Density of Flagellates and Myxophyceae 217 



With the exception of Monaco (which has low pliospliates 

 and hght vahies), all these regions have calcareous flagellates 

 of the genus Cyclococcolithus as prevailing and perennial organ- 

 isms. 



PREVAILING ORGANISMS^CHARACTERISTIC 

 QUANTITATIVE RELATIONS 



In deep layers, nannoplankton is less variable and poorer 

 in species than in the euphotic zone. Unfortunately, very little 

 is known of its distribution in Pacific and in cold seas. However, 

 it is possible to summarize here the valuations in the southern 

 Atlantic (Meteor), in the Mediterranean, and in tlie central 

 Indian Ocean (6). 



The absolute number of cells per ml or liter is the most 

 solid basis for comparisons, but individual cell volume is variable, 

 from 30 ^t^ (smallest flagellates) to 50,000 or more (largest 

 Peridinians and Diatoms). For instance, a palmelloid cell of 

 Cyclococcolithus (mean volume 3600 /'.^) is probably able to 

 give 100 times more food to a Copepod than a small Chromulina 

 (30 to 45 /'^). The following percentages, therefore, indicate 

 the mean approximate proportion in the total volume of unicell- 

 ular organisms: all groups are listed according to decreasing 

 values of that mean volumetric percentage, only in the aphotic 

 zone : 



1. Coccolithaceae of the Genus Cyclococcolithus (Fragilis Group) 



Very common and perennial, from the surface of warm seas 

 to their greatest depths. Numerically: 3 to 4,000 cells per ml, 

 the mean being between 50 and 250 cells. Volume: 65 to 95 per 

 cent of the total phytoplankton, prevalent almost everywhere. 

 Only very rarely did water samples contain no trace of Cyclo- 

 coccolithus (of 1,000 samples in the deep Mediterranean, only 

 one had no trace: at 3,000 m in the Matapan trench, near Greece). 

 C. fragilis occur in water from 70 to 105 per cent oxygen satura- 

 tion, 



2. Syracosphaeraceae, and True CoccoHthus (Pelagicus Groui)) 



Common, with at least 30 species and 7 genera in the deep 

 sea, but, generally, they make up only 10 to 25 per cent of the 

 volume, and 5 to 400 cells/ml. They are much more oligotrophic 



