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Small copepod genera play an important role in marine food webs and biogeochemical fluxes but have been neglected in many studies. This is the first study determining biomass, carbon consumption and egestion rates of small- (2 mm PL) copepods along a cross-shelf transect in the southern Benguela upwelling system. Calanoids contributed on average 55 ± 19% to total copepod abundance and 82 ± 13% to total copepod biomass. Small-sized Oithona spp. (114–119 mg C m−2 d−1) and Clausocalanidae/Paracalanidae (87–263 mg C m−2 d−1) as well as large-sized Calanoides natalis (47–193 mg C m−2 d−1) were the dominant consumers at the most inshore stations. Small- and medium-sized copepodite stages of Metridia lucens were also important, especially towards the continental slope. At offshore stations, Clausocalanidae/Paracalanidae, Oithona spp., Pleuromamma spp., Calanus agulhensis, Acartia spp., C. natalis and M. lucens were dominant consumers. Hence, usually small- and medium-sized copepods dominated total copepod ingestion and egestion, emphasizing that inadequate representation of small copepods will lead to significant underestimations and misinterpretations of the functioning of zooplankton communities and finally to inadequate biogeochemical models.

The variability of the South Atlantic meridional overturning circulation and meridional heat transport measured across 34.5°S during 2013–2017 differs significantly between observational and ocean reanalysis estimates. Variability in an ocean reanalysis ensemble and an eddy-resolving reanalysis is similar to an altimeter-based estimate, but smaller than energy-budget and mooring-based estimates. Over 1993–2020, there is no long-term trend in the ensemble-mean overturning and heat transport, although there are inter-model differences, whereas the altimeter-based and energy-budget estimate transports increase over this period. Time-mean overturning volume transport (and the depth of maximum overturning) across 34.5°S in the ensemble and observations are similar, whereas the corresponding mean heat transports differ by up to 0.3 PW. The seasonal cycle of these transports varies between estimates, due to differences in the methods for estimating the geostrophic flow and the sampling characteristics of the observational approaches. The baroclinic, barotropic and Ekman MOC components tend to augment each other in mooring-based estimates, whereas in other estimates they tend to oppose each other so the monthly-mean, inter-annual and seasonal MOC anomalies have a greater magnitude in the mooring-based estimates. Thus, the mean and variation of real world South Atlantic transports, and the amplitude of their fluctuations, are still uncertain. Ocean reanalyses may be useful tools to understand these differences and the mechanisms that control volume and heat transport variability in the South Atlantic, a region critical for determining the global overturning pathways and inter-basin transports.

Phytoplankton play a central role in the planetary cycling of important elements and compounds. Understanding how phytoplankton are responding to climate change is consequently a major question in Earth Sciences. Monitoring phytoplankton is key to answering this question. Satellite remote sensing of ocean colour is our only means of monitoring phytoplankton in the entire surface ocean at high temporal and large spatial scales, and the continuous ocean-colour data record is now approaching a length suitable for addressing questions around climate change, at least in some regions. Yet, developing ocean-colour algorithms for climate change studies requires addressing issues of ambiguity in the ocean-colour signal. For example, for the same chlorophyll-a concentration (Chl-a) of phytoplankton, the colour of the ocean can be different depending on the type of phytoplankton present. One route to tackle the issue of ambiguity is by enriching the ocean-colour data with information on sea surface temperature (SST), a good proxy of changes in three phytoplankton size classes (PSCs) independent of changes in total Chl-a, a measure of phytoplankton biomass. Using a global surface in-situ dataset of HPLC (high performance liquid chromatography) pigments, size-fractionated filtration data, and concurrent satellite SST spanning from 1991 to 2021, we re-tuned, validated and advanced an SST-dependent three-component model that quantifies the relationship between total Chl-a and Chl-a associated with the three PSCs (pico-, nano- and microplankton). Similar to previous studies, striking dependencies between model parameters and SST were captured, which were found to improve model performance significantly. These relationships were applied to 40 years of monthly composites of satellite SST, and significant trends in model parameters were observed globally, in response to climate warming. Changes in these parameters highlight issues in estimating long-term trends in phytoplankton biomass (Chl-a) from ocean colour using standard empirical algorithms, which implicitly assume a fixed relationship between total Chl-a and Chl-a of the three size classes. The proposed ecological model will be at the centre of a new ocean-colour modelling framework, designed for investigating the response of phytoplankton to climate change, described in subsequent parts of this series of papers., This work is supported primarily by a UKRI Future Leader Fellowship (MR/V022792/1). Additional supports from the UK National Centre for Earth Observation (NCEO), the Simons Foundation Project Collaboration on Computational Biogeochemical Modeling of Marine Ecosystems (CBIOMES, 549947, Shubha Sathyendranath), and Royal Society International Exchanges 2021 Cost Share (NSFC) grant (IEC NSFC 211058) are acknowledged. Astrid Bracher and Vanda Brotas are funded by the European Union’s Horizon 2020 Research and Innovation Programme (N810139): Project Portugal Twinning for Innovation and Excellence in Marine Science and Earth Observation (PORTWIMS). Astrid Bracher is also funded by the ESA 656 S5P+Innovation Theme 7 Ocean Colour (S5POC) project (No. 4000127533/19/I-NS). The AWI in-situ data are supported by the Helmholtz Infrastructure Initiative FRAM. Tarron Lamont and Ray Barlow acknowledge funding, logistical, and administrative support from the South African National Department of Forestry, Fisheries and the Environment (DFFE), Bayworld Centre for Research and Education (BCRE), and the South African National Research Foundation (NRF grants: 129229 and 132073). Fang Shen is funded by National Natural Science Foundation of China (No. 42076187 and No. 41771378) for the sampling of in-situ data in eastern China seas. The Atlantic Meridional Transect (AMT) is funded by the UK Natural Environment Research Council (NERC) through its National Capability Long-term Single Centre Science Programme, Climate Linked Atlantic Sector Science (grant number NE/R015953/1) to Plymouth Marine Laboratory. This work contributes to the international IMBeR project and is contribution number 386 of the AMT programme. The authors would like to acknowledge all the contributors who have shared in-situ data to the public domains, including the Western Channel Observatory, TARA Ocean, Rothera Research Station, NASA SeaBASS, ADON, Government of Canada, DATAONE, BCODMO, EDI Data Portal, PANGAEA, and BODC, and all the scientists and crew who were involved in the collection of in-situ data are sincerely appreciated. We thank NOAA for providing daily and monthly OISST (version 2) SST data; ESA for providing monthly SST-CCI (version 2.1) SST data; ESA for providing monthly OC-CCI (version 5.0) Chl-a climatology data; GEBCO for providing bathymetric data (GEBCO2021 Grid). The authors also thank Hongyan Xi for providing comments and suggestions on the early version of the manuscript.

Located between the sub-Antarctic Front and the Antarctic Polar Front, the Prince Edward Islands (PEIs) provide an essential breeding ground for top predators and are an ideal location to investiga...

Port Alfred upwelling, located on the southeast African shelf, lies on the inshore edge of a western boundary current, the Agulhas Current. It is a semi-permanent upwelling covering a width of 15-100 km. The mechanisms responsible for these cold water events have not been studied in depth due to the poor sampling of Port Alfred. Previous studies have relied on in situ measurements and remote sensing datasets and suggested that this upwelling was current-driven while other studies claimed that it could be wind-driven. Recently, it was also argued that it could be due to a combination of the current and wind forcing. In this study we aimed at identifying the dominant forcing drivers triggering this upwelling. Using the daily simulations of a CROCO model of a horizontal spatial resolution of ~2.5km, we explored its spatial and temporal characteristics. Building up from previous studies, we investigated several potential hypotheses from a modelling perspective. Some of these forcing mechanisms included the strength of the current, wind, the meandering of the current and the eddy. By performing a combined principal component analysis, we tested each hypothesis and found that a combination of current strength, meander and wind would lead to a stronger upwelling event, while some of the forcing mechanisms could trigger a weaker cold event individually. This phenomenon is known to cause an increase of nutrients at the surface, setting the ideal conditions for primary production, a significant oceanographic process for biodiversity. Shedding more light on these cold water events and the main drivers allows oceanographers to focus more attention on Port Alfred in the future. It will also reinforce policymakers to consider Port Alfred as a marine protected zone.

The KwaZulu-Natal sardine run, popularly known as the “greatest shoal on Earth,” is a mass migration of South African sardines from their temperate core range into the subtropical Indian Ocean. It has been suggested that this represents the spawning migration of a distinct subtropical stock. Using genomic and transcriptomic data from sardines collected around the South African coast, we identified two stocks, one cool temperate (Atlantic) and the other warm temperate (Indian Ocean). Unexpectedly, we found that sardines participating in the sardine run are primarily of Atlantic origin and thus prefer colder water. These sardines separate from the warm-temperate stock and move into temporarily favorable Indian Ocean habitat during brief cold-water upwelling periods. Once the upwelling ends, they find themselves trapped in physiologically challenging subtropical habitat and subject to intense predation pressure. This makes the sardine run a rare example of a mass migration that has no apparent fitness benefits.

Some upper trophic level predators, such as marine mammals, seabirds and large predatory fish have been shown to be useful sentinels for marine ecosystems. Through their demography, diet and foraging behaviour, ecosystem changes associated with climate change can be monitored. The local marine ecosystem at the Prince Edward Archipelago in the Southern Indian Ocean is heavily influenced by the position of the dynamic sub-Antarctic front (SAF) that varies inter-annually in terms of its influence on the local environment. If the SAF migrates southwards, as climate change models have predicted, the abundance of autochthonous and allochthonous prey found within the vicinity of the archipelago will decrease and increase, respectively. We investigated the diet of an inshore forager at the archipelago, the gentoo penguin Pygoscelis papua, as a possible sentinel to the expected changes in the prey assemblages in local waters, due to the southward migration of the SAF. We collected stomach content samples from gentoo penguins, using the water off-loading technique, monthly over three years (1994–1996) and then annually during 2012, 2014 and 2015. Prey assemblages within the diet was found to coincide with the proximity of the SAF to the Prince Edward Archipelago as well as the annual life cycles of prey. Due to the plasticity these penguins exhibit in their diet and foraging behaviour, we suggest that they are important sentinel species for the local marine ecosystem of the Prince Edward Archipelago. Regular sample collection from these birds, albeit stomach content samples and/or tissues for stable isotope analysis, should be collected so that variability in the ecosystem can be easily monitored.

Ecosystems within the Southern Ocean, such as the Prince Edward Islands (PEIs), fulfil a pivotal role in sustaining rich environments with large populations of top predators. Many of these have already undergone dramatic shifts in response to changes in climate, highlighting the crucial need to enhance our understanding of drivers of oceanographic variability in the region, as well as their impacts on biological communities. Daily averaged bottom temperatures and water-column current speeds from two moorings in the inter-island region, between April 2014 and April 2018, present the first continuous set of high temporal resolution in situ measurements at the PEIs. These data were investigated to determine the influence of passing deep-sea mesoscale eddies and fronts on hydrographic conditions on the inter-island shelf. Warming/cooling events of the order of 0.5–2 °C, sometimes lasting longer than 30 days at a time, concomitant with changes in current speed and direction were associated with advection of waters into the shelf region from passing anticyclonic/cyclonic eddies. Similarly, the impact of frontal movement was quantified by increased current speeds throughout the water column when the southern branch of the sub-Antarctic Front (S-SAF), or the northern branch of the Antarctic Polar Front was in close proximity to the islands. When the S-SAF was located north of the PEIs, bottom temperatures were overall lower due to the stronger influx of Antarctic surface and intermediate waters. In contrast, when the S-SAF was close to or south of the PEIs, bottom temperatures were elevated as a result of the occurrence of larger proportions of warmer, more saline surface and intermediate sub-Antarctic and even Subtropical waters. Notably, the predominance of westerly flow in the southern portion of inter-island region suggested the perpetual existence of a Taylor column which was at times enhanced by the juxtaposition of some eddies.

Eastern Boundary Upwelling Systems (EBUS) are well-known for their high productivity and fishery yields. However, being scarcely sampled and poorly represented in global models, their role as CO2 sources and sinks to the atmosphere remains elusive. Here, we present a compilation of shipboard measurements over the past two decades, showing how the Benguela Upwelling System (BUS) in the southeast Atlantic Ocean acts as a CO2 source in the north and CO2 sink in the south. Surface warming of upwelled waters increases the partial pressure of CO2 (pCO2) and outgassing in both regions, but in the south, the biologically-mediated drawdown of CO2 exceeds this warming effect. Here, the biological carbon pump owes its stronger impact on pCO2 to higher shares of upwelling source waters carrying preformed nutrients supplied from the Southern Ocean. Their formation increases pCO2 in surface waters and counteracts human-induced invasion of CO2 in the Southern Ocean. However, their utilization in the BUS compensates for over 20% of the CO2 loss occurring in the Atlantic sector of the Southern Ocean. This emphasizes the role of the BUS as key to improve our understanding of the ocean’s response to climate change and the future evolution of CO2 in the atmosphere.

Variations in the Meridional Overturning Circulation (MOC) are known to have important impacts on global scale climate phenomena including precipitation patterns, surface air temperatures, coastal sea level, and extreme weather. The MOC flow structure in the South Atlantic is thought to control the stability of the entire global MOC system. Given this importance, significant resources have been invested on observing the MOC in the South Atlantic over the past decade. Multiple years of full-depth daily observations from moored instruments at 34.5°S are used to calculate the meridional transports near the western and eastern boundaries, as well as the basin-wide interior transports, via geostrophic methods. These transport estimates are combined with Ekman transports derived from satellite wind products to yield daily estimates of the total meridional transports. Analysis of the MOC volume transport using all available moored instruments from 2013 to 2017 allows us to quantify for the first time the daily volume transport of both the upper and abyssal overturning cells at 34.5°S. The structure of these flows is characterized in unprecedented detail; no statistically significant trend is detectable in either cell. Abyssal-cell transport variability is largely independent of the transport variability in the upper-cell. Analysis of this new data set is crucial for improving our understanding of the temporal and spatial scales of variability that governs MOC related flows, and for disentangling their respective roles in modulating its overall variability.

We present the first-ever daily time series of the full-depth volume transports by the overturning circulation cells at 34.5°S., The Meridional Overturning Circulation (MOC) is a primary mechanism driving oceanic heat redistribution on Earth, thereby affecting Earth’s climate and weather. However, the full-depth structure and variability of the MOC are still poorly understood, particularly in the South Atlantic. This study presents unique multiyear records of the oceanic volume transport of both the upper (~3100 meters) overturning cells based on daily moored measurements in the South Atlantic at 34.5°S. The vertical structure of the time-mean flows is consistent with the limited historical observations. Both the upper and abyssal cells exhibit a high degree of variability relative to the temporal means at time scales, ranging from a few days to a few weeks. Observed variations in the abyssal flow appear to be largely independent of the flow in the overlying upper cell. No meaningful trends are detected in either cell.

The variability of the Atlantic Meridional Overturning Circulation (AMOC) has considerable impacts on the global climate system. Past studies have shown that changes in the South Atlantic control the stability of the AMOC and drive an important part of its variability. That is why significant resources have been invested in a South (S)AMOC observing system. In January 2017, the RV Maria S. Merian conducted the first GO‐SHIP hydrographic transect along the SAMOC‐Basin Wide Array (SAMBA) line at 34.5°S in the South Atlantic. This paper presents estimates of meridional volume, freshwater (MFT), and heat (MHT) transports through the line using the slow varying geostrophic density field and direct velocity observations. An upper and an abyssal overturning cell are identified with a strength of 15.64 ± 1.39 Sv and 2.4 ± 1.6 Sv, respectively. The net northward MHT is 0.27 ± 0.10 PW, increasing by 0.12 PW when we remove the observed mesoscale eddies with a climatology derived from the Argo floats data set. We attribute this change to an anomalous predominance of cold core eddies during the cruise period. The highest velocities are observed in the western boundary, within the Brazil and the Deep Western Boundary currents. These currents appear as a continuous deep jet located 150 km off the slope squeezed between two cyclonic eddies. The zonal changes in water masses properties and velocity denote the imprint of exchange pathways with both the Southern and the Indian oceans. Key Points: ● Overturning maximum is 15.64 ± 1.39 Sv; Meridional heat and freshwater transport are 0.27 ± 0.10 PW and 0.23 ± 0.02 Sv, respectively ● Excluding the mesoscale eddies from the section increased the meridional heat transport by 0.12 PW ● The distribution of water masses and currents reflects the favorable position of the section for observing

At the sub-Antarctic Prince Edward Islands (PEIs), a diverse ecosystem is supported by enhanced phytoplankton biomass levels, commonly referred to as the island mass effect. Phytoplankton responses to varied environmental conditions during austral summer (December 2008) and autumn (April–May 2013–2018) cruises were characterised by both surface and subsurface chlorophyll a (chla) maxima. While subsurface chla maxima were generally near the base of the upper mixed layer (zuml) during the summer cruise, such maxima were located well below zuml during autumn cruises. Non-photochemical quenching resulted in severe underestimation of surface in situ chla. Substantially low Fv/Fm and σPSII in the surface layers during December 2008 confirmed the photoinhibitory effects of high light exposure during the day and suggested damage to phytoplankton photosystem II reaction centres. Physiological measurements revealed that phytoplankton communities were probably not limited by nitrate or iron. However, some nutrient stress may have resulted from low surface silicates, which showed comparatively more daytime depletion during December 2008 than April–May 2013–2015. While the spatial distribution of in situ phytoplankton biomass during December 2008 was reminiscent of the island mass effect, congruent satellite observations surprisingly revealed that chla in the open ocean surrounding the PEIs was in fact much higher than chla on the island shelf. Our findings suggested that upwelling and retention, and consequent phytoplankton biomass accumulation, usually associated with a Taylor column structure at the PEIs may have been disrupted by enhanced offshore advection stemming from the interaction of mesoscale eddies and fronts with the PEI shelf. However, more detailed investigations are required to properly understand the spatial and temporal variations of phytoplankton biomass and their physiological responses to environmental fluctuations.

The California Current System (CCS) has two independent seasonal modes of upwelling variability, summer and winter, driven by different atmospheric processes. The variability of upwelling winds during winter is particularly important as strong, episodic events, driven by atmospheric teleconnections with the equatorial Pacific that are active in this season, impact ecological systems along the west coast of North America. Given the importance of upwelling seasonality to ecosystem function, we hypothesize that the Benguela Current System (BCS) shows similar independent seasonal modes of upwelling variability. To test this hypothesis, compare modes of variability between systems, and investigate potential drivers, we use an upwelling index derived from NCEP2 wind data (1979–2014) for the northern, southern, and Agulhas Bank areas of the BCS. In the northern and southern BCS, only one mode of upwelling variability is observed: year-round in the north and during the austral spring and summer (October through April) in the south. The Agulhas Bank region shows two modes of seasonal variability. Based on this 35-year dataset, summer upwelling modes in both the CCS and BCS appear to have similar decadal-scale variability. The other modes of variability (winter mode in the CCS and the non-seasonal second mode in the BCS) are correlated with year-to-year variability in the positioning of regional oceanic high-pressure systems. The leading mode of upwelling variability in the Agulhas Bank region, in the austral summer/fall, is highly correlated with sea level pressure as well as sea surface temperature in the equatorial Pacific, in a spatial and seasonal pattern (boreal winter) resembling the El Nino-Southern Oscillation. Across the CCS, modes of upwelling variability are similar to one another, while modes differ between regions in the BCS. This difference could lead to regional mismatches in favorable ecological conditions. In contrast with the spatially synchronous winter variability influencing the entire CCS ecosystem, substantial regional variation in the BCS may have strong effects on ecosystem functions, especially for species (e.g., small pelagic fish) that migrate between the Agulhas Bank and other areas of the BCS.

The influence of climate variability on Eastern Boundary Upwelling Ecosystems (EBUEs) is evident through changes in productivity and shifts in species' distributions, yet to date, metrics of upwelling variability appropriate for comparative ecosystem studies have yet to be implemented. Here, we present synoptic-scale upwelling indices to quantify inter-annual to decadal variations in Ekman transport, at temporal and spatial scales relevant to the biota of EBUEs, and apply them to the Benguela Upwelling System (BUS). From 1979 to 2015, interannual, decadal-scale, and unidirectional variability in upwelling was observed, including a significant recent decrease in upwelling in the northern BUS, and a significant increase on the Agulhas Bank. These trends are associated with changes in the number of upwelling days and events in these regions, and correspond to a shift in the meridional positioning of the South Atlantic High pressure system.

An investigation of surface phytoplankton communities was undertaken on the shelf of the northern Benguela upwelling ecosystem during austral autumn (May) and spring (September), along latitudinal transects at20º S and 23º S, from 2 to 70 nautical miles offshore, as well as on a zigzag grid located between these transects. Microscopic identification of the phytoplankton and CHEMTAX analysis of pigment biomarkers were used tocharacterise the community composition. During May 2014, warmer, more-saline water with a shallower upper mixed layer corresponding to periods of less-intense offshore Ekman transport was encountered on the shelf. Satellite imagery indicated high phytoplankton biomass extending for a considerable distance from the coast, and CHEMTAX indicated diatoms as dominant at most of the stations (52–92%), although dinoflagellates were dominant at some inshore localities (57–74%). Species of Chaetoceros, Bacteriastrum and Cylindrotheca were the most abundant, with abundance of the Pseudo-nitzschia ‘seriata-group’ being particularly high at a number of stations. In September 2014, more-intense wind-forcing resulted in a deeper upper mixed layer and stronger upwelling of colder, less-saline water. Elevated phytoplankton biomass was confined close to the coast, where diatoms accounted for most of the population (54–87%), whereas small flagellates, such as prasinophytes, haptophytes and cryptophytes, as well as the cyanobacterium Synechococcus, dominated the communities (58–90%) farther from the coast. It is hypothesised that stronger upwelling and deeper vertical mixing in September of that year were not conducive for widespread diatom growth, and that small flagellates populated the water column by being entrained from offshore onto the shelf in the upwelled water that moved in towards the coast.Keywords: Benguela upwelling ecosystem, CHEMTAX, hydrography, pigment biomarkers, SE Atlantic, species composition, surface waters

International audience; Demographic parameters of wild animals are often closely associated with their foraging distribution and behaviour, and understanding these attributes can assist in identifying causes of population changes. The Indian Yellow-nosed Albatross (Thalassarche carteri) is endangered but little information is available on its at-sea distribution and behaviour. It breeds only in French (Iles Amsterdam, St Paul, Kerguelen and Crozet) and South African (Prince Edward Island, PEI) territories in the south-west Indian Ocean, with PEI supporting about 20% of the global population. This study aimed to investigate the at-sea distributions of adult Indian Yellow-nosed Albatrosses provisioning chicks at PEI and to compare them with distributions of Yellow-nosed Albatrosses breeding at other localities. Using satellite transmitters, we identified two areas that were particularly favoured for foraging. Parents whose partners were brooding small chicks frequently moved north-east of PEI to shallow, productive waters where cold, nutrient-rich water upwells and results in enhanced levels of chlorophyll-a. By contrast, parents with older chicks that could be left unattended often foraged along the Agulhas Bank where eddies and shear forces promote vertical mixing. The at-sea distribution of birds breeding at PEI was located between those reported for Indian Yellow-nosed Albatrosses breeding at Ile Amsterdam and Atlantic Yellow-nosed Albatrosses (T. chlororhynchos) breeding at Gough Island, so that birds from these localities may face different threats at sea. Our study is the first to highlight key feeding areas for Indian Yellow-nosed Albatrosses from PEI and to demonstrate partitioning of foraging grounds by Yellow-nosed Albatrosses from different localities.

The three-component model of Brewin et al. (2010) computes fractional contributions of three phytoplankton size classes (micro- (> 20 μm), nano- (2–20 μm), picophytoplankton ( 1 mg m− 3 was limited to shelf regions along the coasts of Southern Africa and Madagascar, while values 50% of the total Chla in these regions with little change throughout the year. The AR shelf differed, with picophytoplankton dominating in summer, and micro- and nanophytoplankton the rest of the year. In the open ocean domains of the NB, SB, and AB regions, nanophytoplankton dominated for most of the year, with picophytoplankton being more prevalent during summer and autumn. In contrast, in the AR open ocean, nanophytoplankton were dominant only during winter and early spring, whereas picophytoplankton dominated throughout the year in the MC open ocean. The refined model characterised previously unknown spatial and temporal changes in size structure in various ecosystems around Southern Africa.

A comprehensive in situ dataset of chlorophyll a (Chl a; N = 18,001), net primary production (NPP; N = 165) and net community production (NCP; N = 95), were used to evaluate the performance of Moderate Resolution Imaging Spectroradiometer on Aqua (MODIS-A) algorithms for these parameters, in the South Atlantic Ocean, to facilitate the accurate generation of satellite NCP time series. For Chl a, five algorithms were tested using MODIS-A data, and OC3-CI performed best, which was subsequently used to compute NPP. Of three NPP algorithms tested, a Wavelength Resolved Model (WRM) was the most accurate, and was therefore used to estimate NCP with an empirical relationship between NCP with NPP and sea surface temperature (SST). A perturbation analysis was deployed to quantify the range of uncertainties introduced in satellite NCP from input parameters. The largest reductions in the uncertainty of satellite NCP came from MODIS-A derived NPP using the WRM (40%) and MODIS-A Chl a using OC3-CI (22%). The most accurate NCP algorithm, was used to generate a 16 year time series (2002 to 2018) from MODIS-A to assess climate and environmental drivers of NCP across the South Atlantic basin. Positive correlations between wind speed anomalies and NCP anomalies were observed in the central South Atlantic Gyre (SATL), and the Benguela Upwelling (BENG), indicating that autotrophic conditions may be fuelled by local wind-induced nutrient inputs to the mixed layer. Sea Level Height Anomalies (SLHA), used as an indicator of mesoscale eddies, were negatively correlated with NCP anomalies offshore of the BENG upwelling fronts into the SATL, suggesting autotrophic conditions are driven by mesoscale features. The Agulhas bank and Brazil-Malvinas confluence regions also had a strong negative correlation between SLHA and NCP anomalies, similarly indicating that NCP is forced by mesoscale eddy generation in this region. Positive correlations between SST anomalies and the Multivariate ENSO Index (MEI) in the SATL, indicated the influence of El Niño events on the South Atlantic Ocean, however the plankton community response was less clear. UK Natural Environment Research Council | Ref. NERC; NE / L002434 / 1 European Space Agency | Ref. AMT4SentinelFRM (ESRIN / RFQ / 3-14457 / 16 / I-BG) European Space Agency | Ref. AMT4OceanSatFlux (4000125730/18 / NL / FF / gp) NERC International Opportunity Fund Grant Satellite estimates of marine net community production in the South Atlantic from Sentinel-3 | Ref. SemSAS; NE / P00878X / 1 P&D ANP / BRASOIL | Ref. 48610.011013 / 2014-66 Oceanographic Institute of the University of São Paulo | Ref. FAPESP 2015 / 01373-0 Oceanographic Institute of the University of São Paulo | Ref. CNPq 442926 / 2015-4 Oceanographic Institute of the University of São Paulo | Ref. FAPESP 2014 / 50820-7 Oceanographic Institute of the University of São Paulo | Ref. CNPq 565060 / 2010-4 NERC | Ref. NE / R015953 / 1

Hydrographic data collected in cyclonic eddies in the Mozambique Channel and Basin revealed notable differences in temperature and salinity at a depth of 100 m, the upper mixed layer, the nitracline depths, and vertical distribution of chlorophyll-a (Chl-a). Differences in temperature and salinity did not show any consistent patterns. In contrast, the differences in the upper mixed layer, nitracline depths and the vertical Chl-a profile appeared to be driven by combined effects of eddy dynamics (i.e. shoaling of isopleths) and the seasonal variation in light availability and mixing conditions in the upper layers. Cyclonic eddies studied during austral spring and summer in the Mozambique Channel exhibited shallower upper mixed layers and nitracline depths, and deeper euphotic zones. Distinct subsurface Chl-a maxima (SCM) were associated with the stratified conditions in the upper layers of these eddies. In contrast, a cyclonic eddy studied during mid-austral winter in the Mozambique Basin had a shallower e...

Summary Climate change and fisheries are transforming the oceans, but we lack a complete understanding of their ecological impact [1–3]. Environmental degradation can cause maladaptive habitat selection, inducing ecological traps with profound consequences for biodiversity [4–6]. However, whether ecological traps operate in marine systems is unclear [7]. Large marine vertebrates may be vulnerable to ecological traps [6], but their broad-scale movements and complex life histories obscure the population-level consequences of habitat selection [8, 9]. We satellite tracked postnatal dispersal in African penguins ( Spheniscus demersus ) from eight sites across their breeding range to test whether they have become ecologically trapped in the degraded Benguela ecosystem. Bayesian state-space and habitat models show that penguins traversed thousands of square kilometers to areas of low sea surface temperatures (14.5°C–17.5°C) and high chlorophyll- a (∼11 mg m −3 ). These were once reliable cues for prey-rich waters, but climate change and industrial fishing have depleted forage fish stocks in this system [10, 11]. Juvenile penguin survival is low in populations selecting degraded areas, and Bayesian projection models suggest that breeding numbers are ∼50% lower than if non-impacted habitats were used, revealing the extent and effect of a marine ecological trap for the first time. These cascading impacts of localized forage fish depletion—unobserved in studies on adults—were only elucidated via broad-scale movement and demographic data on juveniles. Our results support suspending fishing when prey biomass drops below critical thresholds [12, 13] and suggest that mitigation of marine ecological traps will require matching conservation action to the scale of ecological processes [14].

The Benguela Upwelling system (BUS) is the most productive of all eastern boundary upwelling ecosystems and it hosts a well‐developed oxygen minimum zone. As such, the BUS is a potential hotspot for production of N2O, a potent greenhouse gas derived from microbially‐driven decay of sinking organic matter. Yet, the extent at which near‐surface waters emit N2O to the atmosphere in the BUS is highly uncertain. Here we present the first high‐resolution surface measurements of N2O across the northern part of the BUS (nBUS). We found strong gradients with a three‐fold increase in N2O concentrations near the coast as compared with open ocean waters. Our observations show enhanced sea‐to‐air fluxes of N2O (up to 1.67 nmol m−2 s−1) in association with local upwelling cells. Based on our data we suggest that the nBUS can account for 13% of the total coastal upwelling source of N2O to the atmosphere.

The first in situ continuous full‐water‐column observations of the Eastern Boundary Currents (EBCs) at 34.5°S in the South Atlantic are obtained using 23 months of data from a line of Current and Pressure recording Inverted Echo Sounders (CPIES) spanning the Cape Basin. The CPIES are used to evaluate the mean structure of the EBC, the associated water masses, and the volume transport variability. The estimated northward time‐mean Benguela Current absolute geostrophic transport is 24 Sv, with a temporal standard deviation of 17 Sv. Beneath this current the time‐mean transport is southward, indicating the presence of a deep‐EBC (DEBC), with a time‐mean transport of 12 Sv, and a standard deviation of 17 Sv. Offshore of these currents, the shallow and deep flows are more variable with weak time‐means, likely influenced by Agulhas Rings transiting through the region. Hydrographic data collected along the CPIES line demonstrate that the DEBC is carrying recently‐ventilated North Atlantic Deep Water (NADW) as it flows along the continental slope. This is consistent with a previously hypothesized interior pathway bringing recently ventilated NADW from the Deep Western Boundary Current across the Atlantic to the Cape Basin. The observations further indicate that much of the DEBC must recirculate within the basin. Spectral analyses of the shallow and deep EBC transport time series demonstrate that the strongest variability occurs on time scales ranging from 30 to 90 days, associated with the propagation of Agulhas Rings, and from 2 to 15 days linked to coastally‐trapped waves. Plain Language Summary Heat and freshwater transported by the overturning circulation in the Atlantic has been shown to have significant influences on global climate. Boundary current variations in the Southeast Atlantic have furthermore been shown to play a major role in the overturning circulation. This study presents, for the first time, full‐depth, daily observations of the eastern boundary currents at 34.5°S based on moored instruments. The 23 months of observations (velocity and water mass properties) reveal the presence of two distinct eastern boundary currents, with equatorward flow in the upper layer and poleward flow in the deep layer. The variability of both currents is characterized, in unprecedented detail, and is shown to be dominated by large anticyclonic eddies transiting across the moorings. The observations further indicate that much of the deep current must recirculate within the Cape Basin, with a smaller portion of the flow participating in the global overturning circulation.

Water column currents and bottom temperature from September 2005 to September 2006 across the narrow shelf region off Port Edward on the east coast of South Africa, combined with satellite observations, highlighted the dominant influence of the Agulhas Current (AC) on shelf circulation. Stronger southwestward flow occurred across the shelf during austral summer and autumn suggesting an intensification and shoreward movement of the AC during these seasons. In situ moored observations also showed that the AC extended as far inshore as 3.2 km from the coast. Weak and non-significant correlations between wind and current, and between wind and bottom temperature, indicated minimal influence of local wind forcing on current variations and temperature responses. A total of 11 northeasterly flow events, ranging from 1 to 15 days, were observed, with only 4 events associated with the passage of mesoscale cyclonic eddies along the inshore edge of the AC. The remaining events all suggested small-scale movements of the AC resulting in enhanced Ekman veering in the bottom layers further offshore, and full water column current reversals close inshore. Contrary to expectation, no clear seasonal variation in bottom temperatures was evident, and temperatures and currents were poorly correlated, since both warming and cooling occurred during southwesterly and northeasterly flows. Substantial short-term fluctuations of up to 4 °C were associated with shoreward/offshore movements of the AC, and southward propagation of eddies. Advection of warmer AC waters around eddies and southward advection of waters upwelled further north served to complicate the observed temperature responses.