COMPARISON OF THE BENGUELA CURRENT WITH OTHER UPWELLING REGIONS 283 

 current below the Benguela current, flowing southwards and converging with the edge of the 

 continental shelf and bringing water of low oxygen content southwards from the equatorial region. 

 A similar current, better defined, is found off California (Sverdrup and Fleming, 1941) and also 

 below the Peru current, although in the latter case the circulation is more complex. We have mentioned 

 the inclusion of the subtropical convergence in the Peru current. At this convergence, subantarctic 

 water sinks and flows northwards along the coast below the warmer subtropical waters. Off the Peru 

 coast one can identify the southward flowing compensation current, but in 20 S. it meets the north- 

 ward flowing subantarctic water and is forced under the latter but continues to flow south. A salinity 

 maximum and the low oxygen content are also identifiable to at least 35 S. 



The outstanding feature of these compensation currents is their low oxygen content, which in the 

 Benguela current is traceable to the origin of the water in the minimal oxygen layers of the equatorial 

 parts of the oceans. Sverdrup and Fleming (1941) have shown that a further reduction of oxygen 

 takes place with the local decomposition of organic matter. Off the California coast the level of the 

 oxygen minimum lies at a greater depth than off Peru and South-west Africa. Water with an oxygen 

 content of less than i-o ml./l. was rarely found at depths of less than 250 m., even at the inshore 

 stations off California, whereas both in the Peru current and Benguela current upwelling brings the 

 low oxygen water up on to the shallower reaches of the continental shelf. 



It is clear that there are very close similarities between the abnormal conditions found off South- 

 west Africa, and the 'Callao Painter' and 'Aguaje' off Peru. At the north end of the Peru current 

 periodic incursions of warmer water invade the region where cool upwelled water is usually present. 

 These are referred to as the 'El Nino' current, and Gunther (1936) states that 'one finds in place of 

 the cool Peru Current diverging from the shore, hot poorly saline water of the Equatorial Counter 

 Current flows southwards and converges with it. The rise in temperature kills fish and plankton 

 which then decompose and emit sulphuretted hydrogen on an enormous scale. This is the Callao 

 Painter. . .'. While the 'Callao Painter' or 'Aguaje' 1 occurs in conjunction with the 'El Nino', it can 

 also be associated with the convergence of warm oceanic water with the coast, and in this respect 

 we find a close parallel with the conditions off Walvis Bay on survey I. 



We have expressed the opinion (p. 199) that the low oxygen content of the upwelled water, 

 accentuated under conditions of stagnation or reversed flow of the current, must be a very important 

 factor in the catastrophes off South-west Africa. The coincidence of low oxygen and the manifesta- 

 tions of the Callao Painter is strongly suggestive of a similar sequence of events off the coast of Peru 

 (Burtt, 1852; Currie, 1953 b). 



Perhaps it is significant in this respect that records of pronounced abnormal conditions and fish 

 mortalities are rare on the California coast, where the low oxygen water does not normally appear to 

 be elevated to the proximity of the sea-surface on the continental shelf. 



The frequency of records of green diatomaceous mud from the Peru coast (Neaverson, 1934) lend 

 further support to the theory that conditions on the sea-bed of the continental shelf must be virtually 

 anaerobic. Otherwise the chlorophylls and associated pigments would be oxidized to give the sedi- 

 ments a brownish or grey appearance. 



Deposition of diatomaceous mud occurs at a much greater depth off Peru than off South-west 

 Africa. Off South-west Africa the diatom mud occurs principally between depths of 50 and 150 m., 

 though it extends to greater depths at the landward end of the Walvis Ridge. The sediments in the 

 deep water beyond the continental shelf are mainly globigerina ooze. Off Peru, however, Neaverson 



1 Evidently some confusion attends the use of the name 'Aguaje'. In the New Velasquez Dictionary it is denned as 'A 

 current in the sea, persistent or periodical ; e.g. the Gulf Stream ' ! Dr Wooster points out to us his impression that ' Aguaje ' 

 used in Peru refers to discoloured water of biological origin, only occasionally associated with the 'Callao Painter'. 



