One generalisation which can be made is that the importance of nutrient regeneration in the supply processes increases northwards in the Benguela system.
Whereas Thermocline (Central) Water of South Atlantic origin contains fairly similar concentrations of nitrate and phosphate as South Indian Ocean Thermocline Water, the silicate content of the latter is only about half of the former. This is significant for the nutrient chemistry of the Agulhas Bank region, which can according to Lutjeharms et al. (1996) be divided into distinct nutrient “provinces,” the eastern off shore part of the Bank being dominated by nutrient-poor Subtropical Surface Water and the western and inshore areas influenced by nutrient-rich South Atlantic Thermocline Water. Although upwelling occurs inshore in the lee of prominent capes, the Agulhas Bank system is strongly stratified in summer, particularly so in the eastern part. As a consequence nutrient concentrations tend to be higher in winter over the Bank as a whole in winter when the shallow seasonal thermocline breaks down. Readers requiring further information are referred to an excellent overview of the nutrient dynamics of the Agulhas Bank in Bailey and Rogers (1997).
Published work suggests that silicate is the limiting nutrient at times in the northern Benguela, while nitrate is on occasions limiting in the southern part of the system. This is perhaps counter intuitive when it is considered that the Namibian shelf sediments are rich in biogenic silica and Agulhas Current/Bank water (which leaks into the southern Benguela) relatively poor in silica. Considerable advances have been made during the past decade with respect to the assimilation of ammonia and nitrate by phytoplankton using stable-isotope nitrogen fifteen (N 15) incubation techniques by authors such as Probyn et al. (1990), Probyn (1992) and Waldron and Probyn (1992).
A significant contribution to the understanding of nutrient dynamics and production was made during the second phase of the Benguela Ecology Programme (1987-1992) with the recognition of the importance of the microbial loop and the application of N 15 labelling techniques to distinguish between newly incorporated nitrogen (NO3-N or N2) and metabolically recycled nitrogen (NH4-N or dissolved organic-N, i.e. urea).
Using satellite-derived sea surface temperature imagery, Waldron and Probyn (1992) estimated the potential new production in the Benguela system, and the first of these authors subsequently went on to calculate new production during the 1980s using sea level as a proxy for upwelling. Following on from work on carbon pathways in the southern Benguela upwelling system (Waldron et al. 1998), Dr Waldron has kindly generated a nitrate-nitrogen driven pathway for the Benguela as a whole for the purpose of this overview. This is given in Fig. 10 and illustrates simply and dramatically the import and export fluxes in shelf waters, in particular the nitrate-driven new production and the role of the shelf sediments as a sink. This will be discussed further in the section dealing with the foodweb and carbon budget.
Sulphur Perhaps one of the most obvious features of the marine chemistry of the northern Benguela is the odour of hydrogen sulphide gas which is associated with “sulphur eruptions”. These periodic eruptions are common in the general vicinity of Walvis Bay, usually during late summer when upwelling is at a minimum i. e. under quiescent conditions.
Sulphide formation results when anaerobic biological breakdown of organic substances and bacterial reduction of sulphate which is present in sea water and in the interstitial water in the marine sediments occur. As biological reduction of sulphate is an anoxic process, sulphide formation can occur where there is above average oxygen consumption or poor circulation - circumstances which may be associated with the decay following a major bloom of phytoplankton in an embayment or even along an open coast under quiescent conditions. These suboxic or anoxic conditions are common in the northern Benguela shelf waters and underlying organically rich sediments, and this provides suitable environmental conditions for the formation of sulphides by sulphate reducing and anaerobic bacteria.
As hydrogen sulphide which may be formed in the process can be extremely toxic, even at very low concentrations, mass mortalities of marine organisms are often associated with the “sulphur eruptions”’ compounding the effect of the already depleted oxygen content of the sea water. Records of fish kills in the Benguela resulting from sulphide production go back at least as far as 1928, but the occurrence of sulphide was not positively identified until later (Copenhagen 1934). Sulphurous fumes are often present in the atmosphere at coastal sites in central Namibia and may penetrate 60km or more inland. Their corrosive effect on iron and steel and tarnishing of paintwork, brass and silver is visibly evident in Walvis Bay and Swakopmund. During the sulphide events, mud islands may appear and disappear, the smell of rotten eggs pervades the air, and the sea takes on an appearance of boiling – hence the term “sulphur eruptions”. The most recent widescale sulphur eruption occurred in the Walvis Bay/Swakopmund area during March/April 1998 and was characterised by a strong odour of hydrogen-sulphide. The sea along the coast turned a milky-turquoise colour for as far as the eye could see.
The main area within which the sulphur eruptions occur is the so-called “azoic zone”, and free sulphur can be present in sediments from this area. Even 100km further south, concentrations of sulphide as high as 65mg/l may occur in interstitial water in sediments (Bailey 1979). Hydrogen sulphide concentrations of about 1ml/l have been measured in the northern Benguela. In the southern Benguela, eg. St Helena Bay, even though sediments may smell of sulphide, there are very few published measurements of free hydrogen sulphide. However, during the autumns of 1994 and 1998 a strong odour of hydrogen sulphide persisted for about a week in the atmosphere around Cape Town, as a consequence of decaying phytoplankton blooms in the St Helena Bay area, suggesting that sulphide events may be more common in the southern Benguela than hitherto appreciated.
In spite of the occurrence of sulphide in Benguela sediments and shelf waters, surprising little research has been conducted on the subject. Apart from the relevance of such studies to investigation and prediction of mass mortalities of marine life, public awareness and interest is high. Moreover, what is perhaps not well known, but may be extremely important in terms of industrial development along parts of the west coast, relates to the solubilization of heavy metals. For example mercury sulphide which is insoluble in normal oxygenated (oxic) water and sediments and is generally regarded as environmentally harmless, can be transformed to polysulphides of mercury in the presence of low concentrations of sulphide, and goes into solution – thereby becoming a serious environmental hazard.
Other aspects of marine chemistry
In their comprehensive review of the chemical oceanography of the Benguela system, Chapman and Shannon (1985) also discussed other aspects of the redox chemistry, including that of iodine and biomine, in addition to nitrate/nitrite and sulphate/sulphide as couples. (An important reference work on the subject is Price and Calvert 1977 which complements an earlier publication by these authors viz. Calvert and Price 1971). Chapman and Shannon also reviewed published work on the minor elements, which in contrast to the oxygen and nutrient distributions and dynamics have received very little attention in the Benguela. Minor elements include inter alia the alkali elements (potassium, rubidium and lithium), barium, and heavy metals. Many of the pioneering measurements of metals such as copper, iron and manganese, which are trace nutrients, were made by Orren (1969, 1971). As part of a South African marine pollution study, these and trace metals such as cadmium, nickel, lead and zinc were measured in water and sediments along the coast during the 1970s and 1980s eg. Eagle et al. (1982). Their results were summarised by Shannon and Chapman (1985) as was the work of authors such as Chester and Stoner (1975) on the concentrations of metals in dust and surface water particles collected in the Benguela region. Excluding urban environments, most of the measurements on metals in sediments have been from the organically rich Namibian shelf region, and it would appear from the available data that the concentrations of metals in these sediments are higher than observed in many other marine sediments.
Man-made chemicals such as chloro-fluoro-carbons (“CFCs”) have been detected in the deeper water masses present in the South-east Atlantic. CFCs such as CFM-11 and CFM-12 are useful tracers of water mass age and movement, and in the southern African context of the exchange of waters between the South-east Atlantic, North Atlantic and Indian Oceans.What the results also show is that even the deep bottom waters in the Southern Hemisphere have potential to be contaminated by activities of industrial Northern Hemisphere countries and can no longer be considered as pristine.
In conclusion it must be noted that chemical oceanography has for the past two decades been very much a “Cinderella discipline” in southern African countries, and in spite of the importance of chemical processes in regulating the biology of the Benguela ecosystem, this has apparently not been adequately recognised by the fisheries and environmental agencies.
PLANKTON AND THE FOODWEB
The literal translation of the word “plankton” is “wanderer”, and the term applies to a spectrum small neutrally buoyant (“free floating”) organisms in the sea which have little or no power of locomotion, and which drift with the currents. These small microscopic plants and animals provide the primary food sources for a host of marine species and constitute the early building blocks in the marine food chain or, more correctly, the foodweb. The microscopic unicellular plants which occur in surface layers of the ocean are known collectively as phytoplankton, while their animal counterparts are referred to as zooplankton. Although the most common components of zooplankton are only a few millimeters in size, zooplankton includes much larger floating animals such as jellyfish. Useful reviews of plankton in the Benguela system are those of Shannon and Pillar (1986) – now a bit dated – and Hutchings and Field (1997). Perhaps the most readable introduction to the zooplankton of the Benguela Current region is Gibbons (1999).
Phytoplankton and primary production
Like all plants, phytoplankton require sunlight in order to photosynthesise, and accordingly most of the organic productivity associated with these floating algae takes place near to the sea surface. Phytoplankton can for all practical purposes be divided into two categories viz. diatoms, which have no power of self propulsion and which have an outer skeleton of silica, and those which have small hairs or flagella which enable some weak motion viz. flagellates. The latter are generally more fragile than diatoms and are associated with less-turbulent and more stratified waters. Diatoms are characteristic of turbulent, nutrient-rich upwelled water. In upwelling systems the biomass of diatoms is generally much higher than the biomass of flagellates.
The Benguela is generally regarded as a diatom-dominated system. This perception is to some extent an artifact of past sampling, which has tended to miss the very small cells or nanoplankton. (The productivity of nanoplankton is regulated by regenerated nitrogen – see Section 3.2 – and historically nanoplankton have been undersampled in the Benguela). Both the northern and southern Benguela share many similar species assemblages, with Chaetoceros, Nitzschia, Thalassiosira, Rhizosolenia being endemic throughout the region. There are, however, essential differences between the north and the south, some of which are linked to the atmosphere/ocean dynamics (e.g. nutrient supply, turbulence and stratification). The diatom Delphineis karstenii (Fragilaria karstenii) is restricted to the north, while Skeletonema costatum is found predominantly in the southern Benguela, evidently having been most abundant in the early-mid 1960s and mid 1980s (both warm periods in the system). The large cell Coscinodicus spp. commonly occurs in areas of high turbulence. Over the Agulhas bank, the species assemblages are more cosmoplitan than along the west coast. Microflagellates are common in the central area of the northern Benguela e.g. Gymnodinium and Peridinium spp.
Phytoplankton abundance both from net and bottle sampling and chlorophyll a measurements highlights the dichotomy between the northern and southern parts of the system, with low values around 27-28S (the base of the Lüderitz upwelling cell), and high values downstream of the cell, and also further south associated with downstream areas of the other upwelling cells (Namaqua, Columbine, Cape Peninsula). Off Namibia during periods of active upwelling, highest concentrations of phytoplankton occur offshore (50km), and during quiescent periods in a narrow band close to the coast. A characteristic difference between the northern and southern Benguela is that concentrations of chlorophyll a are generally higher off Namibia than off South Africa, being more uniformly distributed in the former with less well defined chlorophyll fronts at the oceanic boundary.
The advent of ocean colour satellite imagery in the late 1970s provided new insights into the distribution and large scale dynamics of phytoplankton blooms. The Coastal Zone Colour Scanner (CZCS) on the Nimbus-7 satellite in fact resulted in a quantum leap in knowledge and understanding of the global distribution of phytoplankton, and southern African oceanographers played an important role in the early validation of CZCS data and application of satellite-derived ocean colour imagery in the study of upwelling ecosystems and fisheries. After about ten years (which is a very long time for a satellite sensor to function) the CZCS eventually failed, and it wasn’t until very recently that a new and much more sophisticated generation of colour satellites were launched and became operational. Of these, the Sea-viewing Wide-imaging Field-of-View Sensor (SeaWiFS) launched in August 1997 is the best-known and most user-friendly. Fig. 11 is a snapshot of the distribution of plant pigments in the Benguela between 16 and 32 S from SeaWiFS.
In the southern Benguela chlorophyll is determined by wind cycles and displays significant seasonality. Maximum concentrations tend to occur 20-80km offshore. (Blooms following periods of active upwelling can extend 100km or more offshore). Chlorophyll a concentrations in recently upwelled water, maturing upwelled water and aged upwelled water are about 1, 1-20 and 5-30 mg/m3 respectively. Over the Agulhas Bank phytoplankton production is largely controlled by thermocline/nutricline dynamics and the area is characterised by a deep chlorophyll maximum layer, at a depth of about 40m.
The total primary production in the Benguela system is approximately the same as that in the Peruvian system, but substantially greater than off California. Brown et al. (1991) provided an authoritative review of phytoplankton primary production and biomass in the Benguela. Average values are given in Table 2.
Table 2. Average values of phytoplankton biomass and production.