar intervals
from 45 to 71 dbar, coloured by pressure. Vertical black stems indicate
the difference between minimum and maximum values at each time step. This illustrates changes in vertical gradients; a small value indicates a
homogenous BML, while a large value indicates a strong vertical gradient in
the BML. We can clearly see events where the temperature gradient increases
in the BML, showing injection of warmer surface water across the
thermocline.
In the context of a strongly stratified two-layer system, air–sea
exchange does not affect the BML and can be neglected. In this region and on
short timescales (3 days), horizontal advective and diffusive processes can
be neglected for the BML due to the homogeneity of the BML across the central
North Sea . Therefore, for the BML in this region, the
vertical cross-thermocline transport and biological processes dominate
.
As evidenced by the chlorophyll a and supersaturation signatures (Fig. ), the majority of the production occurs within the DCM
and SML. Consequently, the dominant biological processes within the BML are
(i) benthic oxygen demand and (ii) the remineralization of organic matter
which may derive from (a) sinking from the DCM and (b) organic matter
still in suspension issuing from the spring bloom. Net oxygen change is
therefore determined by the balance of oxygen supply from cross-thermocline
mixing and remineralization of benthic organic matter and matter sinking from
the DCM.
Observed change in dissolved oxygen in the BML
Figure illustrates AOU at different depths throughout the
BML. AOU is shown at depths beginning several metres below the pycnocline to
account for tidal vertical displacement. As we are not measuring mean BML
temperature value, vertical displacement of the thermocline does not impact
assessment of vertical temperature gradients. As we are looking at a gradient
rather than absolute values, displacement of the BML simply moves this
gradient up and down but does not affect the slope or extent. There is a
clear trend of uniformly increasing AOU over time, indicating oxygen
depletion, throughout the entire BML. A linear regression of mean AOU
throughout the BML indicates a rate of 2.8 µmol dm-3 day-1 of
DO consumption with a standard error of the regression of 0.3. Three sharp
decreases in AOU are indicated by the vertical dotted line. These relate to
vertical mixing events identified in Fig. and will be
discussed in the following section.
An oxygen consumption rate of 2.8 ± 0.3 µmol dm-3 day-1 is
very large; to reach the observed AOU of 76 µmol dm-3 from
saturation would require no more than 1 month. Continuous consumption at
this rate over the entire summer would very rapidly lead to severe hypoxia.
However, in this region, stratification begins near the end of April
, more than 3 months before the survey occurred.
observed a lower rate of DO consumption, with an
average of 0.4 µmol dm-3 day-1 across the entire summer
season at ND. It is therefore likely that strong reoxygenation events
occurred or the mean rate of oxygen consumption was lower during the first
half of the stratified season and that the glider observed a temporal local
maximum in consumption rates. Despite this, these observations are
representative of specific conditions which occur in the central North Sea
and which are likely a recurring maximum of depletion rates within the summer season.
It therefore follows that consumption of organic matter in the sediment
and pelagically (from the DCM and spring bloom) surpasses the supply of
oxygenated water from cross-thermocline exchange by 2.8 ± 0.3 µmol dm-3 day-1.
The following sections aim to quantify both supply of oxygen and
remineralization rate terms using previous studies and the glider
observations in order to close the budget.
Supply of oxygenated water
To observe the supply of DO to the BML through time, ideally the glider
would act as a Lagrangian platform moving with the water. This requires no
horizontal displacement of the glider relative to the water. Based on the
August 2010 hydrographic survey of , it was expected that
horizontal gradients would be minimal in this region and therefore that any
horizontal displacement would have a negligible impact on observed water mass
properties. However, August 2011 was different to August 2010 in terms of
surface circulation and hydrographic characteristics. Sea surface temperature
and chlorophyll a concentration across the North Sea during the deployment
derived from MODerate Resolution Imaging Spectrometer (MODIS) Aqua data (Fig. ) reveal significant
horizontal variability near the Seaglider transect in surface waters. This
surface variability is clearly shown in Fig. b where the
Seaglider observed a freshening of 0.3 of the SML at the end of the
survey. This change was not reflected in the BML, where the glider observed a
weak but opposing trend. In the BML, the Seaglider observed a 0.1 ∘C
cooling and 0.03 salinification (together equivalent to a densification of
0.035 kgm-3), likely due to increasing Atlantic water influence.
Although both 2010 and 2011 show an overall similar spatial pattern with
depleted oxygen in the northern half; areas of lowest oxygen saturation are
located in different regions (Fig. ). Given the
potentially very high consumption rates, these patterns of oxygen saturation
are mainly a snapshot set of observations of late summer and may not be
representative of the entire season or the previous month. This illustrates
the interannual variability and importance of horizontal transport processes
on broad scales in the North Sea, and also potentially in other shelf seas,
linked to other dominating climate modes. Horizontal transport processes
likely affect seasonal oxygen depletion by influencing BML and SML
temperatures, thereby affecting stratification while also providing nutrients
to sustain productivity and potentially supplying labile organic matter to
the BML.
On timescales similar to those in this survey, it is likely that advective processes
play a limited role in regulating seasonal oxygen depletion; instead,
vertical mixing is assumed to be the largest potential source of oxygen input
into the BML. Seaglider observations reveal a highly stratified water column
with a strong pycnocline spanning approximately 8 m (∼ 30–38 m) with a
density difference of 1.5–2 kgm-3 (Fig. c). Evidence of mixing can be found in changing
properties of the BML as observed by the glider. Here, temperature is used as
a tracer to assess mixing across the pycnocline on short timescales through
the large temperature difference between the SML and BML (Fig. ). Temperature data from below the thermocline are used to
derive the vertical temperature gradient in the BML (Fig. ,
black vertical lines). A small value indicates a homogenous BML while a large
value indicates a strong vertical gradient in the BML. Figure
shows changes in vertical temperature gradients on the order of a few hours
(or hundreds of metres) horizontally. The BML temperature gradient increases
simultaneously to mean BML temperature. This indicates warming of BML water
by mixing across the thermocline and injection of warm SML water into the
cooler BML.
Referring back to Fig. , we observe similar fluctuations
in AOU. The three principal mixing events, illustrated by the grey vertical
lines, correspond to sudden decreases in AOU, indicating sudden and rapid
reoxygenation of the BML. After these reoxygenation events, net oxygen
consumption rates increase within a few hours. The three events over the
3 days show increases in BML DO (decreases in AOU) of approximately 1 to
3 µmol dm-3 per event (2 ± 1 µmol dm-3 day-1).
Figure shows wind velocity and tidal currents along
with bathymetry at the glider's position. There is no obvious correlation
between the mixing events identified in Fig. and
currents. The majority of mixing events tends to occur during periods of
peak wind velocities, but the duration of mixing events does not seem to
match the timescale of wind velocity changes; furthermore, two mixing events
occur in periods of low wind velocities. Mixing events do not seem to
correlate with bathymetry in the region either (Fig. ).
These mixing events are short-term (a few hours or a couple of kilometres)
processes that the Seaglider is able to reveal through its very high-resolution observations. Since wind events do not seem to be responsible,
these events may be linked to a variety of physical processes occurring in
the region. highlighted the importance of internal waves
as a potential source of mixing around the Dogger Bank. Another possibility
is the generation of shear spikes as described by due to
the interaction of wind and tides, explaining the lack of correlation with
either one in particular.
Regardless of the cause, we observe short-lived or very localized
occasional oxygen resupply events leading to an approximate oxygen supply of
2 ± 1 µmol dm-3 day-1. Relating this back to the overall
oxygen budget, we observed a net oxygen consumption rate of
2.8 ± 0.3 µmol dm-3 day-1 accompanied by a gross supply of
2 ± 1 µmol dm-3 day-1 through vertical cross-thermocline
mixing processes.
Tidal currents (m s-1, a), 10 m wind speed with
scale arrows representing 5 m s-1(b), and depth (m,
c) at the Seaglider's location during the survey. The dotted
vertical lines indicate when vertical mixing events were evident. The
vertical black line indicates the transition to a different water mass. Tidal
data were obtained from the Tidal Model Driver (TMD) tide toolbox and OTIS
(OSU (Oregon State University) Tidal Inversion Software) European Shelf Model
. Wind data was obtained from ECMWF ERA-Interim reanalysis
data. Bathymetry was gathered by the Seaglider's on-board altimeter. The
comparison was made by extracting data from the closest point in time and
space to the glider's location from the ECMWF 6-hourly high-resolution data.
Linear interpolation was used in time between the ECMWF data.
Oxygen consumptionBenthic respiration
We now assess the relative magnitude of the different oxygen sinks within the
BML. As the North Sea is a shallow region, vertical transport of organic
matter from the SML and BML to the benthic layer is fairly rapid. On the whole
North Sea scale, report that 17 to 45 % of primary
production is remineralized in the sediments, although the bulk of this
occurs in the Skagerrak and Norwegian Trench area (50 to 70 %) due to
transport processes. On a more local scale, tidal currents resuspend and
transport this organic matter to temporary depocentres. This makes estimating
benthic respiration across the North Sea difficult due to the consequent
spatial heterogeneity. report that particulate
matter from the East Anglian plume and the southern North Sea may settle in
the OG and the southern flank of the Dogger Bank.
directly measured nutrient and oxygen dynamics at the
benthic–pelagic interface at the ND site. DO uptake by the sediment was
determined to be approximately 250 µmolm-2h-1, with
variations linked to organic matter export to the benthic layer. Assuming a
uniformly mixed BML with a height of 38 m (Fig. ), this
amounts to a decline in oxygen concentration of 1.6 µmol dm-3 day-1 consumed throughout the BML from respiration in the
sediment.
Remineralization of DCM-originating organic matter
Primary production estimates for the North Sea range from 40 to
300 g C m-2 yr-1 depending on area and technique (modelling or
empirical), with maxima in the central North Sea area (north of Dogger Bank;
). A transect by north of Dogger Bank
showed whole-water-column-integrated primary production values of 167, 370, and 270 g C m-2 yr-1 for areas classified as Dogger Bank, the
front (located along the northern edge of Dogger Bank), and the stratified
area (the ND site) respectively.
More recently, the importance of the DCM has been reviewed. The DCM
contribution to total primary production for the stratified North Sea during
the summer has been estimated at between 58 and 60 % . Values for new production range from 37 to
66 % produced at the DCM for the whole water column
. As the DCM is sustained by intermittent,
enhanced turbulent fluxes of nutrients from the BML
, it is necessarily located
very near to the pycnocline. Consequently, any mixing also injects highly
labile organic matter into the BML. Furthermore, sinking organic matter
produced at the DCM is exported to the BML immediately; therefore, any
remineralization of organic matter occurs in the BML. This means that nearly
all export production from the DCM is consumed within the BML.
The Seaglider observed DCM chlorophyll a concentrations of 2 to
3 mgm-3 in 2011 (Fig. ). These values are
similar to those found by but lower than concentrations
observed by (ca. 5–10 mgm-3). This is likely
explained by the systematic variation in DCM chlorophyll a concentrations
across the North Sea. The site considered here is located further north than
the site investigated in the study and is not subject to as
much mixing from internal waves fuelling the DCM in the ND region
. Chlorophyll a concentrations at the DCM
fluctuate on a near diurnal cycle (Fig. ). This may be
linked to either fluctuations in light availability or a diurnal change in
wind velocities. Increased wind speed could lead to an increase in
chlorophyll a fluorescence a few hours later by encouraging nutrient supply
to the DCM. Fluctuations in chlorophyll a at the DCM (Fig. ) are not due to quenching as periods of high DCM
chlorophyll a concentrations are present when surface chlorophyll a
fluorescence is affected by high light intensity. Instead, it is possible
that these fluctuations in chlorophyll a fluorescence at the DCM may be
caused by increases in nutrient supply linked to stronger winds that occurred
in the mornings (Fig. ).
We now investigate the export of DCM organic matter to the BML. Figure d shows a similar pattern for BML chlorophyll a
concentration and DCM chlorophyll a concentrations. We observe a delay
between peaks in DCM chlorophyll and BML chlorophyll with BML chlorophyll a
concentrations peaking a few hours later. As we have already observed a delay
between peaks in wind velocities and DCM chlorophyll a concentrations, we
can assume that wind-driven mixing is not responsible for injecting these
large amounts of organic matter into the BML. Neither are these increases in
BML chlorophyll a correlated to tidal velocities or changes in BML
salinity, thus ruling out the influence of horizontal transport processes or
tidal mixing on export of DCM organic matter to the BML. It seems likely that
the wind drives production at the DCM but export mechanisms are driven by
biological processes.
The Seaglider data show pulses, or short-lived increases, in BML chlorophyll a concentrations of 0.15 mgm-3 occurring on daily cycles. We
assume these are events where DCM organic matter sinks to the BML. With an
approximate BML height of 38 m, these pulses equate to
5.7 mgm-2day-1 of chlorophyll a export to the BML. Using a
particulate organic carbon (POC) to chlorophyll a ratio of 50:1, this amounts to an export of approximately
0.285 g C m-2 day-1 to the BML. If all this organic
matter were to be remineralized within the BML, this would equate to
33.58 mmol DO m-2 day-1 or 0.9 µmol dm-3 day-1
(as per the Redfield ratio; ). Although chlorophyll a
is not a direct proxy for POC due to the non-fluorescence of decayed organic
matter and lysed cells, the 50:1 ratio was determined empirically and therefore
also accounts for non-fluorescing organic matter. Based on the variability of
chlorophyll a within the DCM, the calibration of the glider sensor and the
variability of the POC to chlorophyll a ratio (< 25 %), we estimate the
error of our oxygen consumption estimate to be ± 1 µmol dm-3 day-1.
By summing the benthic (1.6 µmol dm-3 day-1) and pelagic
(0.9 µmol dm-3 day-1) oxygen sinks within the budget, we
obtain a gross oxygen remineralization rate of 2.5 ± 1 µmol dm-3 day-1.
Linking these observations back to the initial budget,
we observed a net oxygen consumption rate of 2.8 ± 0.3 µmol dm-3 day-1, a
gross supply of 2 ± 1 µmol dm-3 day-1, and a gross remineralization rate of
2.5 ± 1 µmol dm-3 day-1.
Balance of consumption and supply
Oxygen consumption arising in the water column from recently exported matter
(0.9 µmol dm-3 day-1) and benthic processes (1.6 µmol dm-3 day-1) provides a potential for oxygen consumption of
2.5 ± 1 µmol dm-3 day-1. This is described as “potential”
as the value is likely an overestimate of oxygen consumption rates because it
is unlikely that all of the sinking organic matter is remineralized
pelagically. Much of this sinking organic matter is deposited before being
remineralized, and therefore a portion of the DCM originating consumption
(0.9 µmol dm-3 day-1) is already accounted for in the benthic
consumption (1.6 µmol dm-3 day-1).
Furthermore, no POC : chlorophyll a ratio could be obtained from North Sea
DCM phytoplankton communities in this region. It is likely that in reality
the ratio is in fact lower than 50:1 as DCM phytoplankton communities are
adapted to lower light conditions and contain greater concentrations of
photopigments. If this were the case, it would only reduce the estimate
of oxygen depletion potential from DCM export, reinforcing the implication
that there are unresolved mechanisms contributing to the oxygen depletion.
The 50:1 conversion ratio, as well as the amount of sinking organic
matter, is likely to vary on a seasonal level through changes in community
composition. Species composition and bacterial cycling within the DCM will
also likely impact how labile sinking organic matter is. This estimate would
benefit from further in situ studies of sinking rates and organic matter
export from the DCM.
Our estimates can also be skewed due to aliasing from the glider's travel and
advection. Distinguishing vertical processes such as injection events from
horizontal advection or spatial aliasing from glider travel is critical when
assessing oxygen dynamics. In this study, the temperature and salinity data
allow different water masses to be distinguished; horizontal advection does
occur throughout the bottom mixed layer. Such an event occurs at the end of
the record and is excluded from our study.
Even by accounting for additional organic matter issuing from the DCM and
SML (faecal pellets, lysed cells, zooplankton) and doubling the potential for
oxygen consumption from the DCM, the sum of oxygen consumption potential from
the DCM and the benthic compartment remain only marginally larger than the
total net observed rate of dissolved oxygen consumption within the BML
(2.8 µmol dm-3 day-1). This either implies oxygen resupply
rates much lower than the 2 µmol dm-3 day-1 identified in
Fig. or, more likely, the existence of an additional
oxygen sink relevant to the time and location of the survey which has not
been accounted for.
In the results of this study, observed net consumption is significantly
greater than previously reported values in the literature. As previously
stated, the observed rate of apparent oxygen utilization would lead to lower
oxygen saturations than observed if maintained from the start of the
stratified season until the glider deployment. The implication here is that
the Seaglider surveyed the region during a short-lived or localized increase
in apparent oxygen utilization.
If we look at the budget in a wider seasonal context and ignore the
glider-observed net consumption rate (2.8 µmol dm-3 day-1),
the estimates of oxygen supply and consumption derived above (-2 + 2.5 µmol dm-3 day-1)
are in agreement with past studies of
the North Sea where net seasonal consumption rates of 0.4 µmol dm-3 day-1 were observed . This also points
to the unknown sink being either short-lived or very localized and explains
why the observed net consumption rate does not correlate with previous
studies.
Potential dissolved oxygen sinks
Several processes could account for this unidentified oxygen sink. Bacterial
recycling within the BML is rarely accounted for and could contribute to this
unaccounted for oxygen depletion. In addition, recent work has begun focusing
more on the importance of resuspension events in seasonal oxygen depletion
.
Background benthic DO consumption is limited by oxygen penetration depth and
redox levels . When deep benthic organic matter is
resuspended into an oxic water column, bioavailable surface area is greatly
increased and oxygen is readily available leading to very rapid and
short-lived oxidation of reductants such as nitrite, ammonium, and sulfide.
Organic matter is likely also briefly degraded more rapidly when resuspended
and more surface area is exposed to the oxic BML.
Three particular causes have been identified as providing enough turbulent
energy to the seafloor to resuspend organic matter: tidal resuspension, large
wind mixing events, and trawling
.
Current speeds and storm events sufficient to cause resuspension have been
recorded at both the OG and ND sites
. The influence of
trawling has only recently been investigated but has been shown to
potentially lead to a small (0.5 %) decrease in BML DO in
modelling studies. A strong resuspension event leading to an even greater
decrease in DO was documented by at the Oyster Grounds.
In the 2011 survey data, glider observations of optical backscatter at 650 nm
in the BML show the existence of resuspension events (Fig. e). Glider optical backscatter measurements showed
fluctuations similar to that of chlorophyll a concentrations with strong
increases and rapid declines in BML backscatter. The distance from the
seafloor at the deepest point of the dive prevents the glider from observing
fluctuations of backscatter in the near-bottom fluff layer. Despite this,
there is a clear gradient present with increasing backscatter nearer the
seabed showing that bottom stress does resuspend some organic matter (Fig. e). The peaks in BML optical backscatter do not correlate
with observed cross-pycnocline mixing (Fig. ) but seem to
occur on a diurnal cycle, about 4 to 6 h after daily peak wind speeds
(Fig. ).
Correctly understanding and quantifying resuspension becomes increasingly
important when one considers temporary depocentres. These sites accumulate
organic matter originating from much wider regions through slowing of
currents. These sites have the capacity to rapidly sequester large amounts of
organic matter, not only during peak production but also throughout the
winter. This creates a reservoir of highly labile organic matter which, when
resuspended, leads to very rapid consumption of DO. Depocentres are not
exclusive to the North Sea; they are driven by local hydrography and
topography, acting as hotspots of oxygen depletion in shelf sea environments
around the world.
Conclusions
This study, through the use of an autonomous underwater ocean glider, has
given new insights into the timescales of processes that regulate the supply
and consumption of oxygen across thermoclines and into the BML. The average
observed rate of oxygen consumption rate of 2.8 ± 0.3 µmol dm-3 day-1 is high; this would have resulted in the observed AOU
of 76 µmol dm-3 within only 1 month of the onset of
stratification. The glider observations' high temporal and vertical
resolution reveals the periods when resupply of oxygenated water occurs and
enables these vertical processes to be distinguished from horizontal
processes. Long-term trends in net oxygen consumption hide a more complex
short-term variability reflecting variations in oxygen supply and
consumption. During a period when the BML was isolated, the AOU was shown to
increase significantly over a short 3-day period. The observed vertical
mixing events have no apparent correlation with winds or tides, and remain a
process to be further explored. Optical backscatter readings from the
Seaglider in the BML showed varying suspended sediment, which generally
increased near the bed and varied on diurnal timescales.
recorded average (since stratification) oxygen
depletion rates in 2007 of 0.43 µmol dm-3 day-1 at ND and
0.75 µmol dm-3 day-1 at the OG. We observed export of organic
matter from the DCM with the potential of consuming 0.9 µmol dm-3 day-1, and the study by found rates of
1.6 µmol dm-3 day-1 for the sediment–water oxygen flux in
incubation experiments. These rates highlight two particular aspects of the
mechanisms governing seasonal oxygen depletion in the North Sea. The first is
that consumption along the seafloor plays a predominant role in the
consumption of BML oxygen. This supports the idea that depocentres, such as
the OG, are particularly prone to seasonal oxygen depletion. Advected organic
matter will add to the consumption budget, increasing the ratio of
consumption to supply, whilst the reduced water flow that promotes this
deposition also promotes stronger stratification. The second is that supply
and consumption of organic matter are tightly coupled. Net oxygen depletion
rates averaged over a season are 1 order of magnitude smaller than gross
oxygen consumption and supply rates. A small shift in either gross DO
consumption or supply would have a large impact on net oxygen consumption
rates.
A recent study by observed much greater vertical
mixing than is traditionally thought to be the case within the North Sea.
Using a similar budget-based approach, they identified consumption rates 5
times greater than those measured by (∼2 µmol dm-3 day-1). These rates, as well as high vertical mixing rates,
are in agreement with our findings.
This study has shown the potential of new AUVs in regions such as the North
Sea. This deployment was cut short and only 5 % of battery capacity was
utilized; a full duration glider deployment within the North Sea could last
upwards of 2 months. Despite the deployment lasting only 3 days, the
data revealed small-scale mixing mechanisms that would not be identifiable
with traditional oceanographic methods. A longer glider deployment would
provide more confidence in these estimates of rates and would reveal the
temporal variability of biological processes across an entire season. A
Seaglider performing repeat transects in and out of known depocentres could
provide critical high-resolution observations of subsurface primary
production and organic matter transport in regions of both low and high
biomass accumulation.
A longer survey would also increase the likelihood of observing larger mixing
events (i.e. storm events) and the impact these have on resuspension of
organic matter and subsequent oxygen drawdown. High-resolution observations,
such as those obtained by a glider, are critical to improving shelf sea
ecosystem models. There remain limitations to such models; many processes are
simplified and they do not resolve finer processes due to lack of
understanding and observations, particularly feedback and subsurface
processes. highlight that, in the case of the North Sea,
General Estuarine Transport Model–Biochemical Flux Model
(GETM-BFM)'s
representation of advection, particulate organic carbon transport, and
remineralization requires further knowledge to adequately represent these
carbon and oxygen dynamics. These are gaps that autonomous underwater
vehicles, by providing season-long observations, could fill. These models
could then, in turn, fill the gaps in glider observations to provide a
comprehensive view of the processes occurring in these highly dynamic and
heterogeneous environments. As it is now well established that many shelf
seas undergo seasonal oxygen depletion , it is critical to
increase the presence of persistent observation systems to improve models in
order to provide useful policy advice about future developments.
B. Y. Queste, K. J. Heywood, and L. Fernand designed the study and
provided the Seaglider. B. Y. Queste, L. Fernand, and A. J. Hind collected the data during the
August 2011 survey. B. Y. Queste corrected and interpreted the data and wrote the
paper. All authors contributed significantly to the interpretation of the
data and revisions of the manuscript.
Acknowledgements
This work and Bastien Queste were funded through a NERC Case studentship from
the University of East Anglia and Cefas. The Cefas CASE contribution was made
from Cefas Seedcorn funding. The research leading to these results has
received funding from the European Union 7th Framework Programme (FP7
2007–2013) under grant agreement no. 284321 GROOM (www.groom-fp7.eu).
The authors would like to thank the officers and crew of the RV
Endeavour for their assistance in this work. Wind and atmospheric
data were obtained from the European Centre for Medium-Range Weather
Forecasts (ECMWF). Satellite chlorophyll data were obtained from the Ocean
Biology Processing Group (OBPG) at the Goddard Space Flight Center. We thank
the editor and reviewers for their valuable comments and suggestions which
improved the manuscript.
Edited by:
L. Stramma
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