Eastern Agulhas Bank Research Articles

Currents along the Tsitsikamma coast, South Africa, and potential transport of squid paralarvae and ichthyoplankton

In July 1998, a bottom-mounted Acoustic Doppler Current Profiler was deployed at 36m depth in the centre of the Tsitsikamma National Park on the eastern Agulhas Bank, South Africa. The purpose was to investigate transport of chokka squid Loligo vulgaris reynaudii paralarvae hatched on the inshore spawning grounds (<60m) and ichthyoplankton spawned within the park. Analysis of the first 12 months of data (July 1998–June 1999) shows that surface flow was mainly eastward (alongshore), with a maximum velocity (u-component) of +115cm s−1 and an average of +24cm s−1. Generally, velocity decreased with depth, with a maximum bottom velocity (u-component) of +65cm s−1 and an average of +10cm s−1. Data from a nearby thermistor array show that the water column was usually isothermal during winter (July–September), with bottom flow in the same direction as the surface layer. In summer (December–March), vertical stratification was most intense, and surface and bottom flows differed in velocity and direction. Potential net monthly displacements calculated for three depths (5m, 23m and 31m) indicate that passive, neutrally buoyant biological material (e.g. squid paralarvae, fish eggs and larvae) would likely be transported eastwards in the surface layer for eight of the 12 months, and would generally exceed distances of 220km month–1. Displacement in the bottom layer was more evenly distributed between east and west, with net monthly (potential) transport typically 70–100km, but reaching a maximum of 200km. Wind-driven coastal upwelling, prevalent during the summer, causes the surface layer of the coastal counter-current to flow offshore for several days, resulting in potential displacement distances of 40km from the coast. These results suggest that squid paralarvae hatched on the inshore spawning grounds are not generally transported towards the 'cold ridge', a prominent semi-permanent oceanographic feature of cold, nutrient-rich upwelled water, where food is abundant, and that fish larvae, whether from the surface or bottom layer, are exported beyond the boundaries of the Tsitsikamma National Park.

Chokka squid (Loligo vulgaris reynaudii) abundance linked to changes in South Africa's Agulhas Bank ecosystem during spawning and the early life cycle

AbstractChokka squid biomass and catch are highly variable, likely owing to their links to changes in the ecosystem, which impact spawning and recruitment. A synthesis of basic ecosystem components for the domain in which chokka squid live (i.e. South Africa's west coast and Agulhas Bank) was prepared using published and new data. It included bottom temperature, bottom dissolved oxygen, chlorophyll, and copepod abundance. Alongshore gradients of these indicated that the main spawning grounds on the eastern Agulhas Bank are positioned where bottom temperature and bottom dissolved oxygen are optimal for embryonic development. This location, however, appears suboptimal for hatchlings because the copepod maximum (food for paralarvae) is typically on the central Agulhas Bank some 200 km to the west. Data on currents suggest that this constraint may be overcome by the existence of a net west-flowing shelf current on the eastern Agulhas Bank, improving survivorship of paralarvae by transporting them passively towards the copepod maximum. CTD data and a temporal analysis of AVHRR satellite imagery reveal the copepod maximum to be supported by a “cold ridge”, a mesoscale upwelling filament present during summer when squid spawning peaks. In situ sea surface temperature (SST) data used as a proxy for cold ridge activity demonstrate considerable interannual variability of the feature, especially during El Niño-Southern Oscillation events. Negative linear correlations between maximum summer SST (monthly average) and squid biomass the following autumn (r2 = 0.94), and annual catch (r2 = 0.69), support the link between the “cold ridge–copepod maximum” and the early life cycle of chokka squid, and holds promise for prediction.

Upwelling at the inshore edge of the Agulhas Current

Evidence for upwelling along the landward side of the Agulhas Current is presented. An analysis of old and new hydrographic data, surface temperature observations and satellite measurements show that this upwelling occurs in a tightly circumscribed geographic area. Centred at Port Alfred, it has a lateral range along the Agulhas Current from 85 to 300 km. Intermittent outcropping of upwelled water occurs more than 40% of the time and changes the surface temperatures dramatically. Below the upper layers this upwelling is more persistent and durable. It derives its water from the upper to middle levels of South Indian Central Water. This process may have a profound effect on the nutrient availability, the stratification and the primary productivity of specifically the eastern Agulhas Bank south of South Africa.

Multiple factors affecting South African anchovy recruitment in the spawning, transport and nursery areas

Despite high primary productivity, the yield of pelagic fish in the southern Benguela is relatively low compared to that in the Humboldt system. Part of the constraint may be the ability of pelagic fish to reproduce successfully in a strongly pulsed upwelling environment, where enrichment, retention and concentration mechanisms are less compatible than in Peru-Chile. Anchovy Engraulis capensis spawn upstream of the main upwelling centres on the food-poor, thermally stratified western Agulhas Bank, over a protracted summer season (October–February) when high wind speeds of 7–8 m˙s−1 are prevalent. Eggs spawned farther east, on the central or eastern Agulhas Bank, may be subject to increased cannibalism and advective losses, whereas those spawned farther west could be susceptible to heavy advective losses offshore during periods of strong southerly winds. Copepod concentrations are negatively correlated with spawner biomass on the western Bank and are inversely linked to high rates of gonad atresia in anchovy and reduced subsequent recruitment. There is a restricted area of suitable spawning temperatures (l6–19°C) on the western Bank, and fish outside that range appear to have less successful spawning, affecting recruitment; a reduction of the duration of spawning appears to decrease the chances of good recruitment. Oil content of pelagic catches on the west coast of South Africa suggests that good feeding conditions for adult fish are also favourable for growth of young fish in the nursery area. An inverse correlation between the numbers of recruits estimated in June and the strength of upwelling-favourable winds over the previous summer indicates the importance of advective loss of eggs and larvae in the transport and nursery areas. The presence of Agulhas Rings close to the shelf-edge, predators on juvenile anchovy, and the strength of upwelling on the West Coast could also be important factors affecting recruitment success. There seem to have been changes in the inshore-offshore distributions of spawning anchovy and/or their eggs over the past three decades, resulting in different recruitment responses to south-easterly winds.

CHEMICAL OCEANOGRAPHY AND MARINE GEOSCIENCE OFF SOUTHERN AFRICA: PAST DISCOVERIES IN THE POST-GILCHRIST ERA, AND FUTURE PROSPECTS

SUMMARY In 1895/96. John D. F. Gilchrist was appointed marine biologist to the Cape Colony. During voyages extending as far as Walvis Bay and Maputo, he initiated studies of the marine geology and chemical oceanography of the shelf while mapping substrata for new demersal fishing grounds. The shelf sediments off the East Coast are controlled by wave processes along the inner shelf and by the poleward-flowing Agulhas Current along the outer shelf. In contrast. South Coast sediments of the eastern Agulhas Bank consist of wave-dominated, landward-coarsening modem (Holocene) terrigenous muds to sands on the inner shelf and relict wave-dominated shelly sands on the outer shelf, deposited during Pleistocene lowstands within glacial (hypotbermal) periods. The Agulhas Current also appears to exert a controlling influence over the nutrient chemistry and, hence, primary productivity, on the East and South Coast margins. The surface waters of the Agulhas Current are nutrient-poor and most East Coast areas are consequently considerably less productive than their West Coast counterparts at the same latitude, but the underlying South Indian Central Water (SICW) is nutrient-rich. Recent findings suggest that the Agulhas Current may induce upwelling of nutrient-rich bottom water derived from SICW at sites such as the Natal Bight and off Port Alfred by kinematic upwelling so enhancing the nutrient content of surface waters and increasing the potential for primary production there. A second physical process, which is also thought to be related to interaction of the Agulhas Current and bottom topography, is the dynamic shelf-edge upwelling of SICW onto the shelf along portions of the South Coast where the shelf is wider. It is uncertain whether this is continuous in space or time, but it is possible that the process may, prime bottom waters for wind-induced upwelling in the south-western lee of capes along the South Coast. On the West Coast, the outer-shelf sediment consists of Holocene planktonic-foraminiferal ooze, reflecting the dominating influence of the equatorward-flowing Benguela Current. The middle-shelf sediment often consists of glauconitic sand, whereas the sediment of the inner shelf usually has a landward-coarsening and -thickening wedge of terrigenous muds to sands. Wind-induced upwelling is the dominant West Coast physical process of relevance to the sedimentology and chemistry of the inner shelf and overlying waters. In the southern Benguela this is seasonal, resulting in seasonal variability in the abundance of nutrients and the resultant productivity of surface waters and associated biogeochemical processes, such as the appearance of oxygen-depleted bottom water. There is a northward decrease in the seasonality of these physical and biogeochemical processes along the West Coast, which is reflected in an increase in the reducing nature of the underlying organic-rich sediments between St Helena Bay and Walvis Bay. In the deep-sea environment of the Cape Basin, the clockwise pole-ward flow of both the Antarctic Bottom Water (AABW) and the North Atlantic Deep Water (NADW) is reflected in a major zone of erosion of the sea floor, mantled by abundant ferromanganese nodules, at the foot of the continental rise, which is fed by margin-perpendicular slumps, debris flows and canyon-fed turbidity currents. The currents, driven by Coriolis Force, both swing left (east) into the Agulhas Passage between the Agulhas Bank and the Agulhas Plateau, before parting company in the Transkei Basin, where the AABW is forced eastwards by the northeast-shallowing contours of the Natal Valley. The E-W-orientated Agulhas Drift, a contourite drift, is being deposited on the left (north) side of the AABW. The NADW then heads into the Natal Valley to deposit margin-parallel contourite drifts at the foot of the continental slope as far north as Durban, where the Central Terrace and then the Mozambique Ridge steer the NADW first east and then south back to the mouth of the Natal Valley.

The nutrient characteristics of the Agulhas Bank

The distribution of nutrients over the Agulhas Bank, based on an analysis of historical hydrographic data, demonstrates the existence of three distinct nutrient provinces. The western Agulhas Bank is largely driven by the Benguela system and derives its higher nutrient values from coastal upwelling; the shelf-edge of the eastern Agulhas Bank is under the influence of the Agulhas Current, which only contributes nutrient-poor water at the surface, whereas the major part of the eastern Agulhas Bank is under the influence of the far-eastern Agulhas Bank upwelling cell, which provides nutrient-rich bottom water. Seasonal changes in nutrient distributions over the whole Agulhas Bank are driven by strong vertical stratification in summer and the erosion of this stratification, by vertical mixing, in winter. During summer, nutrient concentrations in the surface layers are characteristic of Subtropical Surface Water; those in bottom water derive from Central Water.

9000 Years of sea-level change along the southern African coastline

Beachrocks are good indicators of sea-level stillstands on microtidal coastlines and data for this sea-level curve is mainly based on dating a series of Holocene beachrocks and planation episodes on the southeast African coastline. Sea-level reached its present level along the southern African coastline at ca. 6500 BP; sea-level then rose depositing a series of beachrocks at an elevation of + 2.75 m. This mid-Holocene sea-level highstand persisted for a period of 2500 years with a + 3.5 m stillstand, dated at 4480 BP, being the highest sea-level reached during the Holocene. In post-Holocene sea-level optimum times, a regression occurred lowering sea-level to its present level at 3880 BP and down to −2 m at approximately 3000 BP. Subsequently, sea-level rose to + 1.5 m at 1610 BP and attained its present level at about 900 BP. The mid-Holocene sea-level highstand appears to have resulted from a combination of isostatic emergence and the steric expansion of seawater relating to warmer ocean temperatures associated with the Holocene hypsithermal event on the eastern Agulhas Bank.

Primary productivity and nitrogen uptake in the subsurface chlorophyll maximum on the Eastern Agulhas Bank

Measurements of primary productivity and nitrogen uptake using 15N-labelled N0 3, NH 4 and urea were performed on the eastern Agulhas Bank during January 1992. Primary productivity normalized to biomass ( P B) was exceptionally large for stations characterized by shallow mixing depths. In addition, P B was not well correlated with incident light. Exclusion of those stations with large PB ratios yielded a light utilization index of 1.43 ± 0.48. Primary productivity was largely regeneration based as indicated by photic zone integrals of the f-ratio ( x = 0.17). Ammonium uptake and regeneration were in approximate balance over the photic zone. However, there was a tendency for vertical displacement of these fluxes at some stations. On average, half the total new production occurred in the Chl max layer. New production was N0 3 limited in the upper photic zone (100−25% light depths) though was independent of N0 3 in the thermocline. Photic zone integrals of new production showed a moderately good relationship ( r 2 = 0.74) with surface N0 3 concentration. Possible mechanisms behind the formation and persistence of the subsurface Chl max are considered. In situ growth is recognized as important; by virtue of the significantly larger f-ratios at the Chl max ( x = 0.20) compared to the upper mixed layer ( x = 0.12), though not to the exclusion of other mechanisms. Herbivory and concomitant NH 4 release and inhibition of N0 3 uptake is proposed as a possible control over the vertical positioning of the Chl max. The application of these results to the estimation of regional primary productivity and new production are discussed. Simple models based on the light utilization index and incident light are not promising though the data set is too small to offer any firm conclusions in this regard. The potential exists for estimating new production directly from near-surface N03 concentration but will need to be substantiated in further cruises.

Sea-surface temperature fluctuations during the Holocene off the south coast of Africa: implications for terrestrial climate and rainfall

Models of southern African palaeoclimate implicate surface atmospheric circulation anoma lies as forcing large-scale changes during the Late Quaternary. The available proxy data are insufficient to test the models since they provide information about temperature and rainfall rather than surface circulation. A conceptual model is proposed which links coastal ocean temperatures with atmospheric circulation and allows a history of surface circulation to be inferred from sea-temperature data. A Holocene sea-surface temperature (SST) record was constructed by measuring the oxygen isotope composition of marine mollusc shells preserved in an archaeological cave deposit on the coast of the eastern Agulhas Bank, southern Africa. Radiocarbon-dating of individual shells allowed definition of the timing and timespan of events in the record. By serially sampling along the growth axis of each shell, information was obtained about intra-annual variability as well as millennial-scale trends. During the early Holocene, the sea surface on the eastern Agulhas Bank was colder than it is at present. Maximum summer and winter temperatures obtained 5800 years ago, exceeding by more than 2°C those recorded in the region today during non-El Nino years. On average, temperatures remained high for the following 1500 years but dropped again during the Late Holocene. At 650 BP, at the start of the 'Little Ice Age' in southern Africa, the surface waters on the eastern Agulhas Bank were colder during both winter and summer. Millennial-scale variability was attributed to the same processes which force interannual SST anomalies in the region today, namely surface winds and the Agulhas Current. Negative temperature anomalies were probably associated with higher volume transport of the Agulhas Current, which forces upwelling over the shelf edge, and with increased prevalence of easterly winds, which drive coastal upwelling. In contrast, a northward displacement of the westerly wind belt, weakening of the anticyclonic circulation and thus suppression of both coastal and shelf-edge upwelling may have been responsible for positive temperature anomalies recorded during the mid-Holocene. The conceptual model predicts that negative SST anomalies are associated with wetter conditions over the southern African interior, whereas warm-water anomalies are associated with dry conditions.

Anchovy spawning in relation to the biomass and the replenishment rate of their copepod prey on the western Agulhas Bank

Anchovy biomass and copepod standing stocks and growth rates on the Agulhas Bank were compared during the peak spawning period (November) in 1988 and 1989. In 1988, copepod biomass over the western Agulhas Bank was low (1,0 g dry mass·m−2) relative to anchovy biomass there (14,7 g dry mass·m−2). In November 1989 in the same area, fish biomass was much lower (5,7 g dry mass·m−2), following a recruitment failure, and copepod biomass was higher (2,4 g dry mass·m−2), possibly as a result of lesser predation by anchovy. By contrast, the eastern Agulhas Bank had a larger biomass of copepods (4–6 g dry mass·−2) and a lower biomass of anchovy during both years. Knowing, from laboratory studies, that a prey biomass of 0,78 g·m−2 is required for fish to obtain their daily maintenance ration, it is suggested that spawning on the western Agulhas Bank was food-limited in 1988. Copepods on the western Bank may be replaced by local growth or transport from the eastern Bank. Growth rates of copepods on the western Bank were 10–50 per cent of maximum in 1988, but total production (c. 100 mg dry mass·m−2·day−1) was low, primarily because biomass was low and less than the rate of consumption by anchovy (243 mg copepod dry mass·m−2·day−1). On the eastern Bank, copepod production exceeded anchovy consumption and it is concluded that the flux of copepod biomass onto the western Bank may be as important as local growth in replenishing copepod stocks there. Feeding conditions for anchovy on the western Agulhas Bank are often marginal compared to the situation on the eastern Bank, and it is suggested that the selection of the western Bank as the major spawning area is related more to the success of transport and survival of eggs and larvae on the West Coast recruiting grounds than to feeding conditions per se.

Spatial features of the near-surface and midwater circulation patterns off western and southern South Africa and their role in the life histories of various commercially fished species

Currents on the South African shelf have been measured on 13 cruises since an Acoustic Doppler Current Profiler (ADCP) was installed on F.R.S. Africana in 1989. Most previous descriptions of the West Coast (Benguela) system, including the baroclinic jet currents off the Cape Peninsula and Cape Columbine, the latter clearly diverging into a dominant offshore arm and a weaker inshore arm, have been confirmed. This baroclinic flow originates along the south-western margin of the Agulhas Bank. Measurements of the inshore features of the Agulhas Current have shown strong WSW – SSW flow aligned with the bathymetry, but with significant shelf-edge north-eastward return flows. The velocity differences between the surface and depth reveal barotropic dominance in all inshore areas and over the central Agulhas Bank. The West Coast outer shelf displays variable baroclinic effects, reflecting the importance of frontal features and cross-shelf flow. SSW flow on the outer eastern Agulhas Bank appears responsible for an offshore loss of eggs from this region. The funnelling of anchovy eggs from the western Agulhas Bank to the West Coast was clear, but this "funnel" was found to have holes through which substantial loss of eggs offshore is also probable. The currents on the Agulhas Bank and the West Coast are discussed with regard to the life histories of various commercially fished species, in addition to anchovy. The study concludes that hypotheses relating environmental factors and recruitment need to consider the variety of potential options in terms of life history contained within the prevailing current system.

Post-Palaeozoic stratigraphy of the eastern Agulhas Bank, South African continental margin

The Agulhas Bank, which forms the continental margin on the southern tip of Africa, consists of a thick Meso-Cainozoic sedimentary sequence (up to 6.2 km) resting on and behind pre-Mesozoic continental acoustic basement. The stratigraphy of this sequence is outlined and its history and facies variations mentioned where they are known. Refraction seismic velocity and bottom sample data indicate a basic three-fold subdivision of the Mesozoic sequence, which can be correlated with the onshore succession in the Algoa Basin. It is separated by a major hiatus from the Cainozoic sediments, which consist of a Palaeogene and Neogene sequence subdivided by another well-defined level of erosion. Various formations within the Cainozoic are defined and named. An outline of the bathymetry of the eastern Agulhas Bank is also given.

Significance of Upper Jurassic Sediments in the Knysna Outlier (Cape Province)

DR DINGLE replies: Dr Jones's stressing of whether the commencement of the formation of the eastern Agulhas Bank dates from the time of rifting or drifting between East and West Gondwana is something of a red herring. As the Agulhas Bank consists of a thick Mesozoic sediment wedge built onto a block-faulted Palaeozoic basement, the age of initiation of the construction of this wedge dates from the first sediments deposited onto the down-faulted basement. These sediments are at least as old as lower Upper Jurassic. The fact that the sediments deposited in the Knysna area during Upper Jurassic times were paralic in nature is also not significant, as it happens. Knysna lay at the northern limit of the Jurassic–Lower Cretaceous sea—it lies in a similar position today—and shallow marine sediments with interfingering lagoonal/terrestrial beds are precisely what one would expect from the establishment of an ocean adjacent to the area.

Topographic and hydrographic effects on catch rates ofAustroglossus pectoralis(Soleidae) on the Agulhas Bank

Catch rates of Agulhas sole Austroglossus pectoralis from research trawls were compared with depth of sea bed, bottom temperature and oxygen concentration. Data were from 10 research surveys conducted during the years 1986–1992 in autumn and spring. Cold bottom water situated between Mossel Bay and St Francis Bay separated two zones of warmer bottom water on the inner central and eastern Agulhas Bank in both seasons. Low to zero catch rates were associated with the cold water, whereas catch rates were higher in warmer water, especially at depths shallower than 75 m. On the inner central Agulhas Bank, catch rates close inshore were considerably higher in autumn than in spring. Conversely, on the eastern Agulhas Bank, catch rates were higher over a larger area in spring than in autumn. Depth of sea bed explained 23–49% of the variability in catch rates within localities and seasons, whereas bottom temperature and oxygen concentration had lesser effects.

A review and tests of hypotheses about causes of the KwaZulu-Natal sardine run

The term ‘sardine run’ is part of the cultural heritage of the South African nation and refers to a natural phenomenon that is well known to the general public but still poorly understood from an ecological perspective. This lack of understanding has stimulated numerous hypotheses, often contradictory, that try to explain why (ultimate factors) and how (proximate factors) the run occurs. Here, we provide a new definition of the term sardine run, review the various hypotheses about the run, and propose ways to test those hypotheses. Where possible, the results of tests that have been conducted thus far are presented and discussed. Our interpretation of the causes is that the sardine run most likely corresponds to a seasonal (early austral winter) reproductive migration of a genetically distinct subpopulation of sardine that moves along the coast from the eastern Agulhas Bank to the coast of KwaZulu-Natal (KZN) as far as Durban and sometimes beyond, in most years if not in every year. This eastward migration is constrained close to the coast by the thermal preference of sardine and the strong and warm offshore Agulhas Current. The run is facilitated by the presence of a band of cooler coastal water and by the occurrence of Natal Pulses and break-away eddies that enable sardine shoals to overcome their habitat restrictions. These enabling mechanisms are most important in the area where the shelf is at its narrowest and feature most prominently off Waterfall Bluff, which has led to the coining of the ‘Waterfall Bluff gateway hypothesis’. Based on the collection of eggs off the KZN coast, sardine remain there for several months and their westward, return migration during late winter to spring is nearly always unnoticeable because it likely occurs at depth as the fish avoid warmer surface waters. Years in which the sardine run is not detected by coastal observers could reflect either its real absence due to high water temperatures and/or other hydrographic barriers, or an eastward migration that is farther offshore and possibly deeper and is enabled by hydrographical anomalies.