ANTARCTIC SURFACE WATER 19 



ice-edge is the seat of very large concentrations. Now if at St. 855 a large concentration 

 had been existing for some months previously, it is obvious that a large amount of 

 nutrient salt must have been removed from the surface water during this period. The 

 phosphate value of 84 mg. is sufficiently low to agree with this large withdrawal, pro- 

 vided a pre-production value of about 130 mg. is allowed at the ice-edge. The silicate 

 value of 3400 mg. requires an explanation. 



Let us consider the utilization of phosphate and silicate by the phytoplankton. An 

 immediate result is the withdrawal of a high proportion of these salts from the photo- 

 synthetic zone of the surface layer. Then follows a consumption of the phytoplankton 

 by the zooplankton, which latter as far as we know does not require much silicate for 

 its nutrition. The zooplankton then excretes the undigested matter including the silica 

 skeletons of the diatoms. We have observed the results of this last process in the area 

 close to the position of St. 855. It can be argued that the zooplankton excretes the greater 

 amount of the silicate which comes to it in the form of its phytoplankton food 

 supply, whereas it utilizes a proportion of its phosphate intake in the formation of 

 phosphoproteins, etc. Thus, temporarily, part of the phosphate originally consumed 

 by the phytoplankton is withheld by the zooplankton, whereas the silicate, or a large 

 proportion of it, is returned to the sea in the form of broken diatom skeletons. How rapid 

 is the re-solution of these skeletons is not known exactly, but a rapid regeneration of 

 silicate must be assumed to explain some of the high silicate values recorded just after 

 phytoplankton maxima. By analogy with the English Channel silicate must be utilized 

 by the plankton more than once in one season (Cooper, 1933). Thus it is probable that 

 the high silicate value in the surface at St. 855 is due partly to regeneration in situ. As 

 will be explained later in this report it is also due to the return to the ice-edge of water 

 of high silicate content at this time of the year (see p. 36). Thus the incidence of a 

 moderately rich phytoplankton at St. 855 with a high silicate content in the surface water 

 is not anomalous. The value of 130 mg. as the phosphate content at the surface at 

 St. 1 361 is quite compatible with the negligible amount of phytoplankton present; the 

 silicate value is on the low side, however, for an ice-edge station in late April. 



Individual figures of phosphate and silicate contents for any region in different years 

 may vary very considerably, and we have an example of this at Sts. 855 and 1361. The 

 concentrations of nutrient salts preceding the main outburst of phytoplankton must 

 vary from year to year. This follows because the amount of these salts returned to the 

 ice-edge or anywhere in the Antarctic is dependent on the amount of nutrient salts that 

 is lost to the Antarctic zone in previous years. The whole conception of the amounts 

 of phosphate and silicate in the Antarctic zone is qualified by three considerations : the 

 speed of regeneration in situ of these nutrient salts is different, the times of the maximum 

 return to the south must be different, and the concentration of these salts anywhere 

 in the Antarctic must show an annual variation. I have discussed the first consideration 

 earlier in this section. The second consideration follows from the different methods of 

 return of phosphate and silicate, which makes coincidence of the times of return ex- 

 tremely unlikely. Around South Georgia, an area in which we have made more observa- 



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