so DISCOVERY REPORTS 



in phosphate. The concentrations of phytoplankton actually found on this line of 

 stations in April do not justify the term autumnal outburst, and it seems safer to 

 assign the reason for the varied phosphate content in this month to a combination of 

 other causes. The surface water in the Scotia Sea is composed of water which has passed 

 through the Drake Passage from the Bellingshausen Sea and of water from the Weddell 

 Sea. There is a difference in the concentration of phytoplankton supported by the 

 Bellingshausen and Weddell Sea waters, and a corresponding difference at the end of 

 summer in the phosphate content remaining in these waters. North of the South 

 Orkneys the isotherms show that the Weddell sea water makes a series of thrusts among 

 the water of Bellingshausen Sea origin, with the result, at this time of the year, of a 

 varied phosphate content in the surface layer. Any phytoplankton activity in the far 

 south of the Weddell Sea will have repercussions in the phosphate content of the water 

 arriving at the South Orkney Islands. The January observations are uniform because 

 they were made at a time too early for them to show any effect of heavy withdrawal of 

 nutrient salt in the far south, and they merely reflect the uniform withdrawal in the 

 Scotia Sea, i.e. in situ. The main outburst farther south does not reach its maximum 

 until late in February, and water arriving at the southern end of our series of stations 

 in March or April will show the effect of the peak production farther south superim- 

 posed on any regeneration occurring in situ. It is interesting to note that the surface 

 phosphate content at St. 1331, the most southerly station and the closest to the South 

 Orkney Islands, is the lowest of all the observations in April, whereas the silicate 

 content at this station is the highest among these observations. This difference in 

 magnitude of nutrient salt content must be connected with a difference in speed of 

 regeneration of phosphate and silicate. 



Between the Falkland Islands and Elephant Island there are two sets of observations, 

 one in March and the other in September, both taken immediately before the April 

 and October data north of the South Orkneys. Owing to the north-easterly trend of the 

 Antarctic convergence there are fewer stations in the Antarctic zone between the 

 Falklands and Elephant Island than north of the South Orkneys. The surface content 

 and average of the 0-100 m. layer are plotted in Fig. 12 for both the Antarctic and sub- 

 Antarctic zone stations. The absence of any zigzag effect in the March curves is striking ; 

 this is in contrast with the curves for the observations to the east, i.e. northward of the 

 South Orkney Islands which were taken in early April within a day or two of the 

 observations south of the Falklands. The uniformity may be due to the fact that the 

 observations in the Antarctic zone south of the Falklands were made in water mainly of 

 Bellingshausen Sea origin, whereas the observations north of the South Orkney Islands 

 are subject to the influence of the Weddell Sea. The phytoplankton concentrations in the 

 Bellingshausen Sea are smaller than those in the Weddell Sea ; thus any effective removal 

 of nutrient salt in the Weddell Sea will have a much greater effect on the nutrient salt 

 content of the water near the South Orkneys than the effect of phytoplankton activity 

 in the Bellingshausen Sea will have on the surface layer at the south end of the line of 

 stations from the Falklands. The surface values of phosphate content at corresponding 



