The essence of these differences is that aquatic, particularly 

 pelagic, systems have a much greater degree of "openness," than do 

 terrestrial systems. The great changes in the biotope in these systems 

 occur primarily under the influence of abiotic factors, and they depend 

 more than terrestrial systems on the arrival of nutrients from without. 

 Furthermore, the concept of the final state (climax) in pelagic systems is 

 less definite than in continental systems; however, this problem is less 

 disturbing if we speak not of the climactic state of the system, but rather 

 of its greater or lesser maturity. 



It should be noted that the exploitation* of aquatic, particularly 

 pelagic, systems, as a rule, is significantly higher than that of terres- 

 trial systems. The extraction of organisms from the primary layer of 

 habitation to depths unsuitable for their existence, the loss of organisms 

 carried away by currents beyond the optimal area of habitation, the 

 inroads of predators, all occur in pelagic systems with great intensity 

 and encompass the overwhelming majority of the plankton population. The 

 stronger exploitation, as is the case in terrestrial systems, retards or 

 prevents the achievement of maturity by the system. From this point of 

 view it is interesting that plankton systems reach relative maturity 

 only in stratified water with weak intermixing, where the loss of phyto- 

 plankton cells from the euphotic zone is minimized. 



One defining aspect of succession in the pelagic zone is the 

 accumulation by the community of energy in the initial stages of its 

 development and the preservation or expenditure of this store of energy 

 in its more mature stages. Naturally, energy losses also occur in the 

 early stages, energy accumulation in the more mature communities, but in 

 the initial stages the processes of assimilation of energy, on the 

 average, prevail over the processes of dissimilation, while, with the 

 development of the community, this picture usually changes. 



We emphasize once more that as pelagic zone ecosystems age and 

 mature, changes occur not only in the biotic component--the community, 

 but also in the biotope itself. First of all, the stratification of the 

 layers of water becomes more stable, facilitating differentiation of the 

 population and increasing the stability of the entire system. 



What are the structural and functional changes which occur in a 

 pelagic community as it matures? A summary of all available information 

 on the succession of pelagic communities would occupy far too much space 

 and is therefore not expedient, particularly if we consider that attempts 

 have already been made in this direction (Margalef, 1958, 1962; Vinogradov 

 et al., 1973; Gueredrat et al . , 1972). Nevertheless, the primary trends 

 in the changes should be analyzed. 



*"Exploitation" of a system refers to any removal of living organisms 

 from the system (Margalef, 1968). 



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