Figure 6. Seasonal fluctuations in sea surface temperature (SST in
blue), instantaneous wind speed (U10n,
ms-1 in grey), F CO2 density (pink), F
CH4 density (green) and F N2O density
(yellow) across the locations of Kunene, Walvis Bay, and Lüderitz. The
coastal stations are positioned to the right of the plot. Note different
scales on y-axis.
Lüderitz in the inner shelf, in contrast, shows less pronounced seasonal
variations in F CO2 densities. The positive F
CO2 densities imply a small yet consistent annual source
of CO2 to the atmosphere (F
CO2(Lüderitz-winter ): x̅ ±
σM: 3.00 ± 0.12 mmol m-2d-1 and F CO2(Lüderitz-summer ):x̅ ± σM: 3.00 ± 0.17) (Figure 6).
The Kunene and Walvis Bay regions both exhibited distinct seasonality in
sea surface CH4 concentrations and corresponding fluxes,
whereas in Lüderitz, the seasonal fluctuations in sea surface
CH4 concentrations and their corresponding fluxes are
less pronounced, with flux densities consistently remaining low
throughout the year (Figures 6 and S2). During our campaigns, we
determined the maximal sea surface CH4 concentration at
Walvis Bay during summer campaign with an average ofx̅summer ± σM:
72.45 ± 5.63 nmol L−1 and as high as 700
nmol L−1 in the inner shelf compared to Kunene and
Lüderitz (x̅summer-Kunene ± σM:
3.30 ± 0.02 nmol L−1 andx̅summer-Lüderitz ± σM:
3.17 ± 0.06 nmol L−1). Also, the inner shelf of Walvis
Bay showed to be a source of CH4 to the atmosphere
throughout the year with the maximum daily net F CH4;x̅ ± σM: 1319.34 ± 65.00 µmol
m-2 d-1 and x̅ ±
σM: 593.37 ± 22.12 µmol m-2d-1 in winter and summer, respectively (e.g. van der
Plas et al., 2007; Sabbaghzadeh et al., 2021). The wider shelf with an
extensive organic-rich diatomaceous mud-belt might fuel seasonal cycling
of CH4 in the Walvis Bay region and with high wind
speeds during winter may contribute to elevates sea-air fluxes (Calvert
& Price, 1983; Mollenhauer et al., 2007; van der Plas et al., 2007;
Sabbaghzadeh et al., 2021). The substantial F CH4density recorded during our surveys is not surprising since
CH4 flux densities as high as
3000.00 μmol m−2 d−1 have been
recorded previously in Walvis Bay in association with large sedimentary
fluxes (Brüchert, et al., 2009; Sabbaghzadeh et al., 2021). Morgan et
al. (2019) also reported the maximum F CH4 density of
3145.00 μmol m−2 d−1 with an average
of 450.00 μmol m−2 d−1 in Walvis Bay
– Lüderitz sections during upwelling events.
In general, the concentrations and flux densities of N2O
demonstrate a closer mirroring relationship with CO2compared to CH4 (Figures 6 and S2). Sea surface
N2O concentrations ranged between 5.00 and 35.00 nmol
L-1, slightly exceeding the maximum of 31.00 nmol
L-1 reported by Arévalo Martínez et al. (2019) in the
region. However, these concentrations fall within the range of 5.00 and
51.00 nmol L-1 as reported for upwelling regions in
the Eastern South Atlantic (Bange et al., 2009; Kock and Bange, 2015).
The computed flux densities consistently demonstrate the region as a
perennial source of N2O to the atmosphere (Figure 6).
Off Kunene, flux densities were significantly higher in winter compared
to summer, with the computed flux being 200 times greater in winter
relative to the summer campaign (F x̅winter ±
σM: 151.42 ± 1.85 µmol m-2d-1 and F x̅summer ±
σM: 0.60 ± 0.05 µmol m-2d-1). The daily average wind speed
(U10n in m s-1) also demonstrated a
marked difference, with x̅winter ±
σM: 12.34 ± 0.08 m s-1 in winter
compared to x̅summer ± σM: 1.22 ±
0.03 m s-1 in summer during our campaigns. The Kunene
region has also been previously documented as exhibiting maximal sea
surface N2O concentrations during the winter months,
attributed to the presence of cold water parcels (13 – 16 °C) and
seasonal upwelling (e.g. Arévalo Martínez et al., 2019). The shelf
topography, incorporating a steep slope in the Kunene area, may
facilitate upward advection, initiating rapid diffusive mixing into the
surface ocean and establishing the Kunene as hotspots of gas efflux
(Sabbaghzadeh et al., 2021). Frame et al. (2014) also reported a maximum
F N2O density of 43.40 µmol m-2d-1 between November and December in the event of
upwelling near Kunene. However, it is important to note that their data
was collected from only one station within the Angola-Benguela frontal
zone at coordinates 17.5°S and 11.25°E. Notably, the F
N2O density reported by Frame et al. (2014) was
considerably lower than the F N2O density observed
across the entire inner shelf of Kunene during the course of this study
in the winter campaign. Unlike this study, Frame et al. (2014) did not
include stations within the Kunene cell. Moreover, in contrast to this
study, the majority of their stations were positioned beyond the 250 km
bathymetric contour. As a result, they missed capturing the inner shelf
variability emphasized by our dataset.
Off Walvis Bay, too, displays a comparable trend in flux densities
between winter and summer seasons with flux being 100 times higher in
winter compared to summer (F x̅winter ±
σM: 23.20 ± 0.90 µmol m-2d-1and F x̅summer ±
σM: 0.29 ± 0.01µmol m-2d-1), serving as a source of N2O
emissions to the atmosphere, with a potentially greater contribution
during the winter relative to summer. Continuous observations from a
ground-based atmospheric observatory for greenhouse gases along the
coastal shelf, spanning approximately from 22°S to 28°S, have documented
an average F N2O density of 52.70 µmol
m-2 d-1. This flux density is
associated with specific upwelling events observed at coastal stations
located between Walvis Bay and Lüderitz (Morgan et al., 2019). These
findings are in good alignment with the recorded F- N2O
density at coastal stations in Walvis Bay (F x̅ ±
σM: 48.20 ± 2.01 µmol m-2d-1) during our winter campaign.
Lüderitz on the other hand, may serve as a less significant source of
N2O emissions to the atmosphere compared to the other
regions in the winter (F x̅winter ±
σM: 4.43 ± 0.08 µmol m-2d-1). Reduced N2O concentrations and a
less pronounced gradient in the vicinity of the Lüderitz cell during the
winter months relative to Walvis Bay have already been reported. This
observed reduction aligns with several contributing factors, including
diminished primary productivity and constrained organic matter supply to
deeper waters (Arévalo Martínez et al., 2018). Our data further supports
this notion by revealing relatively lower N2O
concentrations in the Lüderitz region during the winter (x̅ ±
σM: 11.93 ± 0.05 nmol L-1), in
contrast to the Kunene and Walvis Bay regions (Figure S2). Furthermore,
the surfacing of low N2O waters were identified in
Lüderitz, compared to Kunene and Walvis Bay during our winter campaign
(Figures 3, 4 and 5). However, F N2O was considerably
higher during summer campaign in Lüderitz rather than Kunene and Walvis
Bay (F x̅summer ± σM: 8.65 ± 0.35
µmol m-2 d-1). The daily average
wind speed in the inner shelf of Lüderitz during the summer was also
higher than in the other two regions (x̅Kunene ±
σM: 1.30 ± 0.07 m s-1,x̅Walvis Bay ± σM: 5.70 ± 0.03 m
s-1 and x̅Lüderitz ±
σM: 6.00 ± 0.06 m s-1) which may
explain the higher recorded flux densities in summer in Lüderitz.
However, it is notable that, in general, during our campaigns, we found
that flux gradients were primarily associated with concentration fields
than SST and wind speed (Figures 6 and S2).
Collectively, the positive sea surface gas levels and flux densities are
generally observed across our studied areas indicate that the regions
are sources of gas emissions throughout the year. However, the specific
timing and magnitudes of these emissions exhibit variability,
underscoring the intricate and context-dependent nature of gas dynamics
in coastal environments.
We observed a noteworthy phenomenon during our winter campaign, noting
depleted sea surface gas concentrations in the nearshore core of
upwelled waters. This led to a subsequent decline in flux densities at
multiple stations near the coasts of Kunene and Walvis Bay (see Figures
6 and S2). Importantly, despite no significant difference in mean
temperature (p > 0.05) compared to adjacent stations
located within 30 km offshore, the observed reductions were evident.
These reductions are suggested to be linked to the upwelling of strongly
hypoxic waters near the shelf. With respect to N2O,
these waters are characterized by N2O-depleted
conditions, a result of N2O consumption under
denitrifying conditions. However, a comprehensive exploration of this
phenomenon extends beyond the scope of our current paper and
necessitates more extensive investigation.