Agulhas Bank Research Articles

Seasonal cycling of zinc and cobalt in the south-eastern Atlantic along the GEOTRACES GA10 section

Abstract. We report the distributions and stoichiometry of dissolved zinc (dZn) and cobalt (dCo) in sub-tropical and sub-Antarctic waters of the south-eastern Atlantic Ocean during austral spring 2010 and summer 2011/2012. In sub-tropical surface waters, mixed-layer dZn and dCo concentrations during early spring were 1.60 ± 2.58 nM and 30 ± 11 pM, respectively, compared with summer values of 0.14 ± 0.08 nM and 24 ± 6 pM. The elevated spring dZn concentrations resulted from an apparent offshore transport of elevated dZn at depths between 20–55 m, derived from the Agulhas Bank. In contrast, open-ocean sub-Antarctic surface waters displayed largely consistent inter-seasonal mixed-layer dZn and dCo concentrations of 0.10 ± 0.07 nM and 11 ± 5 pM, respectively. Trace metal stoichiometry, calculated from concentration inventories, suggests a greater overall removal for dZn relative to dCo in the upper water column of the south-eastern Atlantic, with inter-seasonally decreasing dZn / dCo inventory ratios of 19–5 and 13–7 mol mol−1 for sub-tropical surface water and sub-Antarctic surface water, respectively. In this paper, we investigate how the seasonal influences of external input and phytoplankton succession may relate to the distribution of dZn and dCo and variation in dZn / dCo stoichiometry across these two distinct ecological regimes in the south-eastern Atlantic.

Sedimentology of a coastal shelf sector characterised by multiple bedload boundaries: Plettenberg Bay, inner Agulhas Bank, South Africa

The study deals with a large sand body (spit-bar) attached to the eastern tip of the Robberg Peninsula, Plettenberg Bay, South Africa. To date, the bar has prograded about 8 km beyond the tip of the peninsula. The bar top is predominantly composed of medium sand, the upper slope of fine sand, and the lower slope of fine muddy sand. Stratigraphically, the sedimentology thus documents an upward coarsening, calcareous quartz-arenitic depositional sequence. The spit-bar as a whole forms the eastern end of a sediment compartment that is clearly distinguishable from neighbouring compartments on the basis of its geomorphology, the textural characteristics of the sediment, and the distribution of sediment thicknesses. Aeolian overpass across the peninsula appears to have formed a fan-like sand deposit in its rear, which is perched upon the upper shoreface of the bay as suggested by the bathymetry to the north of the peninsula. It forms an integral part of the sediment body defining the spit-bar. The estimated volume of sand stored in the spit-bar amounts to 5.815 km3, of which 0.22 km3 is contributed by the aeolian overpass sand. The sediment sources of the spit-bar are located up to 100 km to the west, where a number of small rivers supply limited amounts of sediment to the sea and numerous coastal aeolianite ridges in the Wilderness embayment have been subject to erosion after becoming drowned in the course of the postglacial sea-level rise since about 12 ky BP. By contrast, the sediment volume in the adjacent compartment B to the north (Plettenberg Bay), which has been supplied by local rivers, amounts to only 0.127 km3. In a geological context, large sand bodies such as the Robberg spit-bar are excellent exploration models for hydrocarbons (oil and gas).

Wind speed and mesoscale features drive net autotrophy in the South Atlantic Ocean

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.

Benthic Foraminifera of the Agulhas Bank coastal shelf in the vicinity of Plettenberg Bay, South Africa: a reconnaissance survey

The study area is located on the inner Agulhas Bank around Plettenberg Bay, southern Cape Province, South Africa (Fig. 1). The Agulhas Bank is a transitional environment between the cold-temperate Benguela Current regime of the south-eastern South Atlantic and the warm-temperate Agulhas Current regime of the south-western Indian Ocean. Three distinct faunal assemblages (A, B and C) were identified in the study area. These are aligned in three consecutive, coast-parallel belts, assemblage A forming the inshore belt, assemblage C the offshore belt and assemblage B the in-between belt. Assemblage A is composed of Textularia-group individuals and Pararotalia sp., Cibicides lobatulus and Planorbulina mediterranensis. It occupies the nearshore belt up to 50 m water depth in sediments composed of very fine, fine and medium sands, with some coarse and very coarse sands. Assemblage B is composed of Bolivina cf. pseudopunctata, Cassidulina laevigata, Ammonia beccarii, Bolivina tortuosa and Bulimina elongata. It occupies water depths from 50–70 m, but may locally extend down to 90 m and, within the bay itself, upward to 20 m in fine and very fine sands containing some medium sand and mud. Assemblage C is dominated by Cassidulina laevigata, Bolivina cf. pseudopunctata, Bulimina elongata and Ammonia beccarii. In contrast to assemblage B which is dominated by B. cf. pseudopunctata, assemblage C is dominated by C. laevigata. Assemblage C is mainly confined to water depths of 70–100 m in sediments dominated by very fine sand (0.063–0.125 mm) containing some coarser sediment and mud. The distribution of the forams with respect to water depth, sediment composition and other environmental parameters suggests that it is mainly controlled by a combination of environmental parameters. No tangible relationship was found between the open shelf foraminifer communities and those of estuaries and lagoons along the South African coast.

Marine climate change over the eastern Agulhas Bank of South Africa

Abstract. The rate of change in the marine environment over the eastern Agulhas Bank along the south coast of South Africa (32–37∘ S, 20–30∘ E) is studied using reanalysis observations for 1900–2015 and coupled ensemble model projections for 1980–2100. Outcomes are influenced by resolution and time span: ∼ 1∘ datasets covering the whole period capture large-scale changes, while ∼ 0.5∘ datasets in the satellite era better distinguish the cross-shelf gradients. Although sea surface temperatures offshore are warming rapidly (0.05 ∘C yr−1 since 1980), a trend toward easterly winds and a locally stronger Agulhas Current have intensified nearshore upwelling (−0.03 ∘C yr−1). The subtropical ridge is gradually moving poleward, leading to a drier climate.

Redescriptions of two species of Sepia (Cephalopoda: Sepiidae) from South African waters: Sepia robsoni (Massy, 1927) and S. faurei Roeleveld, 1972

Two species of cuttlefish: Sepia robsoni (Massy) and Sepia faurei Roeleveld, are redescribed based on sexually mature males and females of both species. They were previously known only from their holotypes: male and female, respectively. They belong to a distinct group of small-sized sepiids, all near-endemics of southern African waters. The knowledge of the systematics and biology of this group is still limited despite the long time since the first description (1875). This is because, inter alia, of their small size: not larger than 4 cm mantle length at maturity. Twenty-one individuals of S. robsoni described here were scattered from Port Nolloth area to the Tsitsikamma coast (bottom depth <37–449 m). Eight known individuals of S. faurei came from the eastern Agulhas Bank (bottom depth 116–184 m). S. robsoni can be identified by extremely thinly calcified cuttlebone (transparent); smooth skin of dorsal mantle and head; tips of first pair of arms thick, finger-like, devoid of suckers. S. faurei can be distinguished by the following combination of characters: thick, heavily calcified cuttlebone but with flat and fused inner cone; skin of dorsal mantle and head very densely covered by characteristic warts; thin, whip-like tips of first pair of arms devoid of suckers. All described specimens of both species are deposited in the South African Institute of Aquatic Biodiversity (SAIAB) and in Iziko, South African Museum (SAMC).

There are three species of Chrysaora (Scyphozoa: Discomedusae) in the Benguela upwelling ecosystem, not two.

Chrysaora (Pèron Lesueur 1810) is the most diverse genus within Discomedusae, and 15 valid species are currently recognised, with many others not formally described. Since Chrysaora fulgida (Reynaud 1830) was first recognised as occurring off the south west (SW) coast off South Africa, the species has been variously synonymised with Chrysaora hysoscella (Linnaeus 1767) and Chrysaora africana (Vanhöffen 1902). Using DNA evidence alongside multivariate tools to analyse quantitative morphometric and meristic data, as well as information from the cnidome, we unambiguously separate C. fulgida from C. hysoscella; we resurrect C. africana as a valid species and recognise a new species, Chrysaora agulhensis sp. nov. Full descriptions of C. fulgida, C. africana and C. agulhensis sp. nov. are provided. The species have different geographical patterns of distribution around the region, with restricted areas of overlap: C. agulhensis sp. nov. is found along the southern coast of South Africa and over the Agulhas Bank, C. fulgida extends from Cape Point in South Africa to southern Angola, and C. africana can be found from southern Namibia northwards to the Gulf of Guinea. The species can be readily separated in the field by a combination of tentacle/lappet number and shape, colour patterns and the form of the oral arms.

Physical ocean-atmosphere variability over the shelf of South Africa from reanalysis products

Inter-relationships between physical ocean-atmosphere processes over the eastern Agulhas Bank are explored using data assimilation by the Hybrid Coordinate Ocean Model (HYCOM), which provides daily fields and time series off the south coast of South Africa (33.75–34.5S, 24.0–26.5E) in the period 2009–2017. Our objectives are: i) to describe and understand the sharp gradients over the shelf, ii) to quantify the temporal controls and lagged uptake of weather events and, iii) to analyze the spatial effects of capes on mesoscale oceanographic structure. Sea surface temperature (SST) and zonal winds (U-wind) were cross-correlated with salinity, currents, waves and atmospheric variables such as heat budget components, air pressure (SLP), air temperature. Correlations were made over daily and annual time scales to identify the leading ocean-atmosphere interactions and the strength of processes.At daily time scales weak correlations were found between SST and a variety of ocean-atmosphere parameters, suggesting that multiple processes affect the thermodynamic condition. SST showed most correlation with radiation and heat budget components (r = −0.35, 0.39), SLP (r = −0.30) and meridional currents (r = −0.26). In contrast, the simultaneous correlation of SST with U-wind was weak (r = −0.08). We attribute this to: i) delayed response, ii) opposing effects of coastal upwelling and summer heating, and iii) competing shelf-edge processes.Over the mean annual cycle, correlations between SST, U-winds, salinity, radiative fluxes and V-currents were strong, indicating how kinematic and thermodynamic controls conspire to induce seasonality. SST was well correlated (r < −0.82) with SLP, long-wave radiation, salinity and wave height, but weakly correlated with currents. U-wind was correlated significantly with evaporation, radiative fluxes, curl (wind shear), meridional currents and wave period.Case studies of transient cold and warm events south of South Africa are described to reveal the large-scale atmospheric forcing of local SST. In addition, comparisons between the HYCOM reanalysis time series and independent insitu data are provided as a measure of confidence in global data assimilation systems, which open new opportunities for mesoscale oceanographic research in coastal zones.

Microplastics in offshore fish from the Agulhas Bank, South Africa

The extent and type of microplastic (MP) contamination in South African open ocean marine resources is unknown. This study aims to report on MP ingestion in seven commercially targeted fish species from the Agulhas Bank, south of South Africa. MPs were found in all seven species sampled (N = 105) (Trachurus capensis, Merluccius capensis, Merluccius paradoxus, Etrumeus whiteheadi, Scomber japonicus, Chelidonichthys capensis and Argyrozona argyrozona). MPs were recorded in 86.67% fish sampled, with abundances ranging from 2.8 to 4.6 items/fish. Most MPs were fibres (95.14%), black (38.11%) and ranged from 1000 to 500 μm (35.55%) in size. There was no difference in microplastic concentration in relation to distance from shore (p > .05). This is the first record of MPs in offshore fish from southern Africa and the results indicate that more research is required to assess the extent of MP contamination in the region.

Seismic stratigraphy of the inner to mid Agulhas bank, South Africa

The inner to mid continental shelf of the Agulhas Bank, which forms part of the Palaeo-Agulhas Plain, is scattered with Pleistocene deposits. Their wide lateral extension is the expression of a flat underlying substrate, availability of accommodation space, and depositional processes in response to glacio-eustatic sea-level change. We present seismic sub-bottom profiles up to 30 m deep, sediment cores up to 5 m in length and Pleistocene deposits that date back to Marine Isotope Stage (MIS) 7 from the inner to mid shelf between the Breede River in the West and Plettenberg Bay in the East. Radiocarbon and optically stimulated luminescence dates are integrated with microfossil analysis into a seismic stratigraphic model comprised of twenty Quaternary facies units within two depositional sequences bounded by shelf-wide unconformities. Sequence Boundary 1 (SB1) corresponds to the erosional unconformity with bedrock and SB2 to the MIS 2 glacial lowstand. Incised palaeo-river channels are associated with both sequence boundaries and cored deposits also mapped seismically from estuarine, lacustrine and fluvial systems are grouped to represent the lower floodplain. The most pervasive stratigraphic pattern in these shelf deposits is made up of the depositional sequence remnant of the Falling Stage Systems Tract (FSST) forced regression from MIS 5e–2. The other dominant stratigraphic group is the Transgressive Systems Tract (TST) associated with the Post-glacial Marine Transgression. The TST makes up an almost equal proportion of deposits in both sequences in the sedimentological record as the FSST, despite the shorter temporal span of the TST. A Wave Ravinement Surface marks the rise in sea level from the Last Glacial Maximum in a landward direction.

Geological and soil maps of the Palaeo-Agulhas Plain for the Last Glacial Maximum

The South African Cape South Coast is bordered by one of the broadest continental shelves in Africa. The Agulhas Bank, inshore shelf and presently exposed coastal plain make up the Palaeo-Agulhas Plain (PAP), though our area of study extends beyond this limit and as far inland as the first mountain belt. Quaternary sea levels have been significantly lower than at present for ∼90% of the Pleistocene, exposing a terrestrial ecosystem on what is now the submerged shelf. The presently drowned component makes up 94% of the total area of the PAP. Past work has hypothesised a contrast in character of this submerged landscape when compared to the subaerial environment. Here, we assimilate newly-acquired geophysical and geological datasets to produce geological- and soil maps from the Last Glacial Maximum on a scale of 1:750,000, covering an area of ∼55,000 km. Three broad geomorphic zones are defined, including the Western section from Cape Agulhas to Cape Infanta, the Central section from Cape Infanta to Knysna and the Eastern section extending eastward of Knysna. We demonstrate that Mesozoic sedimentary deposits crop out near the surface on this current-swept shelf and soils derived from siltstone and shale bedrock are prominent when the coast is up to 64 km distant from the modern shoreline at its maximum point. Beyond this, weathered limestone dominates the substrate sequences on the Agulhas Bank. We show that the submerged landscape was a unique terrestrial environment and that there is no exact modern-day analogue in the region other than a small (∼70 km2) area located at the edge of the Agulhas Plain near Cape Agulhas, and map major contrasts in the geological, topographic and edaphic nature of the landscape from the onshore to the offshore. The expansion of this plain is coupled with exaggerated floodplains, meandering shallowly incised rivers and wetlands. The submerged shelf is dominated by fertile soils compared to the dissected onshore belt, and extensive calcareous dunefields extending up to 10 km inland from their associated palaeoshorelines covered much of the emergent shelf. Sedimentary bedforms may have obstructed or slowed drainage as suggested by leached palaeosols and carbonate mixing observed in petrographic thin sections and grain mounts. The data show a low-relief “plains” landscape, which contrasts strongly to the topographically complex contemporary coastal foreland.

The chronological, sedimentary and environmental context for the archaeological deposits at Blombos Cave, South Africa

The site of Blombos Cave (BBC) is well known for archaeological remains that have advanced our understanding of the development of modern human behaviour during the Middle Stone Age (MSA). Occupation of the cave occurred against a backdrop of landscape-scale environmental and sedimentary processes that provide the broader context for finer-scale interpretations of the site-formation history and archaeological patterns detected in the cave deposits. Aeolian and palaeosol sequences are abundant in the vicinity of BBC and these provide a partial view of the past landscapes available to the inhabitants of the cave. An important extension to the palaeo-landscape around BBC currently lies submerged on the Agulhas Bank, as sea levels were lower than at present for the entire period of human occupation of BBC. In this paper, we revisit the optically stimulated luminescence (OSL) chronology for the full sequence of sediment deposition inside BBC, increasing the number of dated samples to a total of 40 and revising the period of MSA occupation to between 97.7 ± 7.6 and 71.0 ± 5.7 ka (uncertainties at 95.4% probability). We describe the geological successions at four main areas around BBC, estimate the time of sediment deposition using OSL, and describe and interpret three seismic profiles on the Agulhas Bank, offshore of BBC. By correlating these onshore and offshore geological sequences with the sedimentary deposits inside BBC, we place the archaeological record within a landscape-scale chrono-stratigraphic framework to examine how environmental changes may have regulated the presence or absence of humans in the cave and surrounding terrain between about 100 and 70 ka.

Progressive delays in the timing of sardine migration in the southwest Indian Ocean

Phenological shifts represent one of the most robust bioindicators of climate change. While considerable multidecadal records of plant and animal phenology exist for the northern hemisphere, few noteworthy records are available for the southern hemisphere. We present one of the first phenological records of fish migration for the southern hemisphere, and one of the only phenological records for the southwest Indian Ocean. The so-called ‘sardine run’ – an annual winter migration of sardines, northeast of their summer spawning grounds on the Agulhas Bank off the coast of Durban, South Africa – has been well documented in local newspapers given the importance placed on fishing and fishing-tourism in the region. An analysis of the first arrival dates of sardines reveals a 1.3 day per decade delay over the period 1946–2012. Although this phenological shift reveals a poor association with sea surface temperatures (SST), it coincides with a poleward shift in the position of the 21 °C mean annual SST isotherm – the threshold temperature for sardine populations. The timing of sardine arrivals near Durban corresponds closely with the number of mid-latitude cyclones passing over the Durban coastline during the months of April and May. The strength of the run is strongly associated with ENSO conditions. The complex suite of factors associated with this phenological shift poses challenges in accurately modelling the future trajectory for this migratory event.&#x0D; Significance:&#x0D; &#x0D; The sardine run, a significant event for tourism and fisheries, is occurring progressively later in the year.&#x0D; The incidence of failed sardine runs has increased in frequency over the period 1946–2012.&#x0D; These changes in fish migration phenology coincide with a poleward shift in the position of the 21 °C mean annual SST isotherm.&#x0D; The strength in the sardine run is strongly associated with ENSO conditions.&#x0D;

Multidecadal Wind Variability Drives Temperature Shifts on the Agulhas Bank

AbstractThe Agulhas Bank is an important area for the spawning of small pelagic fish and other species. Here, within a NEMO ocean model, we investigate changes in temperature over the Bank on multidecadal time scales. In agreement with previous observational studies, a shift to colder temperatures is found in 1997. The model also simulates an earlier shift from colder to warmer temperatures in 1966. These shifts are coastally confined and shown, using a climatologically forced model run as a control, to be driven by a north‐south migration in the large‐scale wind belts, rather than by changes in downward heat fluxes or changes in the Agulhas Current itself. The zonal wind changes on the Agulhas Bank show a significant relationship with the Southern Annular Mode, showing some promise for future predictability of cold and warm regimes on the Agulhas Bank. Thus, while the Agulhas Current has been shown in previous work to have a large impact on intra‐annual and interannual temperature variability, this work shows that multidecadal variability in temperature on the shelf is likely to be wind forced.

Lagrangian pathways in the southern Benguela upwelling system

The effect of ocean currents on fish eggs and larvae during their journey from spawning to nursery grounds in the Southern Benguela upwelling system is poorly understood. The survival and successful transport of fish eggs and larvae results from complex biological and physical processes. This study focuses on the advective processes, more specifically on how the dynamics and characteristics of the ocean currents shape the Lagrangian pathways in the Southern Benguela. A mesoscale eddy resolving interannual (1989–2010) simulation of the region, with a horizontal resolution of 7.5 km, is used to study processes impacting the connectivity between the western edge of the Agulhas Bank and the west coast upwelling region. A set of Lagrangian experiments are conducted with particles being released within the top 100 m of the water column along an across-shore transect off Cape Point (34∘ S). Transport success is given by the ratio of the number of particles that reach St Helena Bay (32∘ S) over the total number of particles released. The model results show a strong seasonal cycle in transport success which is governed by the complex three-dimensional structure of the along-shore jets, their variability, together with the wind-induced Ekman drift. Transport success is most efficient in spring when the Benguela Jet consists of one coherent intensified single-core branch that flows over the 300 m isobath, and when wind-induced Ekman transport favours the retention of particles within the jet. At this time of the year, the pathway leading to successful transport is located inshore, with 90% of particles released inshore the 300 m isobath being successfully transported to St Helena Bay in <15 days. This pathway is also characterized by low eddy kinetic energy values. During the upwelling season (December–March), transport success becomes less efficient, and less sensitive to the initial across-shore position of the particles. The inshore route no longer dominates, as the majority of particles follow offshore pathways. The Benguela Jet shifts offshore and splits into several branches due to the shoaling of the poleward undercurrent. The entrainment of particles within the offshore branch of the jet is favored by the dominating offshore wind-induced Ekman transport. Particles trapped in the offshore branch get exposed to higher mesoscale variability. Their northward progression is slower, which leads to journeys generally exceeding 20 days. This study shows that successful transport from the Agulhas Bank to the west coast upwelling region cannot be attributed to only a simple wind induced modulation of the jet. It explores how the seasonal modulation of the Benguela Jet, poleward undercurrent and offshore Ekman transport combine together with the turbulent off-shelf eddy field to set-up the characteristics of transport success.

A novel approach to assess distribution trends from fisheries survey data

Climate change and fishery impacts modify the spatial distribution of marine species. Understanding and predicting changes in distribution is important for adaptation by fishers and the management of fishery resources and biodiversity. However, identifying such trends is challenging given the variability inherent in trawl survey data. We apply a novel two-step approach to identify fish distribution trends from trawl surveys. First, species-specific average locations (mean latitude and longitude centre of gravity) and extent (effective area occupied) were estimated within a spatio-temporal delta modelling framework. The resulting time series and associated variance estimates were then passed to a multivariate Bayesian state-space model to estimate average trends over the study period. We applied this two-stage approach to three decades (1986–2016) of demersal trawl research survey data from the Agulhas Bank of South Africa to quantify distributional changes in 44 commonly caught fishes (chondrichthyans and teleosts). Across the entire assemblage, average trends showed a westward (alongshore) shift in location and a reduction in the extent of populations. At the species level, six taxa showed a location trend towards the west or south-west, and three shifted towards the east or north-east. The area occupied by species showed two taxa that had a decreasing trend in spatial extent and one species that was expanding. The mean westward and contracting trends of the assemblage were interpreted as likely signals of climate forcing, whereas the eastward shift of three species may be linked to fishing impacts. A lack of knowledge of subsurface oceanographic changes in the region challenges interpretation of the distribution changes and is identified as a research priority. We recommend additional research regarding causal drivers of distribution shifts, specifically to attribute observed changes to climate, fishing, and inter-annual environmental variability.

Long‐Term Trends in Phytoplankton Chlorophyll a and Size Structure in the Benguela Upwelling System

AbstractThe Benguela Upwelling System (BUS) is among the most productive ecosystems globally, supporting numerous fisheries and ecosystem services in Southern Africa. Sea‐viewing Wide Field‐of‐view Sensor and Moderate‐resolution Imaging Spectroradiometer‐Aqua chlorophyll a (Chla) concentrations between September 1997 and February 2018 were used to investigate long‐term trends in phytoplankton biomass and size structure (microphytoplankton [&gt;20 μm], nanophytoplankton [2–20 μm], and picophytoplankton [&lt;2 μm]) in the Northern Benguela, Southern Benguela (SB), and Agulhas Bank (AB) shelf and open ocean regions of the BUS. Trends in upwelling and correlations with Chla and size structure were examined. Increasing Chla and microphytoplankton trends occurred in the Northern Benguela shelf and open ocean, while decreases were evident on the SB shelf in all seasons. In the SB open ocean, small increases occurred during austral winter, with a decrease in spring. On the AB shelf, increases in Chla and microphytoplankton occurred in summer with decreases during the other seasons. Patterns differed in the AB open ocean, with increases in winter and spring and decreases in summer and autumn. Although R2 values indicated that linear trends accounted for a reasonable portion of the variance, and most trends were statistically significant, they showed only small changes on the shelf domains and little to no change in the open ocean. Strong correlations between upwelling, Chla, and the size classes were observed, but distinct seasonal differences occurred in each region. This is the first 20‐year analysis of phytoplankton biomass and community structure in the BUS and provides a baseline against which future changes can be monitored.

Fifty-five-year longevity for the largest member of the family Sparidae: the endemic red steenbras Petrus rupestris from South Africa

A previous study that explored the age and growth of red steenbras Petrus rupestris (Valenciennes, 1830), a large sparid (family Sparidae, seabreams or porgies) endemic to South Africa and reported to approach 2 m in length, provided estimates as a moderately slow-growing species with a maximum age near 30 years. The stock is considered collapsed and a fishing moratorium was imposed in 2012, resulting in this species being assessed as Endangered by the IUCN. One consideration in addressing population status is validation of life-history traits. In this study, estimates of age for red steenbras from thin-sectioned otoliths were reassessed visually in terms of both the original ages and revised estimates using a different age-reading pattern. The revised ages exceeded the original ages by up to 30 years, with a maximum estimated age of 55 years from a well-defined otolith section that provided a basis for the revised age-reading procedure. Bomb radiocarbon (14C) dating revealed there was an offset in the timing of the 14C rise on the Agulhas Bank that, when coupled with considerations for regional oceanography, provided support for the revised age-reading pattern and an estimated longevity that exceeds 50 years. These findings were further supported by the fortuitous recapture of a tagged red steenbras that was at liberty for 22 years.

Moored observations of mesoscale features in the Cape Basin: characteristics and local impacts on water mass distributions

Abstract. The eastern side of the South Atlantic Meridional overturning circulation Basin-wide Array (SAMBA) along 34.5° S is used to assess the nonlinear, mesoscale dynamics of the Cape Basin. This array presently consists of current meter moorings and bottom mounted Current and Pressure recording Inverted Echo Sounders (CPIES) deployed across the continental slope. These data, available from September 2014 to December 2015, combined with satellite altimetry allow us to investigate the characteristics and the impact of mesoscale dynamics on local water mass distribution and cross-validate the different data sets. We demonstrate that the moorings are affected by the complex dynamics of the Cape Basin involving Agulhas rings, cyclonic eddies and anticyclonic eddies from the Agulhas Bank and the South Benguela upwelling front and filaments. Our analyses show that exchange of water masses happens through the advection of water by mesoscale eddies but also via wide water mass intrusions engendered by the existence of intense dipoles. These complex dynamics induce strong intra-seasonal upper-ocean velocity variations and water mass exchanges between the shelf and the open ocean but also across the subantarctic and subtropical waters. This work presents the first independent observations comparison between full-depth moorings and CPIES data sets within the eastern South Atlantic region that gives some evidence of eastern boundary buoyancy anomalies associated with migrating eddies. It also highlights the need to continuously sample the full water depth as inter-basin exchanges occur intermittently and affect the whole water column.

Foraging behaviour and habitat use by Indian Yellow-nosed Albatrosses (Thalassarche carteri) breeding at Prince Edward Island

ABSTRACTDemographic 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.