Considering the above facts and data on the variations in quantity, 

 specific composition and predominance of various trophic groups (II.l) 

 with depth, we can attempt to provide a brief description of the structure 

 of the communities of the meso-, bathy- and abyssopelagic zones. 



Mesopelagic zone (20O-750:-100O m) . Most of the population consists 

 of species directly related to the surface zone and feeding in it during 

 their ontogenetic or diurnal migrations. The concentration of zooplankton 

 here, in some cases, particularly in temperate and cold-water regions, is 

 almost as great as the concentration in the surface layers. In addition 

 to the migrating animals, bacteriophages are also important (Radiol aria, 

 Phaeodaria, etc.), utilizing the high concentration of bacteria and single- 

 celled heterotrophs, usually found at depths of 500-700 m (Sorokin, 

 1971a). In addition to migrants (carnivores and herbivores), predatory 

 forms which do not migrate to the surface layers are also significant 

 (see II.l). The communities are highly varied, almost equal in species 

 to communities of the surface zone in the tropics and more varied than 

 communities in the cold-water regions. 



Bathypelagic zone (750^1000-3000 m) . Communities are characterized by 

 comparatively low population and biomass, no greater than 20-30 mg/m3, even 

 in the most productive regions of the ocean, with significant predominance 

 of carnivores over detritophages and herbivores, among which, in the cold- 

 water regions, the upper-interzonal filter feeders are still important. 

 In the upper levels of the bathypelagic zone, they may make up as much as 

 30-40/^ of the mesoplanktonic biomass. 



The main mass of the population consists of predatory forms, 

 utilizing the organic material which enters this zone from the surface 

 zone. However, most of the predators are passive--they do not chase 

 their prey, but rather attract it. The sparsity of the food resources and 

 the need to reduce the expenditures for active and basic metabolism lead, 

 in many groups, to significant morphologic restructuring. As a result, the 

 fauna differs taxonomically from the surface fauna. This is particularly 

 true of groups present in the surface layers as active predators, in the 

 bathypelagic zone as species which have gone over to passive waiting for 

 food (Birshtein, Vinogradov, 1971). 



The variety of the community is great; many groups of pelagic animals 

 reach high species diversity here. The lower population, with comparatively 

 larger number of species, means that the dispersion of the distribution of 

 individual species is significantly greater than at lesser depths. 

 The increase in the diversity index of communities indicates an increase 

 in stability, which, according to Margalef (1963b), allows the community 

 to continue its existence with a smaller quantity of energy entering the 

 system, i.e., with more limited food resources. 



Abyssopelagic zone . Communities inhabiting depths over 3000 m have 

 been \jery little studied. The biomass and number of plankton are 

 extremely low, and decrease with depth. No group has its maximum species 

 abundance below 3000 m. In contrast to the bathypelagic community, the 



131 



