i84 DISCOVERY REPORTS 



never depleted below a minimum of ca. 50 mg. P2O5 per m.^ Thus if we are to assume 

 that the falling off in production after mid-season is due to lack of these substances, we 

 must also assume that the Antarctic plankton diatoms are incapable of utilizing con- 

 centrations of phosphate and nitrate considerably above the winter maximum in the 

 English Channel. This is extremely improbable, and it would seem fairly certain that 

 generally speaking nutrient salts are not at any time limiting for phytoplankton growth 

 within the Antarctic surface water. This is doubtless the main reason for the extremely 

 rich production, which will probably be found to exceed that of any other large area in 

 the world, apart perhaps from such local effects as in the Benguela and Humboldt 

 currents, where upwelling from the deeps gives rise to a continuous supply of nutrient 

 materials at the surface. 



The numerous analyses for phosphate, nitrate, and to a lesser extent nitrite, made 

 during the course of the work are given in full in the Station Lists (1930 and 1932); 

 in addition, the mean phosphate values for the surface layer have been included at the 

 head of the phytoplankton tables in this paper. In only two cases has it been found 

 possible to draw any comparison between the amount of phosphate present and the 

 abundance or otherwise of the plankton. These occurred on the two surveys round 

 South Georgia. On the first of these, worked in the abnormally warm summer 1929-30 

 when phytoplankton was scanty, it was observed that the phosphate minimum was 

 regularly found at 20 m. depth at all stations where the phytoplankton was appreciable. 

 This has an obvious bearing on the effect of light penetration. On the second spring 

 survey, when production was at its height, the phosphate minimum occurred at 10 m. 

 or at the surface, and there was some correlation between the heaviest catches and 

 relatively low phosphate values. Taking the mean for the upper 20 m. on the two surveys, 

 however, we find 79 mg. P2O5 per m.^ in summer and 83 mg. per m.^ in the spring. 

 It is obvious that so small a difference cannot have much bearing upon the immense 

 disparity between the phytoplankton observed on the two surveys. 



Clearly the comparatively rapid movements of the surface layers off South Georgia 

 prevent the phosphate content of the water from showing any continual relation to the 

 production on the spot, as in areas forming a practically closed system like the English 

 Channel (Cooper, 1933, p. 678). A clear instance of this was encountered during the 

 plankton survey of January-February 1930 at the outer end of the Larsen line. Here, 

 at St. 313, the mean phosphate value for the upper 20 m. was 105 mg. per m.», and 

 some six million phytoplankton organisms were present in the net haul. At St. 358, 

 worked 10 miles farther to the eastward only a fortnight later, it was very evident that 

 water of an entirely different type was present (see Table II). The phosphate value was 

 60 mg. per m.' and some five million phytoplankton organisms were present in the net 

 haul. With one exception, however, the dominant species were not the same at the two 

 stations. Evidently the phosphate content of the surface waters round South Georgia 

 does not reflect the changes in production in that locality at all closely, but is rather the 

 resultant of the production over the large indefinite area farther to the south and west, 

 where the various types of water have their origin. Even if sufficient seasonal data were 



