Figure 1. The measurement lines (A) and distinct sampling locations (B)
throughout the seasonal campaigns, encompassing winter (black), autumn
(yellow), and summer (red). The underway measurements are only available
for winter and summer campaigns.
2.2 Discrete Water Column Measurements
Water samples were collected using 10 L Niskin or FreeFlow bottles
(HydroBIOS) attached to a CTD/Rosette system (SBE 911plus, USA) during
all expeditions. The carbonate system is typically assessed using pH,
total alkalinity (AT), seawater partial pressure of
CO2 (p CO2), and total dissolved
inorganic carbon (CT), of which at least two have to be
measured. We sampled for pH and CT, filling two 250 mL
glass ground-stoppered flasks from each depth. During winter and summer
campaigns, pH was measured immediately after collection. The HydroFIA pH
system (4H Jena Engineering) was used, three measurements were executed
from each sample bottle, pH values were calculated on the total scale
following Müller and Rehder (2018), and the average of the three valid
measurements was reported. The accuracy of results was validated through
measurements of in-house buffer solutions after Müller et al. (2018) to
identify any drift. During the autumn cruise, separate samples for pH
and CT were collected and poisoned with 200 µL of
saturated HgCl2 solution immediately after sampling for
later analysis ashore. The precision of pH measurements during all
surveys was 0.002 between the three replicates.
For CT analysis, around 5 ml of each discrete sample was
acidified with 10% phosphoric acid to release carbon content. The
acidified sample flowed through an automated inorganic carbon analyzer
(AIRICA, Marianda) equipped with an infrared gas detector (LICOR 7000)
with a flow rate of 180 ml min-1, using a carrier gas
(99.999% Nitrogen) and finally passing through a NAFION dryer and
Peltier cooler. The LICOR system tracked the released
CO2 content over time, corresponding to
CT in the water sample. The final calculated
CT concentration is determined as the average of three
consecutive measurements from the system. Certified reference materials
(Scripps Institution of Oceanography) were used to eliminate blank
impurities and allow for drift correction, ensuring precision in the
range of 1.50 - 2.00 µmol kg-1 with a maximum
deviation of +/- 3.00 µmol kg-1 in triplicate
measurements. Measurements for the autumn cruise were performed ashore
upon return.
AT and p CO2 were computed
following the methodology outlined by Dickson et al. (2007) during both
winter and summer campaigns. The carbon system calculations were
executed using the CO2SYS EXCEL program. Dissociation constants K1 and
K2 were determined after Millero (2010), and the sulfate contribution
was derived from Dickson et al. (2007). We applied the total boron
parameterization proposed by Uppström (1974). In the autumn campaign,
AT from poisoned samples were measured by potentiometric
titration (using a glass electrode type LL, Electrode Plus, 6.0262.100;
Metrohm AG, Filderstadt, Germany) in the open-cell configuration,
following Dickson et al. (2007). The system underwent calibration using
the same CRMs employed for CT, resulting in consistent
precision. Prior to commencing the measurement sequence, a reference
water was analyzed to determine its precise target value (in µmol
kg-1), allowing for a deviation of up to 5 µmol
kg-1. The measurement result was adjusted to align
precisely with the target value during calculation. Towards the end of
the series, a final standard water was examined to assess changes in the
measuring system’s sensitivity. The calculation of results between
initial and final standard measurements incorporated these adjustments
and accounted for the system’s drift behavior.
For CH4 and N2O, bubble-free sampling
was conducted by using a silicon tube to transfer water from the CTD
Niskin bottles into 200 ml glass vials. These vials were then sealed
with rubber butyl stoppers and poisoned with 200 µL of saturated
HgCl2 solution immediately after. Determination of
CH4 and N2O was carried out by means of
a dynamic headspace method as outlined in Sabbaghzadeh et al. (2021).
This method ensures an average precision better than 0.5% for
CH4 and 0.6% for N2O by purging 20 ml
of the water sample, along with detection limits below 0.3 nmol for both
gases. Measurements were performed by means of a custom-designed purge
and trap system integrated with a gas chromatograph (Agilent 7890B) as
described previously (Sabbaghzadeh et al. 2021). To ensure accuracy and
detect drift, different volumes of standard gases (9.26 ppm ± 0.2%
CH4 and 1992 ppb ± 0.2% N2O in
synthetic air) were measured before and after each set of analysis on a
daily basis. The measurements were consistently maintained within the
calibration limits, thereby assuring the reliability of the results.
Further hydrographic parameters, including seawater temperature,
salinity, and oxygen concentration, were also assessed during the
campaigns with a Seabird SBE911 CTD. The instrumentation featured a
double sensor system with Digiquartz pressure sensor, SBE 3 temperature
sensor, SBE 4 conductivity sensor, and SBE 43 dissolved oxygen sensor.
To mitigate potential artifacts from ship heave, these sensors were
arranged within a tube system, referred to as the TC-duct, ensuring a
continuous flow of seawater. Data were recorded at a rate of 24 scans
s-1 and subsequently processed using Seasoft V2.
Temperature readings were reported on the ITS-90 temperature scale,
while salinity values were obtained through the Practical Salinity
Scale. Prior to the expeditions, sensor calibration was conducted at the
Leibniz Institute for Baltic Sea Research calibration laboratory. During
the expeditions continuous monitoring of sensor stability was carried
out using an SBE 38 thermometer for temperature, and an AUTOSAL 8400B
instrument for the salinity comparison measurements respectively. Oxygen
concentration was calculated utilizing the Sea-Bird Scientific
implementation of the oxygen saturation concentration. Regular samples
were taken in unstratified water layers, and oxygen concentration was
determined using the Winkler titration method. High resolution
hydrographic observations along cross shelf transects were also
performed by a towed CTD system, the ScanFish. This system undulates
vertically between the surface and about 120 m depth, while it is towed
behind the ship. The ScanFish was equipped with a Seabird SBE911 CTD,
consisting of SBE 3 temperature sensor, SBE 4 conductivity sensor, and
SBE 43 dissolved oxygen sensor. Data quality was checked with the
validated data of the standard CTD described above. The fraction of SACW
below the mixed surface layer was estimated from the CTD and ScanFish
data using the water mass definitions and procedure described in details
elsewhere (Mohrholz et al., 2008).
2.3 Continuous Surface Water and Atmospheric Measurements
The p CO2 and dissolved CH4 and
N2O in surface water were also continuously measured
using the Mobile Equilibrator Sensor System (MESS; Sabbaghzadeh et al.
(2021)) during winter and summer expeditions only. In brief, the system
consists of a custom-made bubble-type/showerhead equilibrator and a
control unit, both connected to two Los Gatos Research off-axis laser
absorption spectroscopy (OA-ICOS) analyzers (Model # 908-0011-0001 for
CO2/CH4/H2O and Model #
908-0014-0000 for N2O/CO/H2O). Seawater
was supplied by a deep-well pump (CAPRARI Desert E4XP30-4 with CAPRARI
XPBM1 control unit, ~ 100 L min-1,
Italy) located in the moon-pool at a depth of about 5.6 m on R/V METEOR
during winter and about 6.2 m on R/V Maria S. Merian during summer
campaign. The water flow rate was approximately 5 – 6 L
min-1. Concurrently, the air flow rate was adjusted to
roughly 4 – 5 L min-1. Regular control measurements
of all analyzers was conducted using three standard gases (Table S1)
measured regularly when the ship was on station. This process ensured
data accuracy through recalibration and drift correction. Additionally,
an occasional measurement of a ”zero” gas (Nitrogen 5.00, Linde GmbH,
Germany) was performed during the surveys to identify any system
deficiencies, such as potential leakage. The average sensor drift
observed was around 0.03 ppb d-1 for
CH4 and approximately 0.26 ppb d-1 for
N2O during winter, while it was 0.10 ppb
d-1 for CH4 and 0.24 ppb
d-1 for N2O during summer.
We also conducted measurements of atmospheric CO2,
CH4 and N2O in ambient air (Table S2) at
various locations, employing an inlet situated at a height of 35 meters.
To ensure data quality, a comparison was made with data from NOAA’s
nearest atmospheric sampling station (Station NMB [Gobabeb,
Namibia], located at 23.58°S, 15.03°E). During winter, the calculated
mean dry atmospheric values for CO2, CH4and N2O were x̅ ± σ; 410.50 ± 3.27 ppm, x̅ ±
σ; 1844.81 ± 31.89 ppb and x̅ ± σ; 331.51 ± 0.99 ppb respectively.
These values were compared to NOAA’s mean values of x̅ ± σ; 411.02
± 0.01 ppm for CO2, x̅ ± σ; 1842.62 ± 9.75 ppb for
CH4 and x̅ ± σ; 331.84 ± 0.37 ppb for
N2O. During the summer survey, the ship-borne daily mean
atmospheric gas levels (CO2; x̅ ± σ; 414.16 ± 1.51
ppm, CH4; x̅ ± σ; 1830.85 ± 13.81 ppb and
N2O; x̅ ± σ; 337.52 ± 1.31 ppb respectively)
closely aligned with the atmospheric mean values provided by NOAA
(CO2; x̅ ± σ; 414.10 ± 0.00 ppm,
CH4; x̅ ± σ; 1850.74 ± 7.05 ppb and
N2O; x̅ ± σ; 335.40 ± 0.43 ppb).
3 Data Analysis
The concentrations of dissolved CH4 and
N2O were computed after Sabbaghzadeh et al. (2021). To
further explore the N2O cycle in the BUS, the excess
N2O (ΔN2O) was calculated, representing
the difference between the observed N2O concentration
(N2Oin situ ) and the equilibrium
concentration of N2O with the atmosphere
(N2Oeq .) (e.g. Nevison et al.,
1995 and Weiss and Price, 1980).
We determined gas flux densities (F ) for CO2 in
mmol m−2 d−1 and for
CH4 and N2O in
μmol m−2 d−1 as follows:
F = kw * ΔC (obs . –eq .) (1)
where ΔC represents the difference between gas concentrations in
seawater and the equilibrated atmosphere (C(obs. )- C(eq. )), and k w (in cm
h-1) is the gas transfer velocity computed according
to Wanninkhof (2014). For sea-air CO2 fluxes, the bulk
equation is expressed in terms of the partial pressures of
CO2 in equilibrium with surface water and in the
overlying atmosphere, respectively, denoted as
Δp CO2 or p CO2w –p CO2a Wanninkhof (2014).
To standardize the comparison of GHG emissions in a common metric, we
also computed CH4 and N2O emissions in
terms of CO2 equivalent (CO2-e)
emissions, following the methodology outlined by Eyre et al. (2023);
Total CO2-e source =
CH4CO2-e +
N2OCO2-e (2)
CH4CO2-e (mg CO2m-2 y-1) = CH4flux
(µmol m-2 d-1) *
106 * 365 * 16 * 79.7 (GWP20)
or . 27.0 (GWP100) (3)
N2OCO2-e (mg CO2m-2 y-1) = N2Oflux
(µmol m-2 d-1) *
106 * 365 * 44 * 273 (GWP20,100) (4).
We firstly determined the average fluxes for each sub-region during
winter and summer and subsequently, CO2-e values were
determined using the 20-year and 100-year global warming potentials
(GWP) for each gas (Eyre et al. 2023). The GWP20 values
for CH4 and N2O stand at 79.7 and 273,
while the GWP100 values for CH4 and
N2O are 27 and 273 respectively, according to IPCC
(2021).
The significance of temperature and biological influences onp CO2 variations is gauged through a ratio that
considers both factors (Takahashi et al., 2002). The biological impact
is assessed by the seasonal amplitude of temperature-normalizedp CO2, while the temperature effect is measured by
the seasonal amplitude of the annual mean p CO2,
adjusted for seasonal temperature changes. The fluctuations inp CO2 resulting from biological and thermal
influences are calculated using the following equations, respectively:
p CO2 at Tmean =p CO2obs. * exp [0.0423
(Tmean − Tobs. )] (5)
p CO2 at Tobs. =p CO2mean * exp [0.0423
(Tobs. − Tmean )] (6)
where Tmean and Tobs.represent the annual mean and observed temperatures in °C, respectively.
These calculations are applied separately to coastal and offshore
sampling sites. The biological impact on surfacep CO2 variations, denoted as
Δp CO2Bio. , is also quantified by the
seasonal amplitude of p CO2, adjusted using the
mean annual temperature (p CO2 at
Tmean ),
Δp CO2Bio. = (p CO2at Tmean ) max -
(p CO2 at Tmean )min (7)
where Tmean (max ) and
Tmean (min ) indicate the average seasonal
maximum and minimum temperature. The influence of temperature
fluctuations on the mean annual p CO2 value,
Δp CO2temp , is
Δp CO2Temp. = (p CO2at Tobs. ) max -
(p CO2 at Tobs. )min (8).
The ratio of Δp CO2Temp. /Δp CO2Bio. > 1 signifies
the prevalence of temperature’s impact on the annual meanp CO2, while a ratio < 1 signifies the
prominence of biological effects (Takahashi et al., 2002).
To further investigate the predominant factors influencing
seasonal-spatial outgassing in the nBUS, Principal Component Analyses
(PCA) were conducted, accompanied by the corresponding biplots using
GraphPad Prism 10. The biplots facilitates a comprehensive understanding
of the interplay among different variables and their collective impact
on the observed phenomena. They effectively presenting both sample
scores (depicted by different colors) and variable loadings (represented
by arrows). The methodology includes interpreting Factor 1 in
environmental terms, where it is perceived as a synergy of biological
factors (associated with oxygen and gas concentrations) and physical
factors (highlighted by wind speed). Factor 2 also elucidates the total
variance, primarily associated with salinity and temperature. This
approach aids in grasping the underlying mechanisms influencing gas
emissions. These analyses were carried out individually for each gas,
incorporating ancillary data, and performed separately for the winter
and summer seasons to account for seasonal variations. To address the
diverse measurement scales of the variables, we standardized them using
a correlation matrix, ensuring a uniform scale for comparison. For
determining the number of principal components, we opted for the
parallel analysis method, chosen for its robustness in identifying
significant components.
Additional hydrographic data, including wind speed, air temperature, and
ambient air pressure, were obtained from the DSHIP data system for all
campaigns. Instantaneous wind speeds from the ship’s meteorological data
were normalized to a height of 10 meters above sea level
(U10n) following Large and Pond (1982). Daily average
wind speeds were further computed and reported to enable a seasonal
comparison within our specified regions. Underway hydrographic data,
encompassing sea surface temperature (SST) and sea surface salinity,
were compiled from information acquired through the ship’s
thermosalinograph (Seabird Micro SBE35).
4 Results
4.1 Seasonal Hydrographic Features in Upwelling Events
In general, the intensity of remote equatorial forcing and local
upwelling controls the distinct distributions of dominant central water
masses along the Namibian shelf (Mohrholz et al., 2008, Siegfried et
al., 2019). Strong upwelling leads to robust cross-shelf movement that
restricts the southward flow of SACW along the coast. This process can
result in the ESACW being confined to higher latitudes, exerting a
notable influence and governing the composition of subsurface water
masses in regions like Walvis Bay and Lüderitz, especially during the
winter. Previous interannual mooring observations in the region have
highlighted distinct patterns: ESACW pulses dominate during winter (June
to October), indicating intensified upwelling due to favorable winds. In
contrast, summer (January-February) experiences a prevalence of
nutrient-enriched SACW intrusions, indicating reduced upwelling
(Mohrholz et al., 2008; Junker et al., 2017).
The most significant variation in upwelling intensity was observed along
the inner shelf (within about 20 km from the coastline) of Walvis Bay
during winter. This region experienced temperature fluctuations between
11°C and 14°C, along with salinity changes ranging from 34.90 to 35.06.
In contrast, the uppermost recorded temperature in summer reached
17.50°C, and a salinity of 35.14. ScanFish observations during our
research campaigns also revealed a moderate level of upwelling near
Walvis Bay during winter. This coincided with lower temperatures and
increased oxygen levels. Interestingly, even in the presence of notable
ESACW intrusion, the waters directly over the shelf off Walvis Bay
experienced near-oxygen depletion (< 25.00 µmol
kg-1) during winter. This finding could be attributed
to a strong local demand for oxygen caused by the organic-rich mud belt
area, as indicated by prior research (Monteiro et al., 2006; van der
Plas et al., 2007). Conversely, summer profiles displayed no signs of
active upwelling, with over 50% of SACW and corresponding oxygen levels
of 50.00 µmol kg-1 or lower (Figure S1).
In the winter months, Kunene exhibits hydrographic features
characterized by warm and saline tropical water, leading to subsurface
oxygen depletion. The minimum observed oxygen concentration surpasses 40
µmol kg-1 within the depth range of 200 m to 400 m.
Contrastingly, the autumn season in Kunene reveals a convergence of the
poleward undercurrent with the nBUS, accompanied by a substantial pool
of oxygen-depleted water below 100 m. Lower salinity and colder coastal
waters, associated with an equatorward-directed coastal jet, mark this
season. Off Kunene, oceanic water with elevated salinity and a deep
chlorophyll maximum dominates. As summer arrives, Kunene maintains the
prevalence of warm and saline tropical water, akin to winter conditions,
with continued subsurface oxygen depletion.
The winter hydrography off Walvis Bay is characterized by
well-ventilated surface layers with medium to strong oxygen depletion
below the mixed surface layer, heightened turbidity due to phytoplankton
abundance, and sea surface temperatures (SST) ranging from 12°C to 16°C,
indicative of upwelling (Figure S1). The dominance of ESACW on the inner
shelf contributes to high oxygen content, preventing anoxic conditions.
However, increased oxygen demand in the deep inner shelf during winter
is linked to elevated organic carbon concentrations in the sediment.
Offshore, oxygen depletion extends below 200 m, with the OMZ core at
approximately 300 m, and a substantial SACW fraction (>
40%) at the shelf edge. In autumn, Walvis Bay also exhibits distinct
evidence of coastal upwelling, highlighted by spreading isotherms and
water masses originating partly from tropical regions penetrating
poleward. In summer, offshore SST exceeds 20°C, reflecting the South
Atlantic trade wind system, and significant temperature gradients with a
north-to-south surface salinity gradient are notable south of Walvis
Bay. Despite a general surface salinity gradient from north to south, a
detached salinity minimum occurs at Walvis Bay, potentially due to weak
winds hindering a coastal jet. The central region of the nBUS at Walvis
Bay indicates coastal upwelling, resulting in a thin layer of warm water
near the coast. Oxygen depletion below 200 m persists, with
near-complete exhaustion on the broad shelf, featuring sulfidic waters
in some coastal stations. The colder yet less saline upwelling water
undergoes rapid heating and stabilization, overlaying more saline water
as it drifts offshore with the Ekman transport (Mohrholz et al. 2014).
Oxygen depletion is observed below 200 m, with near-complete exhaustion
on the broad shelf with observed sulfidic waters in a few coastal
stations.
In winter, transitioning to Lüderitz reveals significant changes in
salinity and oxygen levels, indicating a poleward flow near the coast.
Decreased salinity and oxygen depletion suggest a distinct hydrographic
regime, particularly at the southernmost point (27°S), where
stratification signals the presence of an upwelling regime. Oxygen
depletion in the bottom layer of the shelf and a discernible signal of
poleward transport of saltier and oxygen-depleted water near 14°E at
depths of 200 m to 300 m further characterize the winter hydrography off
Lüderitz. In autumn, the hydrographic section off Lüderitz was situated
where the northward-flowing water carried by the poleward undercurrent
has already undergone upwelling. Despite this, salinity maxima at
specific stations, coupled with corresponding oxygen minima, suggest the
persistence of poleward flow reaching Lüderitz. Chlorophyll
concentrations confined to the sea surface, with maximum values near the
coast, indicate active upwelling during autumn. In summer, off Lüderitz,
the variability of salinity on the shelf is considerably lower compared
to Walvis Bay. Less saline water forming the top ocean layer is
displaced far offshore, and prominent oxygen depletion is observed on
the shelf. A hypoxic bottom layer with oxygen concentrations below 40
µmol kg‑1 extending almost to the sea surface near the
coast. These seasonal variations in Lüderitz underscore the diverse
hydrographic conditions, ranging from poleward flow and upwelling in
winter to persistent poleward flow with active upwelling in autumn and
distinct oxygen depletion during summer (e.g. Hutchings et al., 2009).
During our study, the distinct seasonal variations in upwelling patterns
were also evident in the spatial distribution of SST. The SST signals
show that cool upwelling waters covered the entire coast during the
winter season, characterized by the SST values ranging between 11.33 °C
to 15.50 °C. In contrast, during the summer campaign cool water related
to coastal upwelling is observed only south of Walvis Bay (Figure 2).