ANTARCTIC SURFACE WATER 37 



dissolve in the far south and become available to the surface layer in these months is 

 remote, because the main belt of diatom ooze is found near the Antarctic convergence, 

 and the northern region diatoms are most likely to be deposited there. 



In the curve for the northern region a fall of silicate occurs from 1 September to 

 1 January, followed by a rise to 1 April and a fall to 1 May. Dr Hart informs me that 

 in the northern region the figures of phytoplankton concentration show a very slight 

 production in August, which increases by 100 per cent in September, by 300 per cent 

 in October and by 13,500 per cent during November. Owing to the large amount of 

 silicate present in the surface layer in the northern region before the main outburst, 

 very little effect will be shown as a result of consumption by phytoplankton until 

 November when the increase in the concentration of phytoplankton is enormous. The 

 fall in the northern curve from 1 September to 1 November is therefore unrelated to 

 phytoplankton requirements, and it is only after 1 November that a correlation is possible 

 between the fall of silicate content and the increase of phytoplankton concentration. 



In the northern region the phytoplankton is definitely on the wane by the end of 

 December, when the zooplankton must be exerting a greater effect on the concentration 

 of phytoplankton than any continued production. This is partially reflected in the rise 

 of the silicate averages from 1 January to 1 April, i.e. when the phytoplankton production 

 falls off (and it falls off very rapidly) the monthly averages of silicate content start to 

 increase. This increase must be due to regeneration in situ and to the arrival from the 

 south of the Antarctic zone of surface water containing a larger amount of silicate. In 

 the far south the Antarctic surface layer has a much greater silicate content than in the 

 northern region ; owing to the northerly component in the movement of the surface 

 water the northern region will be continuously refreshed by the arrival of water of high 

 silicate content from the south. The effect of phytoplankton production in the far south 

 and in the intermediate zone will, however, tend to reduce the silicate content of surface 

 water arriving at the northern region. The silicate values in the northern region for the 

 0-100 m. layer never approach in any month the high values recorded in the southern 

 region for December to May, and thus we must assume that the difference between the 

 maximum values in the two regions represents the amount of silicate that is utilized 

 by the phytoplankton during the passage of the surface water across the Antarctic zone. 

 This difference is of the order of 1250 mg., and this amount can only be held by the 

 phytoplankton or be lost to the surface layer by sinking of dead diatom skeletons. 



In the northern region curve a fall of silicate is shown for the month of April. This 

 fall may be due to two causes : (1) a small autumnal outburst of phytoplankton or (2) the 

 arrival in the northern region of surface water which has experienced peak conditions 

 of the southern phytoplankton concentration. There is some evidence, but not a great 

 amount, for the first cause, as the catches of phytoplankton show a slight increase 

 in the autumn. As regards the second cause there is no doubt that the activity of 

 phytoplankton in the southern region must have an effect on the silicate content of the 

 surface water, which during its passage northwards across the Antarctic zone has more 

 and more nutrient salts withdrawn by intermediate zone activity. 



