226 Marine Microbiology 



In the aphotic zone, quite analogous results may be shown 

 in 382 deep water samples off Algeria, and 86 samples in central 

 Indian Ocean, 



Role of Probable Deep Currents 



Food is not quickly renewed in great depths, except when 

 permanent strong currents are able to carry organic matter. In 

 the northern occidental basin of the Mediterranean, such cur- 

 rents are rare or absent, and deep plankton is poor, but in the 

 southern Mediterranean, there is a great deep current coming 

 from Asia. Current appears to be maximum in the layer 290 to 

 450 m in the occidental basin, and in the layer 500 to 2,500 m 

 in the oriental basin (see correlation with plankton at station 

 6 and 16 in Fig. 1). The current of 0.7 to 4 knots, measured 

 in the Strait of Gibraltar, seems rather fast. 



Table 4 gives, very approximately, a possible distinction 

 between five kinds of waters in the aphotic zone of the southern 

 Mediterranean. When the surface Atlantic current dives to 200 

 to 400 m (rarely, in very calm weather only), its deep waters are 

 not rich in Cyclococcolithiis. Mixed Mediterranean and Atlantic 

 waters are sometimes rich, but have 61 per cent rather poor 

 samples. Pure Mediterranean occidental waters are generally not 

 rich. It is the oriental high salinity current (38.4 to 39.1%c) 

 which sometimes shows the greatest production of Ctjclococco- 

 lithus (700 to 4,200 cells/ml). 



In the abyssal layer (2,000 to 4,000 m), the effect of prob- 

 able currents is particularly clear. Our eighteen samples with 

 theoretical currents (warmer and more saline) have a mean of 

 2.3 times more Cyclococcolithus, 3.3 times more Exiwiella, and 

 2.6 times more naked flagellates than our 52 samples probably 

 taken outside the oriental current. Our observations are sug- 

 gestive rather than certain. 



In conclusion, the frequent richness of deep water is obvious. 

 At several Mediterranean stations the aphotic zone contains more 

 flagellates and Myxophyceae than the euphotic layer does. The 

 Mediterranean has the advantages of high oxygen and pH values 

 in the deep sea. Even in the Indian Ocean ( and probably in the 

 Atlantic, according to the Meteor results ) , great densities of uni- 



