Plain Language Summary
The Benguela Upwelling System (BUS) in the Eastern South Atlantic is a
key region for the exchange of greenhouse gases (GHG) between the ocean
and the atmosphere. Despite its importance, little direct evidence
exists on the seasonal variability of GHG production and emissions in
this area. Our multi-year study reveals that the northern Benguela
Upwelling System (nBUS) consistently releases significant amounts of
GHG, with winter contributing the most. During this season,
CO2 emissions dominate, making up about 75 – 76% of
the total GHG emissions, followed by N2O (21 – 22%)
and CH4 (3 – 5%) in terms of
CO2-equivalent (CO2-eq) emissions.
However, in the summer, the composition shifts, with more balanced
contributions from CO2, CH4, and
N2O. This highlights the need to consider
non-CO2 GHG when evaluating the role of coastal
ecosystems in climate. Our findings provide detailed insights into the
factors driving spatial and seasonal variations in GHG concentrations
and sea-air fluxes in the coastal upwelling waters off Namibia. This
study is the first to present multi-year measurements of these three
GHG, offering valuable information for understanding the role of the
region in regulating the marine emissions of climate-active gases.
1 Introduction
Over the past century, alteration of land-sea temperature gradients due
to global warming have become apparent. Bakun (1990) proposed that these
shifts in gradients might directly affect atmospheric pressure cells,
leading to changes in wind patterns and subsequently influencing the
upwelling patterns within ecologically important regions. The
environmental drivers of upwelling, such as temperature and wind,
exhibit substantial variability in the Eastern Boundary Upwelling
systems (EBUS) regions, both spatially and temporally (Abrahams et al.,
2021). This directly affects upwelling patterns, with their shifts
affecting the frequency, intensity, and duration of upwelling events.
Temperature fluctuations correlate with changes in detected upwelling,
and variable winds significantly impact upwelling responses (Abrahams et
al., 2021). Linear regression analysis revealed the predominant
influence of wind forcing on upwelling variability within EBUS (Bonino
et al., 2019). Equatorward wind stress and cyclonic wind stress curl
foster upwelling and cause fluctuations in thermocline depth (Bordbar et
al., 2021). However, despite of the high seasonal variability the
spatial pattern and intensity of upwelling favorable wind in the BUS
depict no significant long term trends during the recent four decades
(Bordbar et al., 2023).
Coastal upwelling regions are key sites for the production and emissions
of climate-relevant compounds such as the potent greenhouse gases (GHG)
carbon dioxide (CO2), methane (CH4) and
nitrous oxide (N2O) (e.g. Bakker et al., 2014; Resplandy
et al., 2024). While CO2 cycling is dominated by
phytoplankton activity in the photic zone and the mineralization of
organic material across the entire water column in addition to
thermodynamic changes, CH4 and N2O
pathways are dominated by microbial processes at mid-water depth and the
sediments, respectively. In these regions, waters enriched in nutrients
and GHG are brought close to the surface, triggering both phytoplankton
blooms and the release of these gases to the atmosphere (Capone and
Hutchins, 2013). Yet, the ocean’s contribution to the atmospheric budget
of these GHG is associated with a large range of uncertainty due to
limited in-situ measurements and large intrinsic heterogeneity, whereby
seasonal variability due to coastal upwelling is the dominant factor
(Weber et al., 2019; Yang et al., 2020; Siddiqui et al., 2023).
The Benguela upwelling system (BUS) comprises one of the most productive
marine ecosystems worldwide and it is one of the four major global
coastal upwelling regions. The BUS is driven by southeast trade winds
along the southwest African coast, extending from the Angola Benguela
Frontal Zone (ABFZ) at 14°S to 17°S down to Cape Agulhas at around 34°S
(Brandt et al., 2024). While general upwelling extends up to 200 km
offshore, specific filaments near Namibian coast reach over 600 km
offshore, lasting days to weeks (Muller et al., 2013). These filaments
are vital for carbon transport from coastal upwelling to the open ocean,
serving as a shelf pump (e.g. Lutjeharms and Stockton 1987,
Santana-Casiano et al. 2009).
The upwelling episodes in the Namibian coast are perennial, while
upwelling intensity follows a seasonal oscillation due to the annual
migration of the South Atlantic Anticyclone, with a minimum in winter
(e.g. Hutchings et al., 2009, Morgan et al., 2019, Bordbar et al.,2021).
The main seasonal variation of biogeochemical conditions in the BUS is
related to the enhanced northward inflow of oxygen-enriched Eastern
South Atlantic Central Water (ESACW) in austral winter, and increased
southward transport of oxygen-depleted, nutrient-enriched tropical South
Atlantic Central Water (SACW) in austral summer, which in turn fosters
the development of a pronounced oxygen minimum zone (OMZ) (Muller et
al., 2014; Junker et al., 2017). It is then during summer that anoxic
conditions and sulfidic events over the Namibian shelf and upper slope
occur (e.g. Monteiro et al., 2006; Mohrholz et al., 2008; Ohde and
Mohrholz, 2011; Ohde and Dadou, 2018). A combination of coastal
topography and shelf width creates a number of discrete upwelling cells
like Lüderitz at ~ 27°S which accounts for up to 50 %
of total upwelled water (e.g. Shannon and Nelson, 1996; Duncombe Rae,
2005; Monteiro, 2008). Also, in the presence of strong winds, high
offshore advection and turbulent mixing, Lüderitz establishes a
perennial barrier between the northern and southern Benguela regions
(e.g. Shannon and Nelson, 1996; Duncombe Rae, 2005; Monteiro, 2008).
Other intense upwelling cells include the Kunene at about 18°S, the
Northern Namibia cell at 19°S and the central Namibia cell at 23°S close
to Walvis Bay, where southerly winds cumulate (see e.g. Santana-Casiano
et al., 2009).
Since the industrial revolution, the oceans have attenuated the
atmospheric CO2 increase by acting as a net
CO2 sink, absorbing an average of 2.90 ± 0.40 PgC
yr−1 globally for the last decade (Friedlingstein et
al., 2022). Upwelling regions affect atmospheric CO2levels by either releasing or absorbing the gas from the atmosphere,
depending on biological (production and mineralization of organic
carbon; calcium carbonate production and dissolution) and physical
processes (mixing and thermally-driven processes) (e.g. Borges, 2005; Le
Quere et al., 2014). Certain regions within the BUS were previously
recognized as carbon sinks, characterized by elevated rates of primary
production and sedimentary organic carbon. However, in other areas,
there has been a transition to becoming CO2 sources,
primarily attributed to the upwelling of mineralized organic compounds
(e.g., Carr, 2002; Mollenhauer et al., 2002; Siddiqui et al., 2023).
This shift from carbon sinks to sources is influenced by temporal
oscillations, highlighting the dynamic nature of these ecosystems (Zhang
et al., 2022). The patterns of sea-air CO2 fluxes play a
pivotal role in better understanding the BUS’s carbon source-sink
function and oceanic carbon export.
The BUS is a net source of CH4 and N2O
to the atmosphere (see e.g. Arévalo-Martínez et al., 2019; Sabbaghzadeh
et al., 2021; Mashifane et al., 2022). CH4 sources in
OMZs are anoxic sediments where microbial anaerobic methanogenesis
occurs during organic matter degradation. Accumulated
CH4 in sediments is released to overlying waters due to
pressure and temperature influences, both by diffusion and ebullition.
Coastal upwelling also increases nearshore production and with that
methanogenesis and CH4 emissions as the consequence
(e.g. Naqvi et al., 2010, Bakun et al., 2010, Bakker et al., 2014, Bakun
et al., 2017, Weber et al., 2019). The ”ocean methane paradox” refers to
other CH4 sources, such as methanogenic Archaea in
zooplankton digestive tracts, sinking organic matter with methanogenic
bacteria, and in-situ methanogenesis in the mixed layer (e.g. Reeburgh,
2007; Schmale et al., 2018). These in-situ sources, particularly in
shelf areas, contribute to CH4 outgassing near the
sea-air interface (e.g Damm et al., 2010, Florez-Leiva et al., 2013,
Capelle and Tortell, 2016). Microbial processes, including anaerobic
CH4 oxidation in sediment and aerobic
CH4 oxidation by methanotrophs, act as effective sinks
for CH4, limiting its release (e.g. Kessler et al.,
2011; Heintz et al., 2012). Despite these sinks, a notable amount of
CH4 may escapes oxidation and be released, especially
during upwelling events and in shallow regions.
In regions with steep oxygen gradients, the efficiency of
N2O production through nitrification (microbially-driven
two-step oxidation of ammonia) plunges as oxygen levels decline,
resulting in accumulation of N2O at oxic-suboxic
interfaces. As oxygen becomes even scarcer, N2O is
produced as an obligate intermediate during partial denitrification
(microbially-driven reduction of nitrite). Under nearly oxygen-depleted
conditions, denitrification further proceeds and N2O is
further reduced to N2 gas (Frame et al., 2010; Dalsgaard
et al., 2014). The extent of denitrification stimulation hinges on the
quantity and quality of organic matter transported from the photic zone,
leading to elevated N2 production by denitrifiers (e.g.
Dalsgaard et al., 2012; Jayakumar et al., 2009). Additionally, anammox,
an anaerobic process for conversion of ammonia and nitrite into
N2, is introduced as a possible route for
N2 production, particularly in oxygen-deficient waters
(Kuypers et al., 2005, Kartal et al., 2007; van der Star et al., 2008).
In the BUS, N2O production is dominated by nitrification
and nitrifier-denitrification (Frame et al., 2014), as well as
sedimentary denitrification (Tyrrell et al., 2002).
In order to assess the extent to which spatio-temporal variability in
the nBUS influences concentrations and emissions of GHG, we conducted
extensive shipboard, real-time measurements of dissolved
CO2, CH4, and N2O in the
region across different seasons, covering the main environmental
meridional and zonal gradients. This study presents the first
comprehensive assessment of the major GHG in the region and sets a
baseline upon which future model studies can improve the representation
of sea-air flux variability, in view of the projected ocean warming and
expansion of OMZs.
2 Materials and Methods
2.1 Sampling Locations
Sample collection was conducted during three distinct seasonal campaigns
spanning the years 2019 to 2022 on board the research vessels R/V METEOR
(August - September 2019, austral winter), R/V SONNE (March - May 2021,
austral autumn), and R/V Maria S. Merian (January - February 2022,
austral summer). In this study we focus on the northern part of the BUS
(nBUS), covering the geographical range of 14°S to 27°S, where, the most
pronounced oxygen depletion in the region is observed. The observational
efforts involved acquiring water column measurements along three main
cross-shelf transects: namely, about 18°S in the vicinity of Kunene,
23°S in the central Namibia upwelling cell off Walvis Bay, and 25°S in
close proximity to the Lüderitz upwelling cell (Figure 1). Furthermore,
we conducted simultaneous, high-resolution measurements of
CO2, CH4, and N2O in
surface waters and the overlying atmosphere during the campaigns in
August-September 2019 and January-February 2022.