284 DISCOVERY REPORTS 



(op. cit.) records diatom mud in abyssal depths; it is usually brown, but sometimes retains a greenish 

 appearance (WS 694, 1216 m.). Tentatively one may ascribe this deposition in deep water to the lesser 

 width of the continental shelf off Peru. The persistence of a green colour of organic origin in these very 

 deep water sediments in the Peru Basin is more difficult to understand, for the sediment must be 

 deposited through water which is fairly well oxygenated. 



ORGANIC PRODUCTION IN THE BENGUELA CURRENT 



The 'William Scoresby's' surveys were aimed at obtaining a more detailed picture of water-movements 

 and plankton distribution in the Benguela current than had hitherto been available. What we have 

 obtained, therefore, is a more or less instantaneous picture of what was taking place on the two 

 surveys. Such limited observations, particularly in a region of such great variability, can tell us little 

 about such a complex process as organic production. We can, however, say something of the biological 

 activity which was taking place in the particular water masses and from this obtain some idea of how 

 the biological processes within the current compare with those in the adjacent oceanic waters. 



On the second survey the greatest quantities of phytoplankton were taken close to the coast between 

 24 and 26 S., in an area where active upwelling was taking place. Further offshore the quantities 

 decreased, and this suggests that the most favourable conditions for the proliferation of the phyto- 

 plankton were present in the very (nutrient) rich, cold water entering the euphotic zone. The upwelling 

 water enriched by the local regneration must have had a very high concentration of dissolved inorganic 

 phosphate-phosphorus, and even this profuse growth of phytoplankton had only reduced it to levels 

 still above 1 mg. at P/m. 3 . Further seawards, phytoplankton and phosphate both decreased. 



In the oceanic surface-water offshore there was a secondary increase in the quantity of phyto- 

 plankton, but the amount of phosphate continued to decrease. Qualitatively, however, this offshore 

 phytoplankton was different from that inshore, being dominated by panthalassic or more definitely 

 oceanic species — some of the solenoids in particular for which no doubt the oceanic environment, 

 enriched by lateral mixing from the coastal waters and perhaps also by some divergence seawards of 

 the boundary (see Fig. 96), presented most favourable conditions. 



It seems, therefore, that on survey II we have a situation analogous to that described by Sargent and 

 Walker (1948) in the California current. There they found the richest population, dominantly 

 chaetocerids, in the recently upwelled water. From this there was a succession, in the eddies of 

 upwelling water, to a rather sparse warmer water flora in the upwelled water which had been in the 

 surface for the longest time. On survey II in the Benguela current, the eddies were not so far developed, 

 and what we have reported may be a compression of the succession indicated by Sargent and Walker, 

 between the coast and the boundary region, with a separate and distinct oceanic flora to the west of 

 the boundary. 



On the first survey, in autumn, the quantity of phytoplankton in the oceanic water was considerably 

 less, by four to five times, than in spring, but there is some evidence that at least in places it may have 

 been richer shortly before the survey was made. At stations WS 996 and 997 the very marked phos- 

 phate depletion in the surface-layers, with the presence of a large number of pteropods and numerous 

 faecal pellets, suggests that the low crop of phytoplankton may recently have been much greater and 

 had since been reduced by heavy grazing by the pteropods. 



In the coastal waters, however, in the relatively abnormal conditions of the autumn survey (I) the 

 average numbers of phytoplankton were greater than in spring, and the area of high concentration 

 extended farther from the coast. Chaetocerids again dominated in the richest region but not all of 

 the dominant species were the same. Many more Asterionella japonica, Eucampia zoodiacus and 



