1 86 



DISCOVERY REPORTS 



at 80 and 100 m., for each station. These percentage values are compared with the 

 corresponding phytoplankton catches in Table XIV. The mean of the percentage 

 reductions for each value of the phytoplankton shows a correlation; and, if we may 

 suppose an interrelation at some earlier date between upper and lower layers, it may 

 be said that off Cape Carranza the volume of the phytoplankton varies inversely with 

 available phosphate. 



Table XIV. Relation of phosphate reduction to volume of phytoplankton 



Surface phosphate is measured against the value at 100 m., and the difference, expressing reduction, is 

 given for each station as a percentage of the latter (see text, p. 184). The stations of each line are arranged in 

 order of increasing phytoplankton yield and for stations of similar yield the figures expressing phosphate 

 reduction have been averaged. The figures in heavy type represent observations made in the region of the 

 highly saline warm-water wedge. At Sts. WS 591 and 628 the 50-cm. net was not fished. 



The lack of phosphate off Pichidanque Bay has been attributed to an absence of up- 

 welling; on this line the phytoplankton is also meagre, and if the presence of thick 

 phytoplankton is regarded as evidence of an earlier abundance of nutrient salts, its 

 absence here indicates that upwelling has probably not taken place for some considerable 

 time. 



The small quantity of phytoplankton and the hydrological conditions agree in showing 

 that at Caldera upwelling was of recent date. The high temperature and the minimal 

 phosphate at the surface on the seaward part of this line makes it probable that recent 

 conditions had been preceded by a period of prolonged calm. On the other hand, the 

 water is homogeneous to a depth of 50 m., which is evidence that the calm period was 

 followed by winds of a strength sufficient to cause extensive vertical mixing in the 

 upper layers. The recent occurrence of these winds is consistent with the upwelling 

 inshore; while the fact that upwelled phosphates were diluted and the fact that in 



