IMPORTANCE OF THE GRAZING DOWN FACTOR 339 



because of the abundance of data during the earHer part of the increase. If it were 

 permissible to include the figures for November-December 1933 the average standing 

 crop value would be increased so that one would deduce a grazing intensity somewhat 

 less than in the English Channel. 



These deductions are based on a phosphorus/plant pigment ratio which may be 

 inaccurate for our southern species, though as shown above the error should not be 

 great. Whatever the ratio may be it will not affect the conclusion that grazing intensity 

 is from three to five times greater in the Northern Region (oceanic) than in the South 

 Georgia area (neritic). With regard to the comparison with conditions in the English 

 Channel it is obvious also that if the figure o-o8 mg. P per 1000 units is too high, the 

 greater intensity of grazing down south will be even more marked. A positive cor- 

 rection of 100%, which is not likely to be needed, would still leave us with a greater 

 grazing intensity in the Northern Region than in the English Channel, where the 

 grazing would work out at double that of the South Georgia area. 



It seems clear, therefore, that in actual fact the grazing intensity in the Northern 

 Region is of the order of three times that found in the English Channel. In the South 

 Georgia area it is probably somewhat less than in the English Channel. This would, I 

 think, be considered probable by anyone with extensive experience of collecting plankton 

 in the areas concerned, on the grounds of the relative sizes of the zoo- and phyto- 

 plankton catches. It would also tend to reconcile the facts that while in the Northern 

 Region we have found some evidence that grazing may be the chief cause of the post- 

 maximal decrease in standing crop, in the South Georgia area, in the dense phyto- 

 plankton at the height of the main increase, Professor Hardy finds evidence of the 

 converse effect — animal exclusion. 



While temporary shortage of silica and grazing by zooplankton are probably largely 

 responsible for the post-maximal decrease in the more northerly parts of the Antarctic 

 zone, none of the factors so far examined adequately account for the vastly greater 

 richness of the neritic areas as compared with the oceanic regions. We are left with 

 the hypothesis that extremely small amounts of organic compounds, iron, and 

 manganese (cf. Harvey, 1937, 1939) derived from the land, exert a strongly favourable 

 influence on phytoplankton production. The work of Harvey, Cooper and others at 

 the Plymouth Laboratory during the last few years strongly supports such an hypothesis. 



One important feature of the work described in this paper which cannot be too 

 strongly emphasized is the great importance of the pack-ice in maintaining the flora 

 within the Antarctic zone and in giving rise to what might be termed pseudo-coastal 

 conditions at vast distances from land, where neritic species maintained by the ice 

 flourish for short periods when the latter disperses. This effect of the ice is even more 

 marked than earlier observations led us to suppose, but cannot be fully demonstrated 

 until there is opportunity for more detailed study of material collected from the ice 

 itself during the last six years. 



