Benguela Current

Boundaries

The icy Benguela and the warm, south-flowing Agulhas current do not meet off the Cape of Good Hope (see diagram on the right, above), but there is a body of water off the South African south coast, east and particularly west of Cape Agulhas that consists of eddies from both currents, so that offshore water temperatures along the south coast of Africa vary chaotically.[[File:Upwelling image1.jpg|thumb|right|250 px|The red areas show major upwelling areas. The Benguela Current is on the southwest coast of Africa.]]

Upwelling and primary production

Pulses of upwelling induce biological production. In the Benguela system, phytoplankton growth requires a period of upwelling followed by a period of stratification and relatively calm waters. The phytoplankton bloom usually lags the upwelling event by 1 to 4 days and blooms for 4 to 10 days. In order for zooplankton to have a continuous food supply, the phytoplankton blooms must not occur too far apart. Pulses of upwelling in the Benguela system regularly have a duration of 10 days, an optimal period for biological production. It is estimated that the annual biomass production in the Benguela system is 4.7× gC/y, making the Benguela system 30 to 65 times more productive per unit area than the global ocean average.{{cite journal

While upwelling promotes abundant primary and secondary production in the upper parts of the water column and near the coast, deeper waters with limited oxygen exchange create hypoxic areas called oxygen minimum zones at the coastal shelf and upper coastal slope. The Benguela oxygen minimum zone starts around a depth of 100 m and is a few hundred meters thick. Bacteria that use sulphur rather than oxygen, called sulphur-reducing bacteria, reside in the oxygen minimum zone.{{cite journal

The most abundant fish in the Benguela system are Sardinops and Engraulis. The Southern African pilchard (S. s. ocellatus), was intensely fished in the 1950s and peaked in 1968 with landings over 1.3 million tons. Since then, fishery of the Sardinops has declined and fishery of the Southern African anchovy (Engraulis capensis) has taken over.{{Cite book

Benguela Niño

Similar to the Pacific El Niño, a thick slab of warm, nutrient-poor water enters the northern part of the Benguela upwelling system off the Namibia coast about once per decade. During the Benguela Niño, warm, salty waters from the Angola Current move southward, between 15°S and 25°S. This slab of warm salty water extends to 150. km offshore and to 50. m depth. Heavy rains, changes in fish abundance, and temporal proximity to the Pacific El Niño have been observed; however, the causes and effects of the Benguela Niño are not well understood. One research team has shown that the Benguela Niño is caused by winds in the west-central equatorial Atlantic Ocean that propagate as subsurface sea temperature anomalies to the African coast.{{Cite journal A recent study has demonstrated the importance of local winds in the development of the Benguela Niño off the coast of Namibia and Angola. This local process together with the remote signal from the equatorial regions form the basis of the formation mechanism in which both processes sometimes reinforce each other.{{Cite journal

References