178 



DISCOVERY REPORTS 



PHYTOPLANKTON 



Phytoplankton on the west coast is of interest, not only as the link between the pro- 

 ductivity of the various upwelling centres and the zooplankton, but also in relation to 

 the green colour of the current. In giving a preliminary account of the catches, we had 

 no better measurement than the volume of settled organisms. The method based on 

 chlorophyll estimation, recently introduced into this country by Harvey (1934), was not 

 available to us in 1931. It is well known that as a result of their different shapes, the 

 secretion of slime, etc., different species of diatoms pack differently, and that conse- 

 quently this method can be used only to distinguish major differences in the size of 

 catches. As, however, catches in the Peru Coastal Current varied in volume from less 

 than I to 390 c.c, the method may be employed with some possibility of success. The 

 amount of plankton at each station is given in Appendix I as the volume of settled 

 organisms (to the nearest 25 c.c), taken by the 50-cm. net from 100 m. to the surface. 



When these data are plotted on a chart, the heavier catches are found to occur in 

 groups; they thus formed areas of concentration and did not seem to be scattered at 

 random among the poorer catches. Some twelve such patches of higher concentration 

 can be made out ; they were arranged irregularly from south to north and are indicated 

 in heavy type in Appendix I. While they were more frequently met inshore than off- 

 shore, four of the patches occurred at more than 30 miles from land off Cape Carranza, 

 Caldera, San Juan and Callao. This is interesting, since it has frequently been held that 

 rich phytoplankton is restricted to the green water in the upwelling zone^ (see p. 222). 



If the mean volume of phytoplankton at varying distances from the shore is computed 

 from all samples collected during the survey, it is seen to be greatest at 4-5 miles offshore 

 and less at lesser distances (Table XI). At greater distances the quantity of phyto- 

 plankton first fell off and then reached a second peak at 56-100 miles offshore. As it 



Table XI. Mean concentration of phytoplankton at different 

 distances from the coast 



1 Michael (1921) claims to have identified the seat of greatest phytoplankton production with that of 

 upwelling on the coast of California. We are not prepared, however, to accept the published data as evidence 

 on this point, for the seat of upwelling seems to have been situated not in the region where phytoplankton 

 was examined, but in the neighbourhood of islands at some distance off. 



