Upwelling and Primary Production
Northward winds along the coast result in Ekman transport offshore and upwelling of nutrient rich deep water to the euphotic zone. The intensity of the upwelling event is determined by wind strength. Variations in wind strength cause pulses of upwelling, which propagate to the south along the coast with speeds of 5 to 8 m/s. The pulses are similar to a Kelvin wave, except on a scale of 30 to 60 km instead of 1000 km, and can propagate around the cape depending on wind systems.
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 new production in the Benguela system is 4.7 × 10^13 gC/y, making the Benguela system 30 to 65 times more productive per unit area than the global ocean average.
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 sulpher rather than oxygen reside in the oxygen minimum zone.
The most abundant fishes in the Benguela system are Sardinops and Engraulis. Sardinops ocelata (pilchard) was intensely fished beginning in the 1950s and peaking in 1968 with landings over 1.3 million tons. Since then, the Sardinops fishery has declined and the Engraulis capensis (anchovy) fishery has taken over.
Read more about this topic: Benguela Current
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