FISHERY BULLETIN: VOL. 75, NO. 4 



processes in an attempt to give a more complete 

 explanation of the observed patterns (Wyrtki 

 1962; Bubnov 1966, 1972; Menzel and Ryther 

 1968). 



Proponents of the first mechanism argue that an 

 oxygen minimum layer is formed as a result of 

 biochemical oxidation of organic matter that has 

 accumulated at intermediate depths due to 

 specific gravity relationships between seawater 

 and sinking detritus (Seiwell 1937; Miyake and 

 Saruhashi 1956). Those supporting the second 

 mechanism suggest that an oxygen minimum will 

 be formed at the relatively still boundary between 

 circulating water masses where replenishment of 

 oxygen will be minimal. This view was first ad- 

 vanced by Jacobsen in 1916 and was later sup- 

 ported by Dietrich and Wiist (Richards 1957). 



More recent studies (Redfield 1942; Wyrtki 

 1962; Bubnov 1966, 1972; Menzel and Ryther 

 1968) have stressed the importance of advective 

 processes in the formation of oxygen minimum 

 surfaces. Redfield (1942) hypothesized that the 

 deep oxygen minimum of the Atlantic could be 

 formed by advection along isentropic surfaces of 

 water carrying a heavy load of organic detritus 

 and solutes from high latitude convergence areas. 

 Subsequent oxidation of the organic load forms a 

 minimum. Wyrtki (1962), Menzel and Ryther 

 (1968), and Bubnov (1966, 1972) considered high 

 oxygen consumption as necessary for initial for- 

 mation of low-oxygen water with advective and 

 mixing processes controlling its position and 

 movement. Wyrtki (1962) contended that oxida- 

 tion occurring in the layer of least advection re- 

 sults in formation of an oxygen minimum, which 

 can spread by mixing into other water masses. 

 Menzel and Ryther (1968) and Bubnov (1966, 

 1972) argued that oxygen depleted water will form 

 in specific areas due to high biochemical oxygen 

 consumption and that these waters are then 

 spread by advection and turbulent diffusion. 



Bubnov (1972) stated that the main factors con- 

 trolling the formation of an oxygen minimum are 

 the rate of biochemical oxidation, the density 

 stratification of the water, and the supply of 

 oxygenated water to bottom layers. In the south- 

 eastern tropical Atlantic the presence of one or 

 more of these factors results in highly favorable 

 conditions for the formation of an oxygen 

 minimum. The coastal region off South- West Af- 

 rica has strong upwelling conditions which result 

 in high organic production and subsequent high 

 oxygen consumption (Hart and Currie 1960). 



Though the coastal waters are weakly stratified in 

 comparison with the region of the Congo River 

 effluent, there is, nonetheless, a well-developed 

 pycnocline which inhibits downward-mixing of 

 highly oxygenated surface waters (Visser 1970; 

 Bubnov 1972). In addition, the deep waters of the 

 Angola Basin are somewhat lower in oxygen than 

 those of the western basin of the South Atlantic. 

 This reduces the amount of oxygen which will be 

 mixed into the upper layers by upwelling or turbu- 

 lent diffusion (Bubnov 1972). Taft (1963) and Vis- 

 ser (1970) suggested that the waters to the north of 

 lat. 20 °S off the coast of South- West Africa may be 

 isolated from the highly oxygenated deepwater 

 masses formed at high latitudes, thus inhibiting 

 the renewal of oxygen from this source. 



Because of the favorable conditions for the for- 

 mation of low-oxygen water in the coastal region 

 of South- West Africa, it has been suggested that 

 this area is a source for much of the water that 

 forms the oxygen minimum surfaces in the South 

 Atlantic. Taft (1963) plotted the oxygen and salin- 

 ity distributions on surfaces of constant potential 

 specific volume anomaly for the South Atlantic. 

 On the 125, 100, and 80 cl/t surfaces (a-0 26.81, 

 27.07, and 27.49 g/1, respectively), the isopleths of 

 both oxygen and salinity are zonal at lat. 20°S. 

 The areas of lowest oxygen concentration are lo- 

 cated just north of lat. 20°S off the coast of 

 South- West Africa, strongly suggesting that this 

 region serves as a source area for low-oxygen 

 water which is then transported westward to form 

 the primary minimum at 300-600 m in the study 

 area). 



In a study by Menzel and Ryther (1968), the 

 concentration of dissolved organic carbon in the 

 South Atlantic was found to be essentially con- 

 stant below 400-500 m while the oxygen content 

 varied. Based on this finding, they concluded that 

 oxygen concentrations in the minimum layer will 

 not be further reduced by in situ decomposition of 

 organic matter. They suggest that low-oxygen 

 water is formed off South- West Africa and is then 

 distributed horizontally along isentropic surfaces 

 to form the primary oxygen minimum layer. 

 Changes in the oxygen content occur by mixing 

 with water masses of higher oxygen content, re- 

 sulting in the increase of oxygen concentrations as 

 the water moves farther from its source. 



Bubnov (1972) identified three areas off 

 South-West Africa where waters of very low oxy- 

 gen content are formed (see Figure 1): 1) the shelf 

 region to the south of lat. 17°S, 2) the coastal 



858 



