the 0-100-m layer of the equatorial Pacific were 

 360 x 10 3 cells/ml 1 and 8.10 ug C 1 ' d ', respectively. Such a 

 level of development of bacterioplankton and its biomass is 

 typical of the bacterial population density in the euphotic zone 

 with background upwelling (Sorokin, 1978). According to 

 data reported in the literature, the principal bacterioplankton 

 count and biomass maxima in stratified tropical waters are 

 usually situated at the upper boundary of the thermocline. The 

 formation of the microbiocoenosis in this zone is attributable to 

 the arrival of biogenic substances from lower layers, to the 

 stable existence of the phytoplankton population in the euphotic 

 layer, and to the stable existence of the phytoplankton population 

 in the euphotic layer. This in turn ensures that the 

 bacterioplankton receives a steady and readily assimilable 

 supply of organic matter (Sorokin, 1971b, 1982). 



According to our observations, the vertical distribution of 

 microflora (Figs. 1-3) included several regularly observed 

 maxima occasioned by the delivery of labile organic matter to 

 the zone of maximum phytoplankton synthesis. These lay at 

 15-25 m for Stations 115-117, at the upper boundary of the 

 thermocline (100-250 m). and at a depth of 1.000-1.500 m 

 (Stations 118-120) (i.e., at the boundary of mixing of the 

 Antarctic waters). 



The principal bacterioplankton concentration maximum 

 lay at a depth of 100 m. The microflora counts and biomass 

 were, on the average, 1 .3 times the corresponding values for the 

 euphotic zone. The level of microbiocoenosis development in 

 deeper waters (at depths of 1,000-2,000 m) was lower. The 

 total bacterioplankton counts and biomass value here turned 

 out to be lower than at the upper boundary of the thermocline 

 by a factor of 1.5. The constraints limiting microfloral 

 development in these deeper waters were very probably the 

 lower temperature and high pressure. 



There was a clearcut tendency for the bacterial population 

 density in the 0.5-250-m layer to decrease in the east-to-west 

 direction. Thus, the bacterioplankton counts and biomass at 

 Stations 115 and 116, situated in the eastern part of the 

 equatorial zone, were, on the average, twice as high as at 

 Station 1 20. lying to the west of them. This observation was in 

 keeping with the declining intensity of upwelling from east to 

 west (Vinogradov. 1978). 



Thus, total bacterioplankton counts and biomass in the 

 waters of the equatorial Pacific, which have suffered less of an 

 impact from human activities that other parts of the Ocean, are 

 fully comparable with those observed for oligotrophic and 

 mesotrophic waters. 



As in the quantitative parameters, the functional 

 characteristics undergo considerable change as one moves 

 from east to west. Data obtained in the course of the present 

 study made possible some quantitative assessments of certain 

 functional characteristics of microbiocoenoses. such as 

 microfloral activity, bacterial biomass production rate, and the 

 rate of degradation of organic matter in the equatorial Pacific. 



Analysis of the results (Figs. 1—1) shows that microfloral 

 activity in the 0- 1 00-m layer in the equatorial zone varied over 

 a broad range and had a tendency to increase in the east-to-west 

 direction despite the gradual diminution of bacterial population 



Station 115 



0.25 5 075 



X-..' 



,y 



Figs. 1-3. Vertical distribution of: ( 1 ) water temperature in °C; (2) dissolved 

 oxygen in mg/l (3) total bacteria count in millions of cells/ml: and 

 (4) CO. assimilation by bacteria in darkness. ingCI ' d '. All data 

 refer to the equatorial Pacific. 



density. Thus, the values for the amount of CO, assimilated by 

 bacteria per day under conditions of darkness averaged 1 .5-2.0 

 times higher for Stations 1 19 and 120 than for the more easterly 

 Stations 115-116. Both bacterial biomass production and 

 organic matter degradation also increased. Whereas the bacterial 

 production values in the 0-100-m layer at Stations 1 15 and 

 1 Id averaged 15.2 and 16.6 (.tg C 1 ' d"\ the corresponding 

 values at Stations I 19 and I 20 were 25.5 and 32.5 ng C 1 ' d '. 

 The respiratory uptake of oxygen by microorganisms in the 

 western area was virtually double. The rate of organic mailer 

 degradation increased from 78.7 HgC l' 1 d' 1 at Station 1 16 to 



208 



