MAIN TYPES OF PHYTOPLANKTON 19 



been omitted, and the stations arranged in sequence as they were situated, proceeding 

 seawards from the points of origin after which the Unes have been named. Next, mean 

 values for the temperature, salinity, phosphate content, oxygen content and hydrogen- 

 ion concentration of the upper 20 m. of the sea water are given. Where surface values 

 only are available these have been denoted by an asterisk. The volume of the sample in 

 cc. and the fraction examined (in practice the largest that could be got on to the squared 

 slide without rendering counting impossible through overcrowding) are then quoted. 

 The estimated numbers of diatoms follow with the genera arranged in order of Lebour's 

 (1930) classification, and the species alphabetically. Dinoflagellates, other holophytic 

 organisms. Protozoa and larger animal organisms follow in natural sequence. Where a 

 large variety of dinoflagellates was present, as in the samples from warmer seas, Lebour's 

 (1925) classification has again been followed. The estimated total of diatom frustules is 

 also shown. 



In constructing the various charts and diagrams, great difficulty has been experienced 

 in devising suitable scales, as the diff'erences in numbers are often very great. It is par- 

 ticularly important that the scale should be noted when referring to any of the figures, 

 as it has been found impossible to adhere to the same scale throughout. 



It is evident that in future work in the south, every eff"ort must be made to obtain data 

 on the vertical distribution of the phytoplankton by the centrifuge method as developed 

 by Gran. The difiicuhies of carrying this out in the far south have already been men- 

 tioned, but with the gradual increase in our knowledge of the general distribution of the 

 main associations it should be possible to arrange the method of investigation in such a 

 way that centrifuge determinations can be carried out at a sufficient number of stations. 

 Until such data are obtained it is impossible to extract the full value from the hydro- 

 logical data now available. 



In view of the fact that cultural methods, ever since their introduction by Allen and 

 Nelson (1910), have been used with much success in unravelling the factors influencing 

 phytoplankton production in European waters, it may be mentioned that attempts to 

 obtain cultures of Antarctic plankton diatoms have been made, but these have so far 

 failed owing to the extreme difliculty of maintaining the media at temperatures ap- 

 proaching those to which the organisms are subject in nature. 



THE MAIN TYPES OF PHYTOPLANKTON MET 

 WITH IN THE SOUTH ATLANTIC 



A very fair idea of the main types of phytoplankton present in the South Atlantic may 

 be gained by a study of the material collected at a series of stations at approximately 

 equal intervals from 57^° S lat. to 141° N lat., along the thirtieth west meridian. This 

 was carried out in April-May 1 931 on the homeward voyage, and was the last piece of 

 work done during the first commission of R.R.S. ' Discovery II '. The time of year was 

 somewhat unfortunate, as phytoplankton production was probably at its lowest on some 

 of the more southerly stations, but in the light of previous work it is possible to interpret 



3-2 



