Figure 11. Principal component analysis of the seasonalized data-set
including winter (left panel) and summer (right panel) for all studied
gases. The panels represent the biplots that displays both the scores of
the samples and the loadings of the variables on the principal
components in the same plot. Numbers on the axis represent the factor
loadings of each variable and each principal component i.e. PC.
Factor 2 elucidated ~ 25% of the total variance,
associated with salinity and temperature, albeit exerting a limited
impact on CO2 and N2O emissions. During
the summer season, Factor 1 played a pivotal role, elucidating
approximately 50% of the total variance concerning CO2and N2O emissions. Notably,
Δp CO2, F CO2,
ΔN2O, F N2O, Chla, and oxygen exhibited
a negative correlation with water temperature, particularly within the
Lüderitz region.
We know that local upwelling plays a crucial role in transporting cold,
nutrient-rich, and gas-enriched deep waters to the surface layers in
Lüderitz. This process leads to outgassing while simultaneously
fostering biological activity. Additionally, Factor 2 exhibited an
association with wind speed, although it had a relatively minor impact
on the emissions of CO2 and N2O.
Previous studies have also reported a negative relationship between F
CO2 and SST/SSS in upwelling regions, depending on the
positioning of hydrodynamic features (e.g. Lefèvre and Taylor, 2002).
In the context of CH4, Factor 1 elucidates over 40% of
the total variance in winter, with oxygen displaying a negative
correlation with temperature, salinity, and wind speed. This observation
may be indicative of upwelling events linked to the influx of saltier,
colder, and oxygen-depleted waters in the Kunene and Walvis Bay regions.
Therefore, factor 1 is likely related to oceanographic conditions.
Factor 2, accounting for 33% of the total variance, was largely
dominated by ΔCH4 and F CH4,
representing significant and persistent disequilibria in
CH4 concentrations at the ocean-atmosphere interface in
both seasons. This factor emerged as a critical driver of
CH4 emissions, particularly in the coastal area of
Walvis Bay. A similar pattern was also evident in summer, underscoring
the paramount role of the extensive organic-rich diatomaceous mud-belt
in driving CH4 production and emissions in Walvis Bay
(Mollenhauer et al., 2007; van der Plas et al., 2007; Sabbaghzadeh et
al., 2021).
5 Conclusions
We showed that the nBUS is a perennial, yet, seasonally-varying hotspot
of CO2, CH4 and N2O
production and emissions to the atmosphere. The seasonal variability in
water column and sea surface concentrations of CO2,
CH4 and N2O could be observed across the
major upwelling cells in the region, albeit a meridional gradient with
the Lüderitz cell having a smaller annual contribution.
The primary drivers influencing CO2,
CH4, and N2O fluctuations include
upwelling events, temperature changes, and pronounced biological
effects, particularly during upwelling. Non-thermal effects (likely
biological effects) consistently control p CO2fluctuations across all regions, with coastal areas displaying more
biological influences than offshore locations. Also, the lack of
correlation between CT and salinity in the nBUS suggests
that factors beyond dilution govern CT fluctuations,
emphasizing the complex interplay of physical and biological processes
in shaping carbon dynamics. Despite the shared drivers, each gas
displays unique seasonal variations, with CO2 and
N2O emissions peaking during intensified upwelling in
winter, while CH4 dynamics are influenced by
organic-rich mud-belt at Walvis Bay and other sedimentary sources at
Kunene and Lüderitz, leading to elevated concentrations and sea-air
fluxes throughout the year. In addition, analysis of
CO2-e emissions indicates a winter prevalence of
CO2 emissions, followed by N2O and
CH4. Conversely, summer exhibits a notable shift, with
N2O taking the lead, followed by CO2 and
CH4 in contributing to the total GHG emissions.
Overall, we show the nBUS to be an important region in terms of its
contribution to atmospheric CO2, CH4 and
N2O. The intrinsic spatial and temporal variability of
production and sea-air fluxes of these gases in the region underscores
the need for an improved understanding of their seasonal dynamics and
its relationship with oceanographic variability. Furthermore, ongoing
environmental changes including global warming and associated changes in
oceanic stratification and ice coverage, ocean acidification, expansion
of OMZs, and eutrophication might significantly alter the oceanic
production and consumption of these gases, their distribution patterns,
as well as their sea-air fluxes. An improved representation of how these
processes affect the seasonal dynamics and driving factors of these
gases, might in turn benefit future model projections of climate change
scenarios. To this end, we contend that including CH4and N2O along with CO2 is key when
assessing the impacts of coastal upwelling systems on climate dynamics,
in particular in terms of the quantification of the net radiative
balance of these highly productive ecosystems.