io6 



DISCOVERY REPORTS 



pletely mixed ; the result is a homogeneous layer and the phosphate and silicate contents 

 are equally distributed so that a pre-outburst value of the surface phosphate is probably 

 125 mg. and 3000 mg. for silicate. Thus at St. 1529 in February 1935 as much as 75 per 

 cent of the available silicate and nearly 50 per cent of the available phosphate had 

 been withdrawn from the 0-30 m. layer by the large concentration of phytoplankton. 

 It does not follow that the above proportions of the concentrations of the available 

 nutrient salts are always withdrawn by the main outburst of phytoplankton, nor is the 

 amount utilized in this way the same throughout the Antarctic. All plankton workers who 

 have experienced Antarctic conditions are agreed upon the ' ' patchiness " of the plankton, 

 there, and this unevenness must be reflected in the nutrient salt contents. As an example, 

 data are given in the following table from two stations some miles apart, but in the 

 same hydrological region and made within a day of one another : 



St. 1269, 67 33-8' S, 162 537' W, 18 January 1934 



Depth (m.) 

 P 2 O s mg./m. 3 



Si0 2 mg./m. 3 



100 



117 

 3400 



St. 1271, 65 05' S, 166 08-4' W, 19 January 1934 



Depth (m.) 



P 2 °5 m g-/ m - 3 

 Si0 2 mg./m. 3 



100 

 129 



3900 



The density of the phytoplankton in the 0-50 m. layer was between 12 and 13 times 

 as great at St. 1269 as at St. 1271. 



As we have seen earlier in this report phosphate and silicate leave the Antarctic in 

 the form of plankton, or in the northward flowing Antarctic surface and bottom currents. 

 These nutrient salts must be returned to the Antarctic zone mainly in the warm deep 

 water but the whole account of the regeneration is much more complicated than this 

 statement might suggest. During the time of the main outburst of phytoplankton in the 

 Antarctic zone, not only are the Antarctic surface and bottom waters carrying away 

 large concentrations of plankton and nutrient salts but mortality of phytoplankton from 

 a number of causes including consumption by the zooplankton is also taking place. The 

 consequence is that a rain of debris consisting of dead diatoms, foraminifera, radiolaria, 

 cast skins of crustaceans and excretions by the zooplankton takes place. Not all the 

 water masses in the Antarctic zone have a northerly component of movement; for 

 instance, the warm deep water presses southwards and carries with it any debris that 

 happens to be in or pass into this layer. Later the debris may pass into the Antarctic 

 bottom water which has a northerly movement. The existence of the belt of diatom 

 ooze near the Antarctic convergence is clear evidence of the rain of dead diatoms. 

 When the phytoplankton ceases to be a living entity and the individuals sink or are 

 excreted, regeneration of the nutrient salts is possible. The actual method of regeneration 

 is imperfectly understood but presumably it must be aided by bacterial action in the 



