By Hu Berry
Lying like a liquid conveyor belt along Namibia’s coastal continental shelf, the northward drift and upwelling of the Benguela Current ensures a prolific food production for marine animals and humans. This can, however, be interrupted by climatic shifts that result in devastating consequences for all forms of life, from plankton to people.
Visualise the world’s oceans as a massive chain of interlinking cold and warm currents, which determine winds and weather in their wake. There is no start and no end to this marine cycle; consequently the Benguela, which is part of the South Atlantic gyration or spiral, forms part of a much larger system. For Namibians and their visitors, the Benguela ‘starts’ at our southern coastal boundary and ‘ends’ at the mouth of the Kunene River bordering Angola. Its actual borders are in the south Atlantic, where it interacts with the warm Agulhas Current, and in the north near Moçâmedes, where it meets the tropical Angolan Current. The width of the continental shelf, where the Atlantic dips to more than 400 metres, varies greatly and this has a significant influence on the current. Land and sea temperatures vary, spawning wind and creating currents.
The Benguela’s birthplace is the Antarctic, from where its flow is directed northwards by the rotation of the earth and the southerly wind regime. The parent winds are controlled by the perpetual South Atlantic high air-pressure system and the high-pressure cells that hover over Africa. Wind stress reaches its peak near Lüderitz, encouraging the formation of another type of sea, that of sand and dunes. The higher the winds, the higher the dunes and so sand mirrors wind strength along the Namibian coast. Between Lüderitz and Walvis Bay, where wind storms rage, the ‘sand sea’ of 34 000 square kilometres illustrates this, adding a convincing grand finale at Sossusvlei to create monumental dunes that tower up to 380 metres. The wind abates somewhat near Swakopmund, leaving only miniature dune hummocks in its path, and then gains strength again north of the Uniab Delta, creating the northern dune belt.
Globally important reservoir of biodiversity
If wind can craft dunes it can also move sea surface water and so, opposite the dune fields, the phenomenon of ‘upwelling’ is most evident. Distinct zones of rising cold water columns replace the warmer surface water, which is driven off by wind squalls. These ‘cells’ occur in pulses of about one week in length, bringing oxygen-rich water and nutrients from the depths in contact with sunlight. Upwelling sets the stage for fertilisation of food for single-celled plants. A host of single-celled plant species exploit this ‘window of opportunity’, proliferating exponentially to form dense phytoplankton ‘blooms’. Thereby wind energy transforms into water energy, which translates into food energy, which transmutes into fish production. Moreover, the system is constantly replenished by frequent surges of upwelling. This food chain is structured to favour generalists above specialists. Thus, plant plankton feeds the major marine herbivores like anchovies and sardines (or pilchards), which are adapted to explosive growth, forming dense schools that stretch for kilometres across the sea surface.
Just as there are predators that hunt herbivores on Africa’s savannahs, so sea carnivores pursue their prey. The small pelagics are stalked by a host of hunters, such as barracuda (snoek) and kob (kabeljou). Added attacks come from the sky when gannets arrow their bodies in an aerodynamic dive, impaling quarry with lance-like beaks, and cormorants swim to depths of many metres, feeding to repletion. In synchronised pods pelicans scoop up in their pouches fish that reach the shallows of lagoons. Even if they survive this organised, natural onslaught there are the trawl nets and long lines of fishing boats and baited hooks of surf anglers dangling in the water to lure fish from their salty home into the frying pan. Some shore anglers fish for pleasure, others for food, others for both, but they may not be aware that the combined cost of transport, fishing tackle and bait usually exceeds, by several magnitudes, the value of the occasional fish caught. Indeed, it would be cheaper to purchase the catch at a supermarket and forego the pleasure or frustration that accompanies a good or poor day at the beach.
The Benguela is unique for its upwelling system in that it is hemmed in by two warm-water regimes, which fluctuate south and northwards unpredictably. Influxes of relatively tepid water can have dire effects, such as in 1994 when a major invasion of the Angolan Current resulted in ‘red’ and ‘black’ tides of toxic plankton. All marine life above the primary plant-food chain suffered and tens if not hundreds of thousands of sea birds and seals perished from hunger, their carcasses littering the beaches. This offering provided a glut of decomposing protein for scavenging gulls, jackals and hyaenas. Rock lobsters evaded the oxygen-depleted water by crawling ashore, only to asphyxiate in fresh air. Fishermen too experienced empty nets and trawlers were berthed as fishing factories suspended operations. Recent research reveals that the Benguela has a decadal (10-year cycle) of upwelling intensity. Moreover, when the wind is too strong, too much surface water is blown away and causes turbulence, which decreases food availability. Weak winds result in low upwelling and low enrichment of nutrients. Marine scientists speak of an ‘optimal environmental window’ of moderate upwelling for the system, which appears to coincide with the decadal cycles.
There is another mighty force far from the Benguela biome that can wreak havoc with its finely tuned system and that also influences three other large surface currents, namely the Humboldt, California and Canary currents. When El Niño appears off Peru and northern Chile at Christmastime, the potent influence of the Pacific Ocean is reflected in the Benguela. Because the Pacific girdle spans nearly half the circumference of the earth and has a gigantic heat-storage capacity, it affects the total global atmospheric system. Furthermore, its accumulated heat wanders in enormous cells to many points of the compass. El Niño’s signal reaches the Benguela in the form of a significant increase of upper-air westerly winds that collapse the normal airflows over the South Atlantic. The Benguela becomes enslaved by the Pacific. Unfortunately, the 1970s and 1980s witnessed enhanced El Niño activity at a time when over-fishing was rife in the Benguela. This led to the virtual collapse of the pilchard industry, from which it has yet to recover.
Several seabird species have mirrored this episode. Cape cormorants, previously numbering more than one million, are now estimated at 250 000 and are considered ‘near threatened’. More alarming is the status of African penguins. Their population of 1.5 million a century ago has shrunk to 150 000, a decline of 90%. Their conservation status is ‘vulnerable’, meaning the species faces a high risk of extinction in the wild if present trends continue. The Benguela Current is a globally important reservoir of biodiversity. It is and will remain climate driven, but humans have added another dimension. We are stressing the global natural systems, including the Benguela, and we may not be prepared for the consequences. The Benguela requires moderation in all aspects – sufficient wind, balanced upwelling and judicious fishing. If wind and water regimes were to change significantly in the Benguela, there would be little or no fishing. Humans have changed the natural equation with disastrous effects and now we must correct it.
This article appeared in the 2009/10 edition of Conservation and the Environment in Namibia.
Hu Berry was a scientist, conservationist and specialist tour guide. He was one of Venture Publications' most valued authors. Sadly he passed away in July 2011. To read more about him click here.