The physical northern boundary of coastal upwelling is marked by the Angola-Benguela frontal zone. The temperature and salinity front (series of fronts) is a permanent feature at the surface, identifiable to a depth of at least 200m, and is maintained throughout the year within a narrow band of latitudes, characteristically between 14S and 17S (i.e. close to the Angola-Namibia border). The front generally has a west-to-east orientation, and appears to be maintained by a combination of factors, including bathymetry, coastal orientation, stratification, wind stress and opposing flows of the Benguela and Angola Currents. The southwards migration of the front is most pronounced during late summer when longshore winds in the northern Benguela are weaker and upwelling is reduced. The situation in northern Namibia/southern Angola is in some respects analogous to the seasonal cycle off Peru in the Pacific during some years, when anomalously warm water appears (El Niño). There is a South Atlantic equivalent of the Pacific El Niño and it manifests itself as an episodic extreme warming in the tropical eastern Atlantic and the movement of tropical water southwards and eastwards along the Namibian coast. These Benguela Niños (Shannon et al. 1986) occur on average every ten years and are not necessarily in phase with the El Niño- Southern Oscillation (ENSO) – although some links with the latter are evident. Benguela Niños occurred in 1934, 1949, 1963, 1984 and 1995 and probably also in 1910, in the mid-1920s and in 1972-1974. Although not as frequent or intense as El Niños, like the Pacific counterpart, their impact on the ecosystem and harvested resources in the northern Benguela is enormous.
The characteristic location of the Angola-Benguela frontal zone is shown in Fig. 7. This is a much simplified diagram, as the “front” is often a combination of fronts with convoluted characteristics. (For example see Fig. 8 – which illustrates the surface expression of the Angola-Benguela frontal zone in April 1997).
While the Angola-Benguela Front (more correctly a series of fronts) comprises the northern extent of the main coastal upwelling zone, upwelling can occur seasonally along the entire coast of Angola. There are, in any event, strong linkages between the behaviour of the Angola-Benguela front (and the oceanography of the area to the south of it) and processes occurring off Angola, especially the Angola Dome and the Angola Current. Unless these are considered as an integral part of the BCLME, it will not be feasible to evolve a sustainable integrated management approach for the Benguela. Moreover, there is a well-defined front at about 5S, viz the Angola Front which is apparent at sub-surface depths. It is this front which is the true boundary between the Benguela part of the South Atlantic and the tropical/equatorial Gulf of Guinea system. A northern boundary at 5S would thus encompass the Angola Dome, the coastal Angola Current and the area in which the main oxygen minimum forms and the full extent of the upwelling system in the South-east Atlantic. A pragmatic northern boundary is thus at 5S latitude (see Fig 7) which is in the vicinity of the northern boundary of Angola (Cabinda) and the southern extent of the Gulf of Guinea Large Marine Ecosystem (GOGLME).
The southern boundary of the Benguela system can be considered as the Agulhas retroflection area, typically between 36 and 37S. Like the northern boundary, this warm southern boundary pulsates on a spectrum of time and space scales, and about 10% of the warm tropical Agulhas Current “leaks” into the South Atlantic. As previously explained most of this leakage is in the form of rings (eddies) which are shed from the Agulhas Current as it retroflects. The main trajectory of these shed rings is west-north-west, although departures from this have been recorded, and there exists a well documented case of ring interacting with the Benguela upwelling system and drawing upwelled water off the shelf in the form of a large curved upwelling filament. In addition to rings, there is a small regular leakage of filaments of Agulhas water around the Cape of Good Hope just west of the edge of the shelf and thermal front (which is associated with the upwelling). On occasions there are substantial intrusions of Agulhas Current water into the southern Benguela, of which the best documented case occurred in 1986 and coincided with that year being among the warmest this century in the South-east Atlantic Ocean. Another large intrusion occurred during the summer of 1997/8. Like its counterpart in the north, these Agulhas intrusions appear to affect the living resources in the southern Benguela.
The location of the southern boundary is shown schematically in Fig.7, while the typical extreme complexity of this boundary and retroflection of the Agulhas Current and “leakage” is illustrated in Fig. 4.
The western (offshore) boundary of the Benguela is fairly open ended, but is generally taken as approximately the 0 meridian. As such the Benguela sensu lato includes the coastal upwelling area, the longshore fronts (see below) and the eastern portion of the South Atlantic gyre. By definition then the Benguela Current comprises the total area of equatorward flow in the upper part of the South-east Atlantic Ocean.
There exists over much of the area between Cape Frio (18S) and Cape Point (34S) a well-developed longshore temperature front or fronts, which extends seasonally (in summer) eastwards around the Cape of Good Hope. The oceanic thermal front approximates to the seaward boundary of the general area influenced by coastal upwelling. South of Lüderitz a single front is usually well defined, and although spatially and temporally variable, coincides approximately with the run of the shelf break (edge of the continental shelf). Further north the surface manifestation of the front is more diffuse and multiple fronts are evident on occasions. The meandering nature of the front is evident in satellite imagery (see Figs 4 and 8). Upwelling filaments which have a life span of days to several weeks and which are generally orientated perpendicular to the coast cause the front to become highly convoluted. Opinion as to whether these filaments are randomly distributed or site specific are divided, although most recent evidence points to the latter (which can be explained in terms of bathymetry and system dynamics).
CHEMISTRY AND RELATED PROCESSES
Prior to 1925 very little was known about the chemistry of the South-east Atlantic Ocean and adjacent areas, and it was not until data collected by the German Meteor Expedition of1925 – 1927 were analysed and interpreted that a picture of the large-scale distribution of elements of importance such as oxygen, phosphorous and silica began to emerge. In the Benguela region, although the role of upwelling in the supply of nutrients (the dissolved fertilisers in the sea) was appreciated as early as the 1930s, it was not until the publication of the definitive work of Hart and Currie (1960) that the nutrient chemistry was placed in a proper physical and biological perspective. Subsequent investigations by inter alia authors such as De Dekker (1970), Calvert and Price (1971) and Andrews and Hutchings (1980) resulted in a greatly improved understanding of the chemical-biological processes of importance in the Benguela. The available information on the Benguela chemistry and related processes was reviewed by Chapman and Shannon (1985) while recently Bailey and Rogers (1997) have provided a useful overview of chemical oceanography within the context of marine geoscience in southern Africa.
Before discussing the essential features of processes associated with the chemistry of the Benguela system it is perhaps appropriate to explain some basic concepts so that readers who are not oceanographers or chemists will more easily be able to follow the subsequent discussions. Viewed very simply, macro-nutrients such as ammonia, nitrate, phosphate and silicate are present in sea water at low but significant concentrations. Near the sea surface these nutrients are consumed by microscopic plants (phytoplankton). Photosynthesis takes place, and the phytoplankton multiplies or blooms, absorbing carbon dioxide and releasing oxygen into the water. The phytoplankton is then either consumed by small planktonic animals (zooplankton) and some fish species or otherwise sinks slowly to the bottom. In this process bacteria play an important role, and as the phytoplankton and also faeces and organic material from zooplankton and higher consumers sinks, these decay. During the decay, oxygen dissolved in the deeper sea water layers and sediments is consumed and nutrients are released back into the water column. The overall process results in a lowering of nutrient concentrations in the surface layers and an increase at depth, with the opposite situation applying to dissolved oxygen. Within the context of a system such as the Benguela, it is the upwelling that is thus so important in bringing the nutrient-rich deeper water to the surface where photosynthesis can occur, and this explains why eastern boundary currents are so rich biologically. In the Benguela there is an imbalance between phytoplankton and zooplankton and fish production, and much of the decaying phytoplankton is deposited on the seabed, forming the organically rich diatomaceous sediments which are characteristic of the larger part of the Benguela shelf. In turn, processes which take place at the interface between the sediments and the overlying water and in the water in the sediments (interstitial water) are chemically also very important, in particular for the “regeneration” of nutrients.
Although what has been described is perhaps somewhat of an oversimplification of the complex physical-chemical-biological interactions which occur, it does help to explain the cycle.
3.1 Dissolved oxygen One of the major features of the Benguela region is the occurrence of large areas where very low concentrations of dissolved oxygen are found. Thermocline Water (Central Water) in the South-east Atlantic Ocean, which is the upwelling source water, commonly contains between 4.8 and 5.2 ml/l dissolved oxygen and is about 80 – 85% saturated. In contrast, the shelf waters in the Benguela system frequently contain much lower concentrations, and on occasions at sites such as Walvis Bay the sea can become anoxic at times, particularly at depth, while near the surface during phytoplankton blooms photosynthesis can result in the surface layer becoming supersaturated with oxygen. The occurrence of low-oxygen water is not only important in terms of the chemistry, but plays a key role in controlling the distribution and abundance of several marine species – not only bottom-dwelling (benthos) like rock lobster, but also fish such as hake. To avoid confusion, in the following paragraphs, we shall refer to water having an oxygen content of less than 2 ml/l as “oxygen-deficient” and above 2 but less than 5 ml/l as “oxygen-depleted”.
The first comprehensive account of the large scale distribution of dissolved oxygen in the South Atlantic was provided by Wattenberg (1938), and was based on measurements made during the expedition of the Meteor a decade earlier. His work showed the existence of a wedge-shaped tongue of oxygen-deficient water with its core lying at a depth of 300 – 400m, and extending from its base between the equator and 20S at the African continent, across the South Atlantic. Concentrations lower than 0.5ml/l were recorded in the Benguela region near 15S. The permanence of this major low oxygen feature in the South Atlantic has been confirmed by all subsequent large-scale investigations.
Hart and Currie (1960) and others have demonstrated the existence of an oxygen-deficient layer overlying the continental shelf north of Walvis Bay, and it is clear that oxygen-depleted subsurface water is a characteristic feature of much of the northern and central Benguela shelf. Subsequent studies have shown that low oxygen conditions can exist at times on the shelf further south, for example near the Orange River, in St Helena Bay and even at some sites on the Agulhas Bank. Hart and Currie (1960) suggested that the oxygen-depleted shelf water might be transported southwards from the tropical South-east Atlantic along the edge of the shelf in a deep compensation current, a view supported by De Dekker (1970) and Andrews and Hutchings (1980) who put forward convincing arguments in favour of this, and suggested that the oxygen-deficient water which occurs as far south as the Cape Peninsula at times, originates from Namibia.
The most comprehensive study of dissolved oxygen off Angola and Namibia was that undertaken by Bubnov (1972), and he suggested that the main oxygen minimum in the South Atlantic forms in a broad area off Angola, and is reinforced by processes associated with the Angola Dome. This is shown schematically in Fig. 9. Bubnov, however, calculated that the level of primary production over the Namibian shelf was sufficient to provide the concentrations of oxygen observed in the water column there in the absence of any advection (horizontal movement) from the north, a view supported subsequently by a number of authors. Interestingly, Poole and Tomczak (1999) show that the pseudo age of this oxygen poor water off Angola is 50 years or greater, in contrast to the younger more recently ventilated water south of 30S which is less than 10 years “old”.
Thus, while southward advection of oxygen-deficient and oxygen-depleted water from the Angola Basin via a poleward undercurrent may be an important mechanism controlling the distribution of low oxygen water on and adjacent to the shelf, local processes over the Namibian shelf are probably more important on average as determinants of oxygen dynamics in the Benguela region per se. This view is reinforced by the properties and distribution of the main oxygen minimum layer off Angola which are different to those of the oxygen-depleted/deficient shelf waters off Namibia. Indeed two distinct maxima cores have been observed on occasions, one over the shelf (locally produced) and a deeper one (probably part of the main minimum layer) west of the shelf break. It does seem, however, that the high primary production off Namibia is an important contributor to the main oxygen minimum layer, the latter at times spilling onto the shelf and reinforcing the depletion processes in Benguela shelf waters. There is also evidence that there is a slow southward advection of oxygen-deficient water throughout the Benguela at least as far south as the Cape Peninsula with lowest oxygen concentrations occurring during late summer/autumn in the southern Benguela. The fact is that oxygen-deficient water dynamics which plays a pivotal role in the ecosystem are not well understood. What is known is that substantial interannual variability in the oxygen concentrations does occur (discussed later) and that this is important for fisheries.
A conceptual model of areas where the low oxygen water forms and its movement is given in Fig. 9. Readers are referred to Chapman and Shannon (1987) and Bailey and Rogers (1997) for further information.
The general features of the distribution of nutrients in the Benguela resemble closely those of other upwelling regions. The upwelling water is enriched in nutrients relative to the surface layers and during active upwelling this water reaches the euphotic zone (the biologically productive surface layer which sunlight penetrates) near the shore. Following the establishment of the thermocline, phytoplankton production consumes nutrients in the upper layers, leaving them much depleted, while nutrient re-enrichment occurs below the thermocline as the phytoplankton decay. Chapman and Shannon (1985) pointed out the difficulty in discussing the nutrient status of the whole Benguela because the chemistry is very much site specific, and it is therefore not easy to generalise.
The shelf waters of the Benguela are, however, characterised by elevated concentrations of nutrients in comparison with those in the surface mixed layer of the adjacent oceanic waters, and also in comparison with concentrations in source waters. For example, South Atlantic Thermocline Water contains about 0.8 – 1.5 M (micro-moles) phosphate, but shelf waters have phosphate concentrations typically between 1.5 and 2.5 M, with values as high as 8M having been recorded off Namibia. Local regeneration processes are important throughout the Benguela, but particularly off Namibia. In comparison with the eastern boundary of the Pacific, source waters in the Benguela have lower levels of inorganic nutrients, and consequently a lower potential for new production. (The term “new production” is commonly used by marine chemists and biological oceanographers and can be viewed simply as that based on outside sources of “fertilisers” such as nitrates, unlike “ regenerated production” which results from the locally produced waste material – ammonia and urea based.). Typical concentrations of macronutrients in the Benguela system are summarised in Table 1.
Table 1: Nutrient concentrations (M) in (a) offshore upwelling (b) shelf and (c) oceanic surface waters in three areas of the Benguela, based on published work.