However, in the more productive regions of the lower latitudes in 

 quasi-stable upwelling zones, the trophic structure of the community is 

 significantly different: The food chain consists of a few links, sometimes 

 even a smaller number than in the productive regions in the higher latitudes. 

 These communities, with their short food chains and rigidly determined 

 direction of the flow of matter and energy, yield extremely high production 

 of the final links (fish), but their stability is quite low. The communi- 

 ties cannot react flexibly to significant changes in production of one 

 of the links in the chain and, for example, a sudden decrease in production 

 of the first link (phytoplankton) leads to truly catastrophic results. 

 Examples include the well-known periodic disruption of the structure of 

 the ecosystem of the Peruvian upwelling during "El Nino," causing catastrophic 

 decreases in the catches of anchovy, deaths of hundreds of thousands of 

 fish-eating birds, etc. 



The communities of productive regions in the higher latitudes have 

 special adaptations to the annual (and, of course, seasonal) fluctuations 

 in the production of phytoplankton and therefore, even with significant 

 changes in the population of the primary producers or consumers from year 

 to year, catastrophic disruptions of their structure do not occur. 



Changes in food interactions during development of communities . Marine 

 pelagic communities, like sea-floor communities or continental communities, 

 undergo significant structural changes from the moment of their formation 

 until they reach the mature stage. The initial stages in the formation of 

 communities occur with mineral forms of nutrients present in the photic 

 layer. In this stage, the processes of accumulation of energy in the 

 community prevail over processes of its dissipation. 



As an ecosystem develops, the total content of nutrients in the 

 water decreases to the extent that it causes a decrease in the biomass of 

 phytoplankton and a change in its dimensional and systematic composition. 

 Due to the different rates of maturation and achievement of maximum numbers 

 of different trophic groups in the community, the entire trophic structure 

 changes. 



As the community matures, it is gradually carried downstream. There- 

 fore, the time difference in achievement of the maximum development by the 

 different trophic elements of the community leads to a shift in space, as 

 well. Many observations in tropical regions in the Pacific and Indian 

 Oceans have shown that the zones of maximum population of phytophages 

 are usually long, narrow belts, crossing the oceans along lines of divergence, 

 This, for example, is the distribution of Undinula darwini , the maximum of 

 which in the Indian Ocean is found at the divergence at the boundary of the 

 southern equatorial current and the equatorial countercurrent , and in the 

 Pacific Ocean at the equatorial divergence (Vinogradov, Voronina, 1962, 1954). 



The maximum population of mature carnivorous surface-dwelling copepods 

 ( Euchaeta , etc.), in contrast to the phytophages, is located parallel to 

 the divergence, but displaced from it by 60-90 miles. Furthermore, in 

 the Pacific Ocean, the maximum population of mature \]_. darwini and 



41 



