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.