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A map of Algoa Bay showing the location of underwater temperature recorders (UTR) and two acoustic Doppler current profilers (ADCP) used in this study. The UTRs are each marked by crosses and three letters. A coastal weather station was located on Bird Island and wind data was also available from Port Elizabeth airport. The core of the Agulhas Current is generally located offshore the 200m isobath.
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Six large solitary meanders in the Agulhas Current, so-called Natal Pulses, propagated down the eastern coast of South Africa between 2009 and 2011. Their influence on the coastal waters between Port Alfred and Algoa Bay, on the far eastern Agulhas Bank, was measured by thermistor strings moored at 30–80 m bottom depths and two current metres (30 m...
Contexts in source publication
Context 1
... in the Agulhas Current (Lutjeharms, 2006) with an offshore extent of 30-300 km, propagate south-westwards along the east coast of South Africa (Fig. 1). They generally form off KwaZulu-Natal and to the north, but they may also form down- stream of Algoa Bay ( Lutjeharms et al., 1989). Very large solitary meanders, with amplitudes of 100-300 km (Gründlingh, 1979), are termed Natal Pulses ( Lutjeharms and Roberts, 1988). Natal Pulses occur on average 1.6-1.7 times per year ( Rouault and ...Context 2
... for this study were made by an array of un- derwater temperature recorders (UTRs) and two acoustic Doppler current profilers (ADCPs) located between Cape Recife and Port Alfred (Fig. 1). The shallow UTR moorings were deployed in 30 m bottom depth and the deeper UTRs in 60-80 m bottom ...Context 3
... not conform to the seasonal trend. Over these long periods, cold bottom water of between 10 °C and 16 °C periodically reached the surface (B and C on Fig. 3). These long periods also showed that warm surface waters of be- tween 18 °C and 22 °C (and higher) were recorded in Algoa Bay, even at sites close to the shoreline such as St. Croix (SCI in Fig. 1), for example D in Fig. 3. During events of this nature generally the whole water column was affected by both cooling and warming ...Context 4
... in Fig. 6 are low-pass filtered temperatures from the Algoa Bay Mouth mooring (ABM in Fig. 1) during the six Natal Pulses. The periods over which each plot were drawn covered the time during which the Natal Pulses were offshore of Algoa Bay, from the leading edge of the meanders to the trailing edge (de- fined in Fig. 2). The other deeper (60-80 m bottom depth) moor- ings, such as offshore Port Alfred (PAO), Bird Island (BIO) ...Context 5
... principal axes at 10 m depth at Cape Recife and Bird Island during five Natal Pulses were calculated using the method de- scribed by Kundu and Allen (1976) and plotted in Fig. 10. The currents were then oriented to the principal axes direction of 58/ 238° true north for Bird Island and 178/358° true north for Cape Recife. The principal axis direction were aligned with the local isobaths (as was found by Goschen et al., 2012) so this gave the current speed component parallel to the isobaths (and the shoreline) ...Context 6
... relationship between along-shore and across-shore cur- rents and temperatures is shown in Fig. 11. It is evident that there was great variability in the system, even at periods greater than 30 h to which the data was low-pass filtered. However, it is dis- cernible that at Bird Island the long-shore currents made an abrupt switch to the south-westward near the start of the active upwelling phase, on 14-15 December 2009 (Fig. 10a), ...Context 7
... is shown in Fig. 11. It is evident that there was great variability in the system, even at periods greater than 30 h to which the data was low-pass filtered. However, it is dis- cernible that at Bird Island the long-shore currents made an abrupt switch to the south-westward near the start of the active upwelling phase, on 14-15 December 2009 (Fig. 10a), 24-25 April (Fig. 10b), 27-28 June 2010 (Fig. 10c) and 30-31 August (Fig. 10d). These are marked as arrows on Fig. 11. At Bird Island the currents reached a maximum speed of 80 cm s À 1 during those events. At Cape Recife southward currents also reached a maximum speed of 80 cm s À 1 on 14-15 December 2009 and 21-22 January 201. Since ...Context 8
... It is evident that there was great variability in the system, even at periods greater than 30 h to which the data was low-pass filtered. However, it is dis- cernible that at Bird Island the long-shore currents made an abrupt switch to the south-westward near the start of the active upwelling phase, on 14-15 December 2009 (Fig. 10a), 24-25 April (Fig. 10b), 27-28 June 2010 (Fig. 10c) and 30-31 August (Fig. 10d). These are marked as arrows on Fig. 11. At Bird Island the currents reached a maximum speed of 80 cm s À 1 during those events. At Cape Recife southward currents also reached a maximum speed of 80 cm s À 1 on 14-15 December 2009 and 21-22 January 201. Since these maximum occurred ...Context 9
... great variability in the system, even at periods greater than 30 h to which the data was low-pass filtered. However, it is dis- cernible that at Bird Island the long-shore currents made an abrupt switch to the south-westward near the start of the active upwelling phase, on 14-15 December 2009 (Fig. 10a), 24-25 April (Fig. 10b), 27-28 June 2010 (Fig. 10c) and 30-31 August (Fig. 10d). These are marked as arrows on Fig. 11. At Bird Island the currents reached a maximum speed of 80 cm s À 1 during those events. At Cape Recife southward currents also reached a maximum speed of 80 cm s À 1 on 14-15 December 2009 and 21-22 January 201. Since these maximum occurred near the start of the ...Context 10
... system, even at periods greater than 30 h to which the data was low-pass filtered. However, it is dis- cernible that at Bird Island the long-shore currents made an abrupt switch to the south-westward near the start of the active upwelling phase, on 14-15 December 2009 (Fig. 10a), 24-25 April (Fig. 10b), 27-28 June 2010 (Fig. 10c) and 30-31 August (Fig. 10d). These are marked as arrows on Fig. 11. At Bird Island the currents reached a maximum speed of 80 cm s À 1 during those events. At Cape Recife southward currents also reached a maximum speed of 80 cm s À 1 on 14-15 December 2009 and 21-22 January 201. Since these maximum occurred near the start of the active upwelling period, it is ...Context 11
... to which the data was low-pass filtered. However, it is dis- cernible that at Bird Island the long-shore currents made an abrupt switch to the south-westward near the start of the active upwelling phase, on 14-15 December 2009 (Fig. 10a), 24-25 April (Fig. 10b), 27-28 June 2010 (Fig. 10c) and 30-31 August (Fig. 10d). These are marked as arrows on Fig. 11. At Bird Island the currents reached a maximum speed of 80 cm s À 1 during those events. At Cape Recife southward currents also reached a maximum speed of 80 cm s À 1 on 14-15 December 2009 and 21-22 January 201. Since these maximum occurred near the start of the active upwelling period, it is likely that the Natal Pulse had an ...Context 12
... of upwelling. Bottom temperatures may drop by as much as 9 °C. From this it is inferred that the onshore movement of colder bottom water from the shelf edge is Agulhas Current driven, probably accentuated in the bottom boundary layer by the increase in current speed and diverging isobaths in the measurement area. This is not inconsistent with the Fig. 11. Along-shore (black lines) and across-shore (grey lines) currents at depths of 10 m measured by the Bird Island (a-d) and Cape Recife (e, f) ADCPs during selected Natal Pulses. Temperatures measured throughout the water column at the same sites over the same period are shown as background. Currents were measured at both Bird Island (a) ...Citations
... The Agulhas Current influences coastal regions by forcing cold, nutrient-rich water onto the continental shelf (known as upwelling) or through eddies (Bryden et al. 2005). These meanders or eddies are usually recorded near Durban, South Africa (where the continental shelf is narrow) and travel further south along the Agulhas Shelf to Port Alfred and Algoa Bay, where they are known as Natal Pulses (Goschen et al. 2015). Upwelling is driven by the topographical forcing of the current in conjunction with wind stress (Lutjeharms 2006). ...
... The Agulhas Current waters are oligotrophic and when the current intrudes onto the continental shelf it is hypothesized to suppress primary productivity (Schumann et al. 2005). However, the Agulhas Current can also enhance primary productivity on the shelf through nutrient-rich upwellings (Probyn et al. 1994;Demarcq et al. 2003), which have been recorded around regions such as Richards Bay (Lamont et al. 2014;Lamont and Barlow 2015) and between Port Alfred and Algoa Bay (Goschen et al. 2015). Changes in productivity have a knock-on effect up the food chain and ultimately influence commercially important fish species (Krug et al. 2014). ...
... Fortunato et al. (2012) argued that the mixing between different habitats, in their case driven by upwelling, creates a vertical gradient in the bacterial communities. Mixing is particularly evident between the coast and the Agulhas Current system where the shearing zone responsible for upwelling is a highly turbulent mixed area (Goschen et al. 2015), which may explain this transition. ...
The Agulhas Current is a globally important western boundary current that influences numerous processes (e.g., upwelling, biogeochemical fluxes and distribution of marine taxa) in the southwestern Indian Ocean. The oceanographic processes of the Agulhas Current are well understood, but precisely how they influence coastal ecosystem productivity remains to be elucidated. In the present foundational study, we characterized the bacterial (16S rRNA gene) and phytoplankton (chloroplast 16S rRNA, rbcL, and eukaryotic 18S rRNA genes) community structures of the Agulhas Current system using a metabarcoding approach. All four markers provided consistent data on the bacterial and phytoplankton communities in the Agulhas Current and coastal sites. The study revealed distinct, conserved communities and similar patterns of dominance by taxa adapted to oligotrophic conditions within the Agulhas Current, sampled 2 yr apart. By contrast, there was significant variability in taxonomic diversity and abundance of phytoplankton communities in the adjacent coastal waters that could be linked to localized upwelling. While the Agulhas Current bacterial and phytoplankton communities were diverse and represented many functional groups, the coastal sites were more diatom‐dominated and included genera typically associated with upwelling, for example, Thalassiosira spp. Based on their relative abundance profiles, phytoplankton communities were more responsive to environmental variability than bacteria and may therefore prove more useful in linking ecosystem dynamics to environmental variability in marine systems.
... While seasonal longshore wind stress induces Ekman transport and pumping in the surface mixed layer, the deep~1 m/s Agulhas current on the southeast coast of South Africa can also induce upwelling [16][17][18]. Cyclonic shear draws water off the shelf, independent of wind forcing, and intensifies when the Agulhas current meanders shoreward [19][20][21]. ...
... Coupled with the coastal lows are continental shelf waves that pulse the upwelling [15]. While seasonal longshore wind stress induces Ekman transport and pumping in the surface mixed layer, the deep ~1 m/s Agulhas current on the southeast coast of South Africa can also induce upwelling [16][17][18]. Cyclonic shear draws water off the shelf, independent of wind forcing, and intensifies when the Agulhas current meanders shoreward [19][20][21]. ...
Coastal upwelling that cools sea temperatures and nutrifies the euphotic layer is the focus of this research, motivated by how these processes benefit the marine ecosystem. Here, atmosphere–ocean reanalysis fields and satellite radiance data are employed to link South African coastal upwelling with nearshore winds and currents in the 2000–2021 period. Temporal behavior is quantified in three regimes—Benguela, transition, and Agulhas—to distinguish the influence of offshore transport, vertical pumping, and dynamic uplift. These three mechanisms of coastal upwelling are compared to reveal a leading role for cyclonic wind vorticity. Daily time series at west, south, and east coast sites exhibit pulsing of upwelling-favorable winds during summer. Over the western shelf, horizontal transport and vertical motion are in phase. The south and east shelf experience greater cyclonic wind vorticity in late winter, due to land breezes under the Mascarene high. Ekman transport and pumping are out of phase there, but dynamic uplift is sustained by cyclonic shear from the shelf-edge Agulhas current. Temporal analysis of longshore wind stress and cyclonic vorticity determined that vertical motion of ~5 m/day is pulsed at 4- to 11-day intervals due to passing marine high/coastal low-pressure cells. Height sections reveal that 15 m/s low-level wind jets diminish rapidly inshore due to topographic shearing by South Africa’s convex mountainous coastline. Mean maps of potential wind vorticity show a concentration around capes and at nighttime, due to land breezes. Air–land–sea coupling and frequent coastal lows leave a cyclonic footprint on the coast of South Africa that benefits marine productivity, especially during dry spells with a strengthened subtropical atmospheric ridge. This work has, for the first time, revealed that South Africa is uniquely endowed with three overlapping mechanisms that sustain upwelling along the entire coastline. Amongst those, cyclonic potential vorticity prevails due to the frequent passage of coastal lows that initiate downslope airflows. No other coastal upwelling zone exhibits such a persistent feature.
... Another forcing mechanism for upwelling are meanders (also called pulses or rings) that arise off Natal in the Agulhas current. They are of the order 30km in diameter, and when these arrive at Algoa Bay the leading edge thereof initiates upwelling near Bird Island, and strong SW currents of 0.8m/s [47]. The meanders themselves travel at a typical speed of 10-20 km/day, up to 65 km/day, and the bulk of the Agulhas current travels around them on the seaward side [47]. ...
... They are of the order 30km in diameter, and when these arrive at Algoa Bay the leading edge thereof initiates upwelling near Bird Island, and strong SW currents of 0.8m/s [47]. The meanders themselves travel at a typical speed of 10-20 km/day, up to 65 km/day, and the bulk of the Agulhas current travels around them on the seaward side [47]. For the Algoa Bay, the more intense thermal gradients occur in summer, whereas more isothermal conditions tend to apply in winter due to the prevalence of the SW wind [21]. ...
... Existing models of upwelling are necessarily simplistic [47] and in reality there are likely to peculiarities and progressively ner level ows, as is universally typical for turbulence. In the Eastern Cape location under examination, prior to the cold front there are westerly winds that drive upwelling. ...
Context: Harmonic summation and amplification by winds blowing contrary to currents are known contributions to rogue waves, but the causes of the observed wave steepness, asymmetric form, and non-breaking are poorly understood. The potential effect of bathymetric and meteorological features has not been addressed.
Method: Vortex theory was applied qualitatively to the weather and ocean conditions of the Agulhas region.
Results: Rogue wave formation is attributed to: (1) Wind lee vortices cause steepening of wave leeward face, and suppresses wave breaking. (2) Boundary layer vortices from the meteorological cold front transfer energy to the wind lee vortices thereby enhancing their wave sharpening effect. (3) Agulhas current boundary layer vortices interact with water lee vortices to accelerate a jet of water between them, thereby steepening the wave and enhancing the preceding trough. (4) Bathymetric topology, especially a canyon on the continental slope, generates a vortex in the Agulhas current. This vortex is detached from the canyon by prising of the coastal downwelling current (induced by the meteorological cold front), and combines with the water lee vortex to heighten the wave. (5) Jetting arises when the canyon vortex and the Agulhas current boundary layer vortices pass each other, thereby accentuating wave height, steepness, and asymmetry.
Conclusions: The novel contribution is the provision of a mechanism for rogue wave formation, using vortex theory, that conceptually integrates wave formation, Agulhas sea currents, bathymetric features including submarine canyons, and meteorological cold front weather systems.
... Persistent upwelling occurs at the headwaters of the current near Port Elizabeth, South Africa, while wind-driven instances of upwelling occur in areas where the shape of the coastline allows for it (Lutjeharms, 2007;Schumann, 1982). The Agulhas Current also experiences a phenomenon known as Natal Pulses, which are large meanders beginning in the Natal Bight that then flow southwest along the coast and create cold water eddies close to the coast that cause upwelling (Goschen et al., 2015;van Leeuwen et al., 2000). These pulses occur between 1 and 6 times per year and can influence the ocean climate of the Agulhas Bank for up to 110 days (Krug et al., 2014). ...
... From these observations, we conclude that while primary productivity is greater at site U1479 throughout our record, terrigenous input is greater in the Natal Bight on the same timescale. This is consistent with modern observations and paleorecords of productivity and continental sediment influx for both core sites (Goschen et al., 2015;Gunn et al., 2020;Hood et al., 2017;Krug et al., 2014;Lutjeharms et al., 2000;Meyer et al., 2002;Vinayachandran et al., 2021;Wells et al., 2024). ...
The Agulhas Current is one of the strongest currents in the southern hemisphere and is responsible for the transport of waters along the southeastern coast of Africa. The sediment load of the Agulhas Current and its interactions with other water masses in the region are essential to primary productivity along Africa’s southern coast. Despite this importance, there are still many discrepancies in our understanding of how sediment burial and nutrient variability have been impacted by regional and global changes in climate. The focus of this research is to explore the connections between climate and elemental burial from the Pleistocene through the Holocene, with an emphasis on phosphorus and metal burial over the last 1.1 mya.
Two ocean sediment cores from the International Ocean Discovery Program (IODP) Expedition 361 were selected based on their proximity to the Agulhas Current. Site U1474 lies within the flow of the current, while Site U1479 lies within the Agulhas Ring Corridor, outside of its direct influence. Different downcore elemental concentrations were collected using a combination of X-Ray Fluorescence (XRF) and UV-Visible Spectroscopy. Site U1474 displayed strong dependence on southeastern African hydroclimate changes and orbitial eccentricity. Site U1479 also displayed dependence on these processes, in addition to changes in the strength of Agulhas Leakage and glacial-interglacial variability. Additionally, we explore the teleconnections between both core sites and how processes upstream can affect productivity and terrigenous burial dynamics downstream.
... Macroalgae, particularly the red seaweed Plocamium corallorhiza and other fleshy species dominate the shallow rocky areas ( Fig. 1 detailed in Edworthy et al., 2022). Cape Recife is influenced by upwelling and associated algal bloom events (Goschen et al., 2015;Goschen and Schumann, 2011), as well as anthropogenic inputs from wastewater treatment works (Lemley et al., 2019). The macroalgal reef around Cape Recife is an important nursery habitat for juvenile sparids, including Diplodus capensis (Beckley, 1985;Beckley and Buxton, 1989;James et al., 2024;Mkhize et al., 2024). ...
Fishes have shown varying responses to the decline in seawater pH associated with ocean acidification. Coastal marine species inhabit characteristically dynamic environments which requires physiological adaptation to variability, including fluctuations in pH and associated carbonate chemistry parameters. Our study assessed the response of the early life stages (postflexion) of a common coastal fish species (Diplodus capensis) that is found in coastal nearshore and estuarine habitats along the South African coastline. We assessed their metabolic and behavioural response to a range of pH conditions which covered a high pH (8.02), the lowest pH that they are naturally exposed to (7.75) as well as extremely low pH levels (∼7.75–7.27) exceeding their current range of exposure, which may occur with future coastal acidification. Our findings suggest that this species is metabolically tolerant of acute low pH conditions (down to 7.27 pH) showing no changes in either routine or active metabolic rates. Although our study identified a slight increase in swimming activity in D. capensis larvae exposed to low pH, there was no change in feeding activity. These results suggest that this species may have the physiological capacity to withstand the present and future high pH variability associated with its environments, in the absence of other stressors and ecological interactions. However, the increased swimming activity at low pH may translate into downstream ecological impacts, for which the mechanisms need to be assessed with further research.
... While such associations with a salinity gradient have been previously proposed in other marine species including the blue skate Dipturus batis (Delaval et al. 2022), the copper shark Carcharhinus brachyurus (Klein et al., 2024), and several dolphin species (Barcelo et al. 2022;Pratt et al. 2022), limited information exists regarding the relationships between environmental variables and the behavior of S. zygaena . Across the regional species distribution of S. zygaena , the Agulhas Current has been shown to significantly shape oceanographic conditions along the south-east coast of South Africa (Lutjeharms et al., 2001;Popova et al., 2016;Rouault et al., 2010), with prominent upwelling observed at the northern end of the Natal Bight (Lutjeharms et al., 2000a) and across the Agulhas Bank (Lutjeharms et al., 1996;Goschen et al., 2015). These upwelling processes transport nutrient-rich water to the surface, significantly affecting productivity and nutrient concentrations in the region (Lutjeharms et al., 1996(Lutjeharms et al., , 2000bBarlow et al., 2010). ...
Globally, hammerhead sharks have experienced severe declines owing to continued overexploitation and anthropogenic change. The smooth hammerhead shark Sphyrna zygaena remains comparatively understudied compared to other members of the family Sphyrnidae, and despite its Vulnerable status, a comprehensive understanding of its genetic landscape remains lacking. The present study aimed to conduct a fine-scale genomic assessment of Sphyrna zygaena within the highly dynamic marine environment of South Africa’s coastline, using thousands of single nucleotide polymorphisms (SNPs) derived from restriction site-associated DNA sequencing (3RAD). A combination of differentiation-based outlier detection methods (OUTFlank and pcadapt) and Genotype-Environment Association (GEA) (Redundancy Analysis) analysis in Sphyrna zygaena were employed. Subsequent assessments of putatively adaptive loci revealed a distinctive south to east genetic cline. Amongst these, notable correlations between adaptive variation and sea-surface dissolved oxygen and salinity, in addition to spatial factors were evident. Conversely, analysis of 110, 965 neutral SNP markers revealed a lack of regional population differentiation, a finding that remained consistent across various analytical approaches, including an assessment of isolation-by-distance (IBD) and isolation-by-environment (IBE), genetic clustering analyses (LEA, fastSTRUCTURE, and find.clusters), and a discriminant analysis of principal components (DAPC). These results provide evidence for the presence of differential selection pressures within a limited spatial range, despite high gene flow implied by the selectively neutral dataset. This study offers notable insights regarding the potential impacts of genomic variation in response to fluctuating environmental conditions in the circumglobally distributed Sphyrna zygaena.
... 2-4). Although wind-driven upwelling can occur at any of these locations, the potential of the Agulhas Current to also contribute to coastal upwelling decreases as the continental shelf widens from Port Alfred towards Plettenberg Bay [14][15][16]18 (Fig. 1). ...
... In both WBCs, sharks swam consistently closer to the surface while migrating through upwelling zones, especially when encountering upwelling cells (verified by corresponding records from nearby temperature loggers) compared with when in subtropical and tropical areas, where animals maintained deeper depths, including sharks tagged at Pinnacle Reef, which never moved into upwelling zones (Figs. 5 and 6, and Supplementary Figs. [10][11][12][13][14]. Shallower dive profiles (Extended Data Fig. 1) in upwelling areas presumably minimize the risk of encountering rapid temperature drops, as upwelling does not always reach surface waters 15 . Moreover, three sharks tagged in Breede River showed deviations from rapid, unidirectional migrations through upwelling zones when encountering upwelling cells, seeking refuge in estuaries and bays (Supplementary Figs. 15 and 16 and Supplementary Tables 9-14). ...
... In the case of the Agulhas Current, coastal upwelling can be caused by current forcing as well as by meanders in the current 16,18,30 . Such meanders can be associated with cold-core inshore eddies that travel southwards along the shelf, often called Natal pulses 15 . Current-driven coastal upwelling is most common along the southeast coast of South Africa due the proximity of the shelf edge to the coast 16,18,30 . ...
The impacts on marine species from secular warming and heatwaves are well demonstrated; however, the impacts of extreme cold events are poorly understood. Here we link the death of organisms from 81 species to an intense cold upwelling event in the Agulhas Current, and show trends of increasing frequency and intensification of upwelling in the Agulhas Current and East Australian Current. Using electronic tagging, we illustrate the potential impacts of upwelling events on the movement behaviour of bull sharks Carcharhinus leucas, including alterations of migratory patterns and maintenance of shallower dive profiles when transiting through upwelling cells. Increasing upwelling could result in ‘bait and switch’ situations, where climate change expands subtropical species’ distribution, while simultaneously exposing climate migrants to an increased risk of cold-mortality events at poleward distributional limits. This shows the potential impacts of increased cold events, an understudied aspect of climate change research, and highlights the complexities of climate change effects on marine ecosystems.
... On-shelf mixing of upwelled SAMW with the extant low-density and low-nitrate water (likely Tropical Surface Water) would lower the density and nitrate concentration of the mixture without noticeably changing its isotopic composition, consistent with our observations. Inshore upwelling events along the Agulhas Current are a well-known phenomenon (Goschen et al., 2015;Lutjeharms et al., 2000;Walker, 1986) and are primarily driven by wind and/or mesoscale current meanders (Leber et al., 2017). Before and during our sampling of the ASCA16 transect, the winds were weak and variable (not shown), and the current did not experience a mesoscale meander (which would have displaced it from the coast by ∼100 km; , such that neither mechanism could have facilitated the upwelling. ...
The Agulhas Current, like other western boundary currents (WBCs), transports nutrients laterally from the tropics to the subtropics in a subsurface “nutrient stream.” These nutrients are predominantly supplied to surface waters by seasonal convective mixing, to fuel a brief period of productivity before phytoplankton become nutrient‐limited. Episodic mixing events characteristic of WBC systems can temporarily alleviate nutrient scarcity by vertically entraining deep nutrients into surface waters. However, our understanding of these nutrient fluxes is lacking because they are spatio‐temporally limited, and once they enter the sunlit layer, the nutrients are rapidly consumed by phytoplankton. Here, we use a novel application of nitrate Δ(15–18), the difference between the nitrogen and oxygen isotope ratios of nitrate, to characterize three (sub)mesoscale events of upward nitrate supply across the Agulhas Current in winter: (1) mixing at the edges of an anticyclonic eddy, (2) inshore upwelling associated with a submesoscale meander of the Agulhas Current, and (3) overturning at the edge of the current core driven by submesoscale instabilities. All three events manifest as upward injections of high‐Δ(15–18) nitrate into the thermocline and surface where nitrate Δ(15–18) is otherwise low; these entrainment events are not always apparent in the other co‐collected data. The dynamics driving the nitrate supply events are common to all WBCs, implying that nutrient entrainment facilitated by WBCs is quantitatively significant and supports productivity in otherwise oligotrophic subtropical surface waters. A future rise in energy across WBC systems may increase these nutrient fluxes, partly offsetting the predicted stratification‐induced decrease in subtropical ocean fertility.
... Sea temperatures on the south coast are strongly affected by several types of upwelling, as documented in numerous studies (e.g. Lutjeharms et al. 2000;Roberts 2010;Goschen et al. 2012Goschen et al. , 2015, including several focused on Algoa Bay (i.e. Schumann 1995, 2011;Schumann et al. 2005). ...
... The numbered coastal sections are depicted in Figure 1 then moves eastwards towards Port Alfred (Goschen et al. 2012). Goschen et al. (2015) documented upwelling in the Algoa Bay area caused by meanders in the Agulhas Current (i.e. large solitary meanders or Natal pulses). ...
... As a result, the west coast can be considered cold temperate, the south coast warm temperate, and the east coast subtropical (Cole & McQuaid, 2011;Emanuel et al., 1992;Stephenson et al., 1937). Both topographic and wind-driven upwellings do occur on the east and south coasts, however, with at least one site of semipermanent upwelling on the south coast (Goschen et al., 2015;Lutjeharms, 2006). Upwelling can also be associated with onshore/offshore meandering of the Agulhas Current (Goschen et al., 2015). ...
... Both topographic and wind-driven upwellings do occur on the east and south coasts, however, with at least one site of semipermanent upwelling on the south coast (Goschen et al., 2015;Lutjeharms, 2006). Upwelling can also be associated with onshore/offshore meandering of the Agulhas Current (Goschen et al., 2015). Despite the occurrence of upwelling and large grazer populations, only one study in South Africa has explicitly incorporated both grazing pressure and upwelling (Ndhlovu et al., 2021), and only a few others have explicitly manipulated grazing or nutrient availability (Bustamante et al., 1995;Dye, 1995;Maneveldt et al., 2009). ...
... This upwelling, along with wind-driven upwelling during periods of strong and persistent easterly winds, extends through Algoa Bay and has a lateral range 85-300 km of coastline (Lutjeharms, 2006). The frequency and intensity of upwelling vary seasonally, with cold, nutrient-rich water being detectable for a further 2-3 weeks after upwelling events (Goschen et al., 2015). This region is associated with elevated nutrient values, with nitrate levels exceeding 20 μmol/L compared with <5 μmol/L on the adjacent Agulhas Shelf (Lutjeharms, 2006). ...
Community structure is driven by the interaction of physical processes and biological interactions that can vary across environmental gradients and the strength of top‐down control is expected to vary along gradients of primary productivity. In coastal marine systems, upwelling drives regional resource availability through the bottom‐up effect of nutrient subsidies. This alters rates of primary production and is expected to alter algae–herbivore interactions in rocky intertidal habitats. Despite the potential for upwelling to alter these interactions, the interaction of upwelling and grazing pressure is poorly understood, particularly for warm‐temperate systems. Using in situ herbivore exclusion experiments replicated across multiple upwelling regimes, we investigated the effects of both grazing pressure, upwelling, and their interactions on the sessile invertebrate community and biomass of macroalgal communities in a warm‐temperate system. The sessile invertebrate cover showed indirect effects of grazing, being consistently low where algal biomass was high at upwelling sites and at nonupwelling sites when grazers were excluded. The macroalgal cover was greater at upwelling sites when grazers were excluded and there was a strong effect of succession throughout the experimental period. Grazing effects were greater at upwelling sites, particularly during winter months. There was a nonsignificant trend toward greater grazing pressure on early than later successional stages. Our results show that the positive bottom‐up effects of nutrient supply on algal production do not overwhelm top‐down control in this warm‐temperate system but do have knock‐on consequences for invertebrates that compete with macroalgae for space. We speculate that global increases in air and sea surface temperatures in warm‐temperate systems will promote top‐down effects in upwelling regions by increasing herbivore metabolic and growth rates.
