The four hydrographic stations occupied in the polar 

 waters revealed that the diatom concentration is scanty at 

 the extreme surface, increases at 50 and 100 feet and then 

 decreases until at great depths no diatoms are detected at 

 all. The maximum depth at which a Secchi disc was seen at 

 these four stations is as follows: station 1, 90 feet; station 2, 

 55 feet; station 3, 41 feet; and station 4, 68 feet. From this 

 it can be seen that the euphotic and disphotic zones in these 

 latitudes are very shallow. Diatoms flourish best in some- 

 what subdued light; therefore there is a subsurface diatom 

 maximum. Schimper^ found diatom maxima at 40 to 80 

 meters in the antarctic. Sverdrup, Johnson, and Fleming 4 

 consider that this can be explained only as caused by sinking 

 since photosynthesis cannot occur below 50 meters in these 

 latitudes. The Secchi disc information available, together 

 with the diatom concentrations at the different levels, confirm 

 these comments. The diatom subsurface maximum at these 

 stations was almost certainly between 50 feet and the surface. 



Bathythermograms from these four stations reveal a 

 marked seasonal thermocline (there is no winter thermocline 

 in the antarctic waters) with about a 7 -degree F. drop in 

 temperature at depths from 60 to 130 feet. The presence of 

 this stratification shows that the surface waters there have 

 already spent some time as such. A certain degree of vertical 

 stability, as is indicated here, is necessary for maximum 

 diatom production, otherwise many of the diatoms are carried 

 down beyond the euphotic zone. Thus, although data on the 

 actual nutrient content of the water are lacking, the high 

 diatom population above the thermocline in the euphotic zone 

 (in surface waters which are "aged") is a strong indication 

 that there are still sufficient nutrients for good phytoplankton 

 production. On crossing the Antarctic Convergence to the 

 north, a remarkable drop in phytoplankton population was 

 noted. Because of the stability in this region (figs. 15 and 

 16), vertical circulation is virtually absent and thus the 

 replenishment of nutrients is slow. The paucity of diatom 

 cells here indicates that the vernal "bloom" is over, so 

 surface water nutrients are depleted. The appearance of a 

 summer form such as Rhizosolenia hebetata f. semispina, 

 and the absence of the primitive resting stage Rhizosolenia 

 hebetata, also shows that the population is a summer one. 



72 



