344 DISCOVERY REPORTS 



of the surface layers, and the (interrelated) effects of pack-ice, which was recognized in 

 earlier work, cannot be doubted. They are certainly prime causes in determining the 

 time of the onset of the main increase, and the extent and duration of the autumnal 

 secondary increase in the more northerly parts of the Antarctic zone. However, they 

 do not by themselves explain the post-maximal summer decrease in the more northerly 

 Antarctic surface waters, or the vastly greater richness of the neritic areas. Now that 

 truly oceanic observations throughout the year are available, it is seen that it is only 

 in the neritic areas that Antarctic seas retain their claim to be the richest in the world. 

 Since the physical factors do not sufficiently account for this, explanation must be 

 sought among chemical and biological factors. Among chemical factors there is now 

 some direct evidence that temporary shortage of silica may be in part responsible for 

 the post-maximal summer decrease in both oceanic and neritic areas. The greater 

 richness of neritic areas remains inexplicable unless we assume that minute quantities 

 of inorganic compounds, as iron or manganese, or of organic compounds derived from 

 the land, exert a strongly favourable influence on diatom growth. We have no direct 

 evidence of this, but the growing body of experimental work by Harvey, Cooper and 

 others favours such an hypothesis. 



Among biological factors the effect of the grazing down of the phytoplankton by the 

 herbivorous zooplankton is probably of great importance in the poorer pastures of the 

 open ocean. In neritic areas of exceptionally rich phytoplankton Hardy has shown that 

 the converse effect, ' animal exclusion ' may occur, but there is little doubt that the post- 

 maximal summer decrease in diatoms must be accentuated, and to some extent caused 

 directly by grazing. In the Antarctic zone, and all other areas with marked seasonal 

 changes so far investigated, all available evidence shows that the zooplankton reaches its 

 peak at a distinct interval of time after the phytoplankton maximum. 



ACKNOWLEDGEMENTS 



It is a great pleasure to record my indebtedness to many kind friends on the staff of 

 the Marine Biological Association at Plymouth, where the work was concluded, by 

 kind permission of the Director, Dr S. W. Kemp, after the outbreak of war. I am 

 fortunate in having been able to discuss phytoplankton problems with Dr H. W. Harvey 

 for several years and am particularly indebted to him for information and demon- 

 stration of his methods in 1933 before he had published the account of them. By kind 

 permission of Dr E. J. Allen this enabled us to utilize the method throughout the third 

 commission, two years earlier than would have otherwise been possible. I am parti- 

 cularly indebted to Misses N. G. Sproston and D. R. Dibben, and my wife, for help in 

 preparing the MS. for the press. 



My indebtedness to my colleagues on the Discovery Staff will be obvious from the 

 text, and, as with all our work, the able co-operation of the marine staff, and the patience 

 and skill of the seamen in handling the gear under trying conditions, cannot be too 

 highly praised. 



