2i8 DISCOVERY REPORTS 



these data^ the remarkable fact emerges that off the more southern Palominos Island 

 the April and May temperatures were, on the whole, higher than those off the Guafiape 

 Islands lying some 240 miles to the northward. Such a distribution of temperature 

 would not arise from a counter-current of warm water flowing from north to south 

 along the coast, and it is unlikely, therefore, to be due to a repetition of the Nino current 

 out of season. Lavalle gives no salinity data by which the origin of this water can be 

 established, but the temperature data might be explained if the counter-current came 

 from the open ocean and was an over-developed wedge of the type met with during our 

 survey. 



PHOSPHATE AND ORGANIC PRODUCTION 



In an earlier section, an attempt has been made to correlate organic production on the 

 west coast with the hydrological conditions. A study of the phosphate content has shown 

 that the concentration of nutrient salts at the surface varies according to the extent of 

 the cool water in the different localities (pp. 182-3) ! ^'^'^ t^^t the rich nutrient salts may 

 be identified with upwelling water is shown by comparing Figs. 54-61 with Figs. 18-50, 

 the corresponding sections of temperature and salinity. 



The relation of nutrient salts to the plant life in the sea is best known from the work 

 of European investigators, and in the sub-Antarctic from results recently obtained by 

 the Discovery investigations (Hardy and Gunther, 1935). An examination of this re- 

 lationship in the Peru Coastal Current has shed interesting light not only on the 

 conditions met with at the time of the survey but possibly also for some considerable 

 time in the immediate past. 



Volumetric measurement of settled phytoplankton shows that within 100 miles of the 

 land, catches are on average larger than beyond (Table XI); that in regions of rich 

 phosphate the average catch is larger than in regions of poor phosphate (Table XIII); 

 and that this close relation between the two is suggestive, on the strength of results of 

 Atkins and others, of a dependence of plankton upon phosphate.^ 



This dependence of the phytoplankton upon inorganic salts can be examined more 

 closely, only if it is possible to measure their reduction with the growth of the plankton. 

 Neither the methods employed by Atkins (1923), Gran (1927) or Schreiber (1927) are 

 available to a ship on the move, and an alternative though less accurate method has been 

 considered. It consists in comparing the phosphate concentrations above and below 

 the compensation point at each station, and it assumes that initially the phosphate con- 

 centrations at the surface and at 100 m. are approximately the same or within limits 

 bear the same relation to each other, and that subsequent changes are comparatively 



1 The higher index of the maximum and minimum thermometer appears to have been misread at both 

 islands, but the error does not seem to affect the comparative value of the data. 



- In Table XIII we see incidentally that when the zooplankton exceeds a certain concentration the phyto- 

 plankton is severely reduced. That the phytoplankton has been cropped down by the zooplankton which is 

 here in greater quantity than at any of the other localities listed, is put forward as the most hkely of possible 

 explanations. 



