LIMITING FACTORS 

 Table 21 



183 



* Exclusion of uncountable colonies makes these values too low. 



Taking the material as a whole, it appears to afford a good illustration of facts already 

 well known as regards the northern hemisphere: that the time of the spring increase 

 falls later in the year as one proceeds polewards, until in the highest latitudes there is 

 a continuous period of production after midsummer. This is well shown by the figures 

 from the Bellingshausen Sea. Thus the impression is gained that the phytoplankton of 

 the older Antarctic surface water in comparatively low latitudes shows more marked 

 seasonal variation, and fluctuates from year to year to a much greater extent, than that 

 produced farther south. In the higher latitudes phytoplankton was never observed in 

 anything like such quantities as around South Georgia (53-56° S) in late spring, and 

 to the north-east of the Weddell Sea area (56-63° S) at midsummer. The more southerly 

 phytoplankton was, however, very much more uniform in quantity. 



THE POSSIBLE LIMITING FACTORS 



CHEMICAL FACTORS 



The factors governing phytoplankton production in north European waters have been 

 intensively studied during the past ten years. An authoritative survey of the present state 

 of our knowledge on this subject has recently been published by Gran (1932). As he has 

 pointed out (p. 343), this study has been largelystimulated by the disco very of practicable 

 methods of determining the quantities of nutrient salts present in the sea water by 

 Atkins and Harvey at Plymouth. Accordingly, in considering conditions in the south, 

 the nutrient salts may be dealt with first. 



In north temperate waters Brandt's hypothesis, that the phosphates and nitrates 

 essential for plant life would be periodically exhausted to such an extent as to put a stop 

 to further prodviction, has been well established. This phenomenon, whi-y.h is most 

 marked in waters such as the English Channel after the great vernal diatom increase, 

 was not manifest in the south. In the more northerly parts of the Antarctic Zone the 

 phosphates in the upper layers showed a considerable decrease at mid-season, but were 



