^^^ p ."5 eg 



!0D % 



Fij. 4. Change in role of various trophic 

 groups in the plankton of the northwestern 

 Pacific with depth. 1, Filter feeders-- 

 phytophages and detritophages ; la, filter 

 feeder-phytophages which feed in the 

 surface layers and do not feed at the 

 depth where caught; 2, zoophagous species-- 

 predators and scavengers; 3, euryphages; 

 4, radiolaria and their remains and other 

 groups not considered above. 



plankton is dominated by euryphages, and only in the benthic layers does 

 a significant Quantity of zoophagous polychaetes and amphipods appear, 

 probably associated with the floor. 



In contrast to the eutrophic boreal regions, in the oligotrophic 

 tropical regions among the most important group of plankton--the copepods-- 

 in the upper 100 meter layer zoophages already predominate, while the 

 significance of filter feeders decreases from 40* in the 0-50 m layer to 

 15X in the 200-500 m layer. Beginning in the mesopelagic zone, euryphages 

 predominate, continuing to dominate the entire column of water: in the 

 2000-4000 m layer they make up 76X of the mass of copepods, below 4000 m-- 

 94% (Arashkevich, 1972). 



In both of the areas in question, the relationship of various trophic 

 groups changes rather similarly as we move vertically downward. However, 

 in the boreal area, predators play a significant role in the plankton down 

 to 3000-4000 m depth, whereas in the tropical area, less rich in food, 

 their role is reduced to nil by a depth of 2000 m. The zoophagous macro- 

 plankton is correspondingly distributed: In the boreal waters, a compara- 

 tively high concentration of zoophagous forms is observed down to 3000 m, 

 in the tropics--down to 1500-2000 m. 



Thus, based on the relationship of the various trophic groups in the 

 pelagic zone, we can distinguish rather clearly different layers, approxi- 

 mately corresponding to the surface, moderately deep and abyssal distribu- 

 tion of the planktonic biomass (Vinogradov, 1968). The superficial 

 layer extends down to 100 (or 200) m and is characterized by clear 

 domination or predominance of fil ter-phytophages. Then, from 200-500 



45 



