The Benguela Current Large Marine Ecosystem (BCLME) extends from east of the Cape of Good Hope, northwards to Cabinda Province in Angola and encompasses the full extent of Namibia's marine environment. It is a major coastal upwelling ecosystem and an important centre of marine biodiversity and marine food production.
The Benguela is particularly productive in terms of fisheries resources, but top predators such as seabirds and marine mammals are also abundant. Commercial fisheries and the extraction of non-living natural resources such as oil, gas, diamonds and other minerals, are the focus of industrial activities in the regionn.
Abundance of Life
The Benguela Biome
The Benguela biome is dominated by a coastal upwelling ecosystem, one of four globally that are found along the eastern boundaries of the major ocean basins. These coastal upwelling areas are probably best characterised by their fish populations rather than by their "vegetation", which contrasts sharply with the situation for terrestrial biomes. Indeed, the dominant marine plants (phytoplankton) and major marine herbivores (planktonic copepods) in these systems could be overlooked because they are so tiny. This is not the case for the several species of small fish (especially anchovy Engraulis encrasicolus and sardine Sardinops sagax) that occur in great abundance in the water column that overlies the continental shelf. These fish species are made more noticeable by the fact that they swim together to form dense schools, which can be many kilometres across and close to the sea surface. The immense amount of biomass contained in these fish populations, which support commercially important fisheries, reflects the large productivity of the Benguela biome.
This large production occurs because surface waters are continually fertilised by the upwelling of nutrient-rich deep water. Upwelling occurs when equatorward and offshore-directed winds blow across the sea surface, causing surface waters to move away from the coast and to be replaced by cool, nutrient-rich water from below. The high productivity can be contrasted with the biome's relatively low biodiversity in all the major marine habitats.
This low biodiversity has been attributed to the extremely variable nature of the marine environment at a range of temporal and spatial scales, which favours generalists over specialists. At the northern and southern boundaries of the Benguela, where warm oceanic water from offshore, and warm coastal waters from the Angola and Agulhas Currents respectively mix with Benguela water, there can be localised peaks in biodiversity, but these are also usually associated with reduced productivity. At greater depths, somewhat less is known about the biota, although there are differences between areas, linked to the structure of the sea bottom and the physical and chemical nature of the environment.
This essay highlights some of the characteristic living elements of the Benguela biome, and links these to attributes of the physical and chemical environments.
It's Cold Out There!
From a satellite's perspective in space, the surface waters of the Benguela are colder than those found further offshore, as well as those to the north off Angola and farther east off the coast of South Africa.
The warm, surface, offshore boundary for the system is provided by an oceanic front, and to the north and south the system is bounded respectively by the warm Angola and Agulhas Currents. The cool temperatures at the coast are caused by coastal upwelling. The cold, upwelled water comes in pulses of a few days from depths where there is limited light, so that dissolved nutrients, essential for the growth of phytoplankton and other plants, tend to occur in relatively large concentrations. When a pulse of nutrient rich upwelled water enters the well-lit environment at the surface, conditions are ideal for the rapid growth and division of phytoplankton cells. In the Benguela, these single-celled plants exploit an approximately week-long window of opportunity, during which time there is only limited herbivory, and the plants proliferate to form dense phytoplankton blooms.
Many of these microscopic plants are destined never to be consumed by herbivores, but sink out of the surface waters, or are carried offshore, returning their nutrients to bottom waters and feeding the thick muds that characterise benthic (bottom) habitats, especially off Namibia. The unicellular plants that are consumed in the water column form the basis of a short food chain that is particularly efficient at transferring energy from the primary producers to fish, mainly because food concentrations are high and are efficiently packaged in large cells or chain-forming diatom species.
The episodic but frequent bursts of productivity by phytoplankton make the Benguela a region of sustained high production all year round.
Too Much of a Good Thing
As nutrient levels fall following the first flush of diatom growth, plant communities change to one dominated by dinoflagellates, which are better adapted to grow in low-nutrient conditions, and can move in response to changes in the water column structure.
Sometimes, dinoflagellate populations encounter favourable combinations of environmental conditions that allow them to grow rapidly, out-competing other species. Some of these algal blooms can be harmful to other marine life. They are known colloquially as "red tides", because many of the species give the sea a brownish-red tinge. Their most alarming impact in the Benguela is through the production of toxins, affecting the edibility of shellfish, and impacting aquaculture operations.
These "sea farms" need to be particularly mindful of these natural but disastrous events, because oysters, mussels and abalone are all known to acquire varying degrees of toxicity during such events. Fortunately, not all red tides are toxic, but they can also have significant impacts on marine ecosystems by stripping the surface waters of oxygen. The exceptionally high biomass of dinoflagellates ultimately decays and sinks to the sea-floor, and the enhanced bacterial activity during the decay process consumes all available oxygen.
And Too Little... Stuck In The Mud
Low oxygen water is a natural feature of bottom water in the Benguela biome. Marine creatures that are unable to move to better oxygenated water can be killed, with the best known examples being the rock lobster "walk outs" off the west coast of southern Africa. Many benthic species, however, are adapted to tolerate hypoxia, at least for a while. For example, commercial hake species are known to be able to live in water near the sea bottom where oxygen concentrations can be very low.
The bottom waters are low in oxygen in some shelf areas of the Benguela because there are deep layers of mud, which are continually replenished as dead plants and animals sink down from the surface. Just as bacterial decay strips surface waters of oxygen during red tides, so bacterial activity removes most oxygen from these bottom waters. These muddy areas have few large animals, but are high in nutrients, especially sulphur. Methane and hydrogen sulphide can be produced in these anoxic sediments, which can be metres deep. Hydrogen sulphide gas moves from the sediments to the water column, trapped in methane bubbles, which expand as they rise towards the sea surface.
This bubbling cocktail also removes dissolved oxygen from the water column and marine animals tend to move en masse away from such sulphur eruption areas. Mass mortality of fish and seals has been linked to these events. It is speculated that sulphur eruptions could increase in frequency and extent in the future, linked to climate change in the region.
Life On The Edges
Although most primary production in the Benguela biome occurs through the photosynthesis of microscopic phytoplankton, along the coastal fringes we find the marine equivalent of forests, where massive kelp plants grow attached to rocky substrata and structure a unique ecosystem. Kelps are among the fastest growing organisms in the world, with their fronds able to grow up to 13 mm per day. These phenomenal growth rates are made possible by the high nutrient concentrations in upwelled water at the coast. Kelp beds and their surrounding areas sustain a variety of herbivores, many of which are uniquely adapted to this special environment.
Limpets on the west coast of southern Africa have been shown to actively trap kelp fronds under their shells, allowing them to graze on these large plants, which normally would be either out of reach or hard to grasp in the turbulent zone found between the low and high tide levels. The availability of kelp fronds represents an enormous food subsidy to this intertidal environment and allows limpet densities to average 200 limpets per m2, or greater than 10 kg per m2 on the west coast of southern Africa.
The marine plant and animal communities along the edges of the Atlantic Ocean in the Benguela biome have relatively few species (when compared with other sub-tropical and temperate marine environments), but these species can occur in great abundance. In general, standing stocks of marine organisms in the Benguela decrease from north to south, but species diversity increases. Greatest diversities occur off the south coast of South Africa on the Agulhas Bank.
Here the broad shallow bank provides extensive habitat, in contrast to the deep shelf found off Namibia and parts of the west coast of South Africa. The marine environment in the south is also more stable than on the west coast, especially off Namibia, where upwelling is intensive all year round. One of the most important commercial species in the coastal habitat, found from just below the low tide mark, is the rock lobster Jasuslalandii, which is a seafood delicacy in the region.
There are also a host of bony fish and cartilaginous fish, such as small sharks and rays that occur near the coast, many of them targeted by shore anglers. Indeed, it is interactions with these inshore fish and other edible animals (such as mussels, octopus, and abalone) which probably best characterises the marine experience for many human inhabitants and visitors to the Benguela region.
In the water column from the sea bottom to the sea surface, plants and animals live in suspension. Some of the smallest of these are larvae of animals that will eventually settle to the bottom, but the vast majority spend their entire life in the water column. In the microscopic world of phytoplankton, there are a large number of species, with an incredible diversity of body shapes, sizes and forms.
The vegetable soup provides sustenance for zooplankton, which consists largely of small crustacean animals which filter the plant cells from the water, using modified limbs as nets. These tiny animals are hunted and eaten by other small invertebrates and also by fish, thereby linking the microscopic world, where life cycles can be completed in a few drops of water and plants and animals are at the whim of the ocean currents, to the macroscopic world of highly mobile animals. At the interface between these are colonialjellyfish, often at great abundance, especially off Namibia where they clog nets and can interfere with fishing activities.
Jellyfish also prey on fish larvae, and have the potential to prevent the recovery of fish stocks. At each stage in a food chain, only a small proportion of the energy is passed on to higher trophic levels; the rest is lost through respiration and faeces. One of the main reasons why the Benguela is so productive in terms of pelagic fish, is that the food chain between the primary producers (diatoms) and fish passes through only a single zooplankton level (usually copepods), and some large, chain-forming diatoms may even by consumed directly by filter-feeding fish such as sardines.
Flashes of Silver and Gold
The most abundant fish species in the Benguela are small, schooling fish that can be found in great abundance in surface waters overlying the continental shelf.
Historically, sardine and anchovy have been the most important of these pelagic species. However, redeye round herring Etrumeus whiteheadi is also abundant off South Africa, and in the last decade pelagic goby Sufflogobius bibarbatus has become important off Namibia, where the pelagic ecosystem has undergone major changes. Sardinella species are important in the northern parts of the Benguela region, but they do not reach the large numbers that are found for the other species of bony fish.
Small schooling fish are the hub of the Benguela offshore ecosystem. Known as "forage fish", they are the main food of a host of predators, including fish, squid, seabirds, seals and whales. When pelagic fish numbers fall, the predators suffer, but when they are abundant, the predators thrive. During these times of plenty, it is believed that the pelagic fish have another important influence on the ecosystem – they depress the standing stocks of their own food, primarily microscopic zooplankton.
With a dual role of suppressing the growth of their own food while enhancing the growth of their predators, small pelagic fish in the Benguela are thought to exert "wasp-waist" control on the ecosystem. The term derives from the fact that a small number of fish species occupy a central position in the Benguela system, with many more species both below and above them in terms of the pathways for energy to flow from primary producers. Therefore, the large interannual and interdecadal variations in standing stock of the small schooling fish can exert a strong influence on all components of the food web.
The large abundance of sardine and anchovy in the Benguela has also made them commercially attractive to a substantial pelagic fishing industry which became an important income generator in Namibia and South Africa. However, the short-term potential for economic wealth generation by removing pelagic fish from the sea needs to be balanced with their important ecological roles e.g. as food for other species. Attaining this balance is an important focus of current research activities in the Benguela.
In addition to anchovy and sardine, horse mackerel (maasbanker) is a third very important fish in the Benguela biome. Two species of horse mackerel are found off southern Africa, with the Cape horse mackerel Trachurus trachurus capensis found throughout most of the Benguela region and the Cunene horse mackerel T. t. trecae occurring only in the north, off Angola. Juvenile horse mackerel are mainly found in surface layers, feeding on zooplankton prey, as do anchovy and sardine, and often shoaling with these small pelagic fish.
Large horse mackerel tend to feed in the midwater, and include in their diet polychaete worms, bottom-dwelling and planktonic crustaceans, and other fish such as lanternfish, lightfish and gobies. Large horse mackerel are usually taken by midwater-trawl.
Large, fast-swimming pelagic fish rely heavily on small pelagic fish for dinner. Well-known examples include snoek Thyrsites atun, chub mackerel Scomber japonicus and various species of tuna, although the latter are highly migratory and spend only part of their time in the Benguela region, usually patrolling its outer margins. Snoek are important targets of the handline fishery along the South African west coast and are also caught in bottom trawls. They are opportunistic predators, and thus their availability to fishers is influenced by the distribution of their prey. Top predator fish species like tunas and swordfish are seasonally available to fishers. They are not only prized for their flesh, sought after by local and foreign fishing vessels, but also for their strength and swimming abilities, appreciated by game-fishing enthusiasts.
Found deeper in the water column is an important predator of small pelagic fish, the well-known eating fish, the hake. Two species of hake are found off southern Africa.
They are not easily distinguishable from outer appearances: but named according to the depth of water they tend to inhabit: deep-water Cape hake Merluccius paradoxus and shallow-water Cape hake Merluccius capensis. Although hake are largely considered to be bottom-dwelling (benthic) fish, they spend a substantial part of their time in near-surface waters, rising from the bottom at dusk to feed at night.
Juvenile hake are pelagic in life style, feeding on planktonic crustaceans just like small pelagic fish, and thus competing with them for food. The hake are also interesting in that hake eat hake! Large shallow-water hake are found in the same areas as small deep-water hake and prey heavily on these. However, hake-hake predation is not limited to this dual species interaction, there is also cannibalism within a single species – large hake of one species eat smaller hake of the same species as well.
Hake are opportunistic predators, adapting their diet seasonally and spatially to the availability of fish prey. Since the Second World War, hake have supported a valuable fishing industry in the region. They are caught near the sea bed with bottom trawls during the day.
In the Benguela, perhaps the most "bottom-dwelling" fish species in the truest sense is the sole.Two species occur in the region: west coast sole Austroglossus microlepis and Agulhas sole Austroglossus pectoralis. Soles bury themselves in the sand on the sea floor, spending much of their time simply lying still. An interesting aspect of their life history is that, as they mature, their eyes migrate so that both are located on the right hand side of their heads, facilitating their sandy camouflage trickery. Soles dine on polychaete worms, benthic crustaceans and small molluscs, and are tasty treats for hake, sharks and even seals.
Scouting For Food
The Benguela biome is recognized for its diversity and abundance of marine birds and mammals, which make good use of all that this generous fish restaurant has to offer. For the same reason, these groups are strongly influenced by man's interventions at sea and on land. In the past, the Cape fur seal Arctocephaluspusilluspusillus went through a marked population decrease because of seal harvesting from the early 1800s through the 1900s.
Harvesting still continues in Namibia today. Seal pups aged 7-10 months were prized for their pelts and, more recently, bull seals are being harvested as a source of aphrodisiacs for the Asian market.Sealing was stopped in South Africa in 1990. Fishers commonly view seals as a menace, but scientific studies have shown that competition between seals and hake fishing operations is small in the greater scheme of things.
In recent years, rogue bull seals, unable to secure their own harem of females, have been responsible for killing unsustainable numbers of African penguins Spheniscus demersus, Cape gannets Morus capensis and Cape cormorants Phalacrocorax capensis around breeding islands off South Africa and Namibia. This led to control measures being implemented to deal with individual problem seals and begged the question of which marine species should be given priority in terms of marine resource management.
Fifteen species of seabirds breed on islands or at mainland sites off south-western Africa: African penguin, Cape gannet, four species of cormorant, three species of gull, five species of tern, and the white pelican.
Because of their large population decreases, African penguin and Cape gannet have been classified as Vulnerable species of conservation concern according to The World Conservation Union (IUCN) criteria. Seabirds support important ecotourism industries along South Africa's west coast, making management efforts all the more urgent. Because marine birds and mammals are confined to the vicinity of terrestrial localities for breeding, they are especially influenced by the availability of prey fish within their restricted foraging ranges during the breeding season. A case in point is the African penguin, which has halved in numbers breeding off South Africa in just three years (2002-2005), thought to be explained at least in part by the southward and eastward shift of sardine, an important and nutritious prey for penguins.
A large variety (37 species!) of whales and dolphins, collectively referred to as cetaceans, frequent southern African waters (including the warmer east coast). Cetaceans consist of two groups: toothed whales and dolphins, which hunt relatively large prey (mainly fish and squid), and baleen whales, which use plates or whalebones in place of teeth to filter planktonic food from the water. Southern right Eubalaena australis, Bryde's Balaenoptera edeni and humpback Megaptera novae-angliae whales are baleen whales that were affected in the past by whaling. The most common toothed whales sited in the Benguela biome include the endemic Heavyside's dolphin Cephalorhyncus heavisidii, as well as common Delphinus delphis, dusky Lagenorhyncuhus obscurus and bottlenose Tursiops truncates dolphins.
When viewed through plant pigment-detecting "spectacles", from satellites, the Benguela biome is rich in plant life. If you watch the seas off southern Africa over the course of a full year, you observe the tumultuous, pulsing nature of the environment. Oceanic features form and then dissipate, boundaries move back and forth, and sea surface temperatures increase and decrease. As the physical environment changes, so too do the living organisms within it.
Beneath the sea surface, low oxygen water, inhospitable to all but the hardiest of living organisms, ebbs and flows, and moves along the coast. Animals move away from these inhospitable conditions, and others aggregate to feed or to reproduce or to avoid predation. Still others travel long distances, entering and leaving the Benguela as a brief segment of a much longer journey.
The distinct environments that occur within the Benguela biome do not function in isolation. They are fluid in their locations and in their inhabitants. The biota varies over time in abundance and composition, with some groups being mobile and moving among environments both vertically and horizontally. Continuous change is a feature of the physical environment of the Benguela, and its marine inhabitants are adapted to cope with this. Whether they will be able to cope with future change, and how, are questions that can only be answered once the nature of that change and the dynamics of the biome are better understood.
Written by Colleen Moloney and Lynne Shannon and extracted from the book Benguela Current of Plenty (published by the BCLME Programme in 2008).