Coastal upwelling regions associated with Eastern Boundary Upwelling Systems and the monsoon-driven circulation in the Arabian Sea are the ocean’s most productive ecosystems (Carr, 2001; Messié et al., 2009). They support ∼20% of the global fish catch and are associated with pronounced oxygen minimum zones (OMZ, Pauly and Christensen, 1995; Monteiro et al., 2011). The Benguela Upwelling System in the southeast Atlantic Ocean is the most productive system of all Eastern Boundary Upwelling Systems in terms of primary production (Carr, 2001; Messié et al., 2009). The strong Lüderitz Upwelling cell at 26°40′S divides the region into a northern (NBUS) and southern subsystem (SBUS, Figure 1), which are located off Namibia and South Africa, respectively (e.g., Shannon, 1985; Duncombe Rae, 2005; Hutchings et al., 2009). Even though the intensity of upwelling, as indicated by the wind-driven Ekman transport, is an order of magnitude stronger in the NBUS than in the SBUS, net primary production rates are similar between the two subsystems (Wasmund et al., 2005; Barlow et al., 2009; Verheye et al., 2016; Lamont et al., 2018). Nutrient concentrations, in addition to stable nitrogen and oxygen isotopic ratios, indicate that on-shelf trapping of regenerated nutrients strongly increases the effects of upwelling on the productivity in the SBUS (Flynn et al., 2020). Regenerated nutrients are released during the remineralization of organic matter while preformed nutrients are biologically unused nutrients, which are carried along with sinking surface water masses mostly at high latitudes into the deep sea (Broecker et al., 1985; Ito and Follows, 2005). Sub-Antarctic Mode Water loaded with preformed nutrients feeds the BUS and is further enriched with regenerated nutrients within the upwelling system (Figure 2). This occurs due to the export of organic matter from the euphotic zone into subsurface waters by offshore advecting upwelling-driven plankton blooms and its remineralization within the upwelling source waters (Dittmar and Birkicht, 2001; Tyrrell and Lucas, 2002). Since the remineralization of exported organic matter within the upwelling source waters limits lateral and vertical losses of nutrients from the upwelling system, it is called nutrient trapping and it strengthens the impact of upwelling on the productivity by increasing nutrient concentrations in the upwelled waters. In contrast, lateral and vertical nutrient losses lower the nutrient trapping efficiency and the influence of upwelling on the productivity. The offshore advection of upwelling-driven phytoplankton blooms and the resulting export of organic matter outside the upwelling region represent a lateral nutrient loss. Similarly, the remineralization of exported organic matter above and below the upwelling source waters, as well as the burial of organic matter in sediments, is considered as a vertical nutrient loss.

Since dissolved oxygen is utilized during the remineralization of organic matter, the nutrient trapping efficiency links the productivity of an upwelling system to the intensity of the subsurface OMZ. However, a slowdown of remineralization rates and a decreasing oxygen-inhibition of anaerobic processes at low oxygen concentrations is assumed to prevent the development of anoxia by reducing the nutrient trapping efficiency (Rixen et al., 2019, 2020). Decreasing remineralization rates lower the nutrient trapping efficiency by deepening the main zone of remineralization, which represents a vertical loss of nutrients from the upwelling system. During anaerobic processes oxygen bound to nitrogen is used to drive metabolic processes with the consequence that nitrate and nitrite are converted into elemental nitrogen (N₂). This form of nitrogen is inaccessible for eukaryotic plankton, such as diatoms and dinoflagellates, which dominate the phytoplankton community in the Benguela Upwelling System (e.g., Fawcett et al., 2007; Pitcher et al., 2014; Postel et al., 2014). Hence, a reduction of nitrate to N₂ lowers the nutrient trapping efficiency by decreasing the availability of nutrients within the upwelling system.

In contrast to the OMZ in the NBUS, which is largely driven by the inflow of oxygen-poor waters from the Angola basin, the OMZ in the SBUS is mainly produced locally by the balance between physical oxygen supply and biological oxygen consumption (Monteiro et al., 2006, 2011; Mohrholz et al., 2008; Pitcher et al., 2014). The SBUS OMZ is most pronounced in bottom waters of St Helena Bay (Figure 1). Studies since the 1970s (De Decker, 1970; Bailey and Chapman, 1985) revealed an upwelling-driven seasonality with lower oxygen concentrations and an associated expansion of the OMZ at the end of the main upwelling season in summer (January to April), and higher values between June and October when upwelling is weaker or absent (Bailey, 1991; Pitcher and Probyn, 2011; Pitcher et al., 2014; Lamont et al., 2015). In contrast to many other OMZ in the world’s ocean (Stramma et al., 2008), oxygen concentrations in the SBUS reveal no trend during the last 50 years, but an increase in the frequency of periodic anoxic events was observed (Pitcher et al., 2014). They are often associated with the development of massive blooms of dinoflagellates referred to as red tides (Fawcett et al., 2007; Pitcher and Probyn, 2011; Pitcher et al., 2014). These blooms emerge at the end of the upwelling season and are confined to near shore calm conditions which develop in the shadow of wind-driven upwelling plumes (Pitcher and Nelson, 2006). During such events, the OMZ expands vertically over the entire water column. In contrast to the bottom water OMZ, decreasing oxygen concentrations in the upper water column were inferred from a southward shift of the rock lobster distribution and an increased frequency of rock lobster walkouts in response to anoxic events (Cockcroft, 2001; Cockcroft et al., 2008; Hutchings et al., 2009).

Here we compile data obtained from a research cruise with the German research vessel RV Meteor (M153) and a moored oxygen sensor (Figure 1) to demonstrate that nutrient trapping occurs in a quite stable and narrow strip along the coast, and that its seasonal cycle and response to global changes stabilizes the SBUS OMZ.

The BUS stretches from the north of Cape Frio (∼15°S) to Cape Agulhas (∼35°S), while south-easterly trade winds emanating from the interplay between the South Atlantic Anticyclone and the continental low pressure trough, cause the emergence of distinct upwelling cells along the coast (e.g., Shannon and Nelson, 1996; Veitch et al., 2009; Kämpf and Chapman, 2016). However, South Atlantic Central Water feeds upwelling, which is a Sub-Antarctic Mode Water. It can be considered as a mixture of mainly Antarctic Intermediate Water and Subtropical Mode Water (McCartney, 1977; Karstensen and Quadfasel, 2002; Souza et al., 2018). These water masses are subducted beneath subtropical surface waters north of the Sub-Antarctic Front at about 36°S–54°S and are fed via the South Atlantic Current into the Benguela Current (e.g., Gordon, 1981; Donners et al., 2005). The Benguela Current splits into the Benguela Oceanic Current and the Benguela Coastal Current in the northern part of the SBUS (e.g., Hardman-Mountford et al., 2003; Veitch et al., 2010). In the southern part of the SBUS, the Benguela Current follows the shelf break toward the equator while the Cape Jet flushes the SBUS shelf (Figure 1A). The Cape Jet originates from the Agulhas Current and carries also Indian Ocean Central water into the SBUS (Durgadoo et al., 2017). It has a similar origin as the South Atlantic Central Water and both share almost the same temperature and salinity (θ-S) characteristics (McCartney, 1977; Harms et al., 2019). However, mostly in summer, the Cape Jet splits into two branches at Cape Columbine. The Columbine Front separates the inshore and offshore Cape Jet, whereas the Shelf Break Front separates Benguela Current from the Cape Jet (Shannon, 1985). According to the θ-S characteristics and the resulting density, the wide shelf north of Cape Columbine is flushed by Sub-Antarctic Mode Water and Modified Upwelled Water (Lamont et al., 2015). The latter is upwelled Sub-Antarctic Mode Water that was modified by its interaction with the atmosphere and is subducted beneath Open Ocean Surface Water at the oceanic front (Duncombe Rae, 2005; Lamont et al., 2015). This front separates the Benguela Current from the subtropical gyre and is considered as the outer boundary of the BUS (Barange and Pillar, 1992; Shannon and Nelson, 1996). However, some authors include the Modified Upwelled Water into their definition of South Atlantic Subtropical Mode Water (Flynn et al., 2020), whereas other authors suggest that the core properties of South Atlantic Subtropical Mode Water occurs only in a small wedge between the Sub-Antarctic Mode Water and Modified Upwelled Water at the outer shelf (Lamont et al., 2015).

During the last two decades satellite data indicate no significant trends in productivity (Demarcq, 2009; Verheye et al., 2016; Lamont et al., 2019) but there is a tendency toward an intensification of upwelling in the SBUS (Sydeman et al., 2014; Tim et al., 2015; Wang et al., 2015; Lamont et al., 2018, 2019). Associated with this trend are shifts in the ecosystem structure such as an overall decrease in zooplankton abundance and a shift from large to smaller zoo- and phytoplankton species (Hutchings et al., 2012; Jarre et al., 2015; Verheye et al., 2016; Lamont et al., 2019). However, as mentioned before, oxygen concentrations within the bottom water OMZ reveal no clear trend.

During the RV Meteor cruise M153 shipboard measurements were conducted along a station grid in the SBUS between February 16th and 28th, 2019 (Figure 1B). This included 25 stations, continuous underway measurements by the use of on-board underway systems, as well as one TRIAXUS transect and the deployment of one mooring. CTD casts were carried out to characterize spatial and depth-related changes of the environmental conditions (temperature, salinity, and dissolved oxygen concentrations). In order to study the diurnal cycle, four CTD casts were conducted between February 18th and 19th at station 7 close to the mooring location.

Water samples were collected with Niskin bottles attached to a rosette water sampler. The profiling SEABIRD ELECTRONICS (SBE) 911plus CTD system was equipped with a DIGIQUARTZ pressure sensor, a SBE3 temperature and SBE4 conductivity sensor within a double sensor setup, a PAR sensor (QSR 2300) and a SBE43 dissolved oxygen sensor. In order to calibrate the SBE43, concentrations of dissolved oxygen were analyzed in 63 water samples by using the Winkler titration method.

Additionally, water samples were taken to analyze nutrient concentrations. All nutrient samples were filtered immediately after sampling, using 0.45 μm disposable syringe filters, filled in pre-rinsed 50 ml PE bottles and frozen at −20°C. Subsequently, samples were sent to Germany to be analyzed by means of a continuous flow injection system (SKALAR SAN PLUS SYSTEM) according to methods described by Grasshoff et al. (1999).

The TRIAXUS was equipped with a SEABIRD FASTCAT, SBE 49, AANDERAA OXYGEN OPTODE 4330 F, and a SEASCAN SUNADeep Nitrate Sensor. This high speed remotely operated towed vehicle was towed at a speed of 8 knots with a horizontal offset out of the vessel’s wake. It undulated vertically between 5 and 180 m, depending on the water depth. In order to calibrate sensors attached to TRIAXUS and the underway system, their records were correlated with CTD data collected approximately at the same sites and water-depths (Figure 3). This calibration included additional data obtained in the NBUS, which are not further discussed in this paper.

The mooring was deployed off Hondeklip Bay (17.02°E/30.64°S) and equipped with an oxygen and temperature sensor (Mini Dot Oxygen Sensor, MDO) and Hydrobios MST-12 sediment trap at 95 and 96 m, respectively (∼74 m above the sea floor). The mooring was recovered during a research cruise with the South African research vessel RS Algoa (ALG 269) in October 2020. The mooring recorded dissolved oxygen concentrations and seawater temperatures with a temporal resolution of 10 min (Figure 4). The Hydrobios MST-12 sediment trap was equipped with 12 sample bottles which were filled with a NaCl (35 g/l) and poisoned with HgCl₂ (3.3 g/l) to prevent degradation of the trapped material. The sampling covered the period between March 2019 and May 2020 with an interval of 40 days. After recovery, the samples were shipped to Germany and, according to common sediment trap processing procedures, split into fractions of > and < 1 mm (Haake et al., 1993; Honjo et al., 2008). The > 1 mm fraction contained larger swimmers and the < 1 mm fraction was assumed to represent the gravitationally driven export of particles (= particle flux).

Near real-time satellite-derived concentrations of chlorophyll a and sea surface temperature were obtained from CERSAT (Centre de Recherche et d’Exploitation Satellitaire) at IFREMER Plouzane (France) and CMEMS (Copernicus Marine Environment Monitoring Services), respectively, and used to identify oceanographic features during cruise M153 (Figure 1). Furthermore, satellite-derived sea surface temperature (SST) and primary production rates (PP) were and downloaded from the MODIS-Aqua website in August 2021¹ and the ocean primary production website in August 2020². These data sets cover the periods between 2002 and 2021 (MODIS-Aqua) as well as 2019 (ocean primary production) and were used to calculate monthly means and illustrate seasonal cycles. Therefore, SST and PP were averaged for an area of approximately ± 1° around the mooring site. Additionally, the one arc-minute global relief model data of the Earth’s surface (ETOPO1)³ were downloaded in August 2020 and used to map the 1000 m depth counter line in order to identify the shelf break (Figure 1).

In line with data obtained from the literature, our data indicate that the Upwelling Front strongly reduces the cross shelf exchange and traps northward flowing waters in a shadow zone that emerged downstream of Cape Columbine in summer during the main upwelling season. Nevertheless, the SST seasonality is more strongly modulated by the seasonal cycle of solar insolation, with warming in summer and cooling in winter whereas upwelling of cold water reduces its amplitude by lowering water temperatures in summer. In the south around Cape Columbine this even reversed the seasonal SST cycle as indicated by in situ data. However, the pronounced Upwelling Front separates coastally trapped hypoxic waters from the still better-ventilated shelf waters west of the Upwelling Front, even though an increased stratification in association with enhanced export and decay of offshore advecting upwelling-driven plankton blooms decrease oxygen concentrations on the entire shelf in summer. High nitrate concentrations associated with the low oxygen concentrations and a rapid nutrient consumption in surface waters between the Upwelling Front and the coast indicate an efficient nutrient trapping. It explains the high upwelling-driven productivity and the preferential occurrence of anoxic events toward the end of the upwelling season in the SBUS. However, in winter, a weakening or even a collapse of the Upwelling Front favors the cross shelf exchange, which in addition to a reduced winter stratification and a lower productivity increases the ventilation of the coastally trapped waters. This reduces the risk of anoxia, which again gains momentum with the development of Upwelling Front at the beginning of following upwelling season. The proposed future intensification of upwelling and the observed changes in the ecosystem structure could reduce the nutrient trapping efficiency by favoring the advection of plankton blooms offshore across the Upwelling Front and even the Shelf Break Front. The resulting lower biological oxygen consumption on the shelf could mask the de-oxygenation trend that has been observed in other OMZ including those in the NBUS. Hence, the nutrient trapping efficiency appears to be a key element mitigating, so far, global change impacts on the SBUS and explains differing responses of upwelling systems to global change as seen e.g., in the comparison between the SBUS and NBUS. Underlying mechanisms seem to be the topographically steered and therewith locally varying physical boundary conditions and their influence on the development of frontal zones and the supply of upwelling feed waters as well as biogeochemical response to these boundary conditions. However essential biogeochemical processes, which in addition to physical boundary conditions control the nutrient trapping efficiency such as the export of organic carbon from offshore advecting plankton blooms and its decay in the water column, are still poorly understood. This in addition to the lack of long-term observations stresses the need for further studies in order to better understand the response of Eastern Boundary Upwelling Systems to global change.

The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found below: https://doi.org/10.1594/PANGAEA.935648.

TR, NL, TL, and KP designed the mooring experiment. RK, BM, and JvB conducted the TRIAXUS operations. LM and CS analyzed the data and samples obtained from the mooring. TL additionally evaluated satellite data. TR led the writing of the manuscript. All authors discussed the results and contributed to the writing of the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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