io8 



DISCOVERY REPORTS 



between 200 and 300 m. The upper discontinuity has been formed in the absence of 

 rough weather by an accumulation of thaw water. This layer has given rise to ideal 

 conditions for the growth of a large concentration of phytoplankton and in this 

 connection it is noteworthy that the silicate value at 10 m. is less than that at the 

 surface. Had a sample been taken at 5 m. I think it more than possible that the silicate 

 content there would have been even less. From the phosphate distribution in the upper 

 part of the surface layer it is clear that phytoplankton production has been restricted 

 to the 0-20 m. layer, whilst the character of the phytoplankton catches proves that 

 mortality due to consumption by zooplankton has taken place, i.e. the zooplankton has 

 excreted the undigested siliceous skeletons of the diatoms. Below a depth of 20 m. the 

 phosphate content of the surface layer is as high as 128 mg. which indicates that either 



decomposition of the dead phytoplankton has taken place with instant regeneration of 

 the phosphate or else that no phosphate has been consumed, i.e. the pre-outburst 

 phosphate content was 128 mg. in the surface layer and the outburst was confined to 

 the 0-20 m. layer. The latter supposition is far more likely to be the correct one since 

 the zooplankton requires some of this phosphate for its own growth, whilst the undi- 

 gested siliceous skeletons and other unwanted bodies are excreted. Some recent research 

 work at Plymouth (Harvey, Cooper, Lebour and Russell, 1935) is of great interest in 

 this connection. These workers found that in the English Channel, only a fraction of 

 the phosphate utilized by the phytoplankton during the first half of the year was found 

 as phosphorus compounds in the zooplankton. They consider that much of the organic 

 phosphorus compounds such as phosphoproteins and phospholipins in diatoms may 

 pass undigested through animals and remain in solution in the sea until regeneration 

 to phosphate by the aid of bacterial action takes place after an interval of three to four 

 months. Obviously some future research into the seasonal variation in the nutrient salt 

 content of southern plankton combined with similar data of the sea-water in the photo- 

 synthetic zone would be of very great interest and some attempt will be made to this 

 end in a subsequent cruise of the 'Discovery II'. 



The silicate and oxygen content data at St. 1543 seem to be very instructive in 



