forming the biotopes are related to the physical conditions in which 

 they were formed. Therefore, these biotopes are distinguished by the 

 thermohaline characteristics of the waters, the vertical distribution of 

 water density, the predominant directions of vertical water movement, 

 the transparency of the water, the annual mixing cycles, the quantity 

 and composition of nutrients and many other properties which influence 

 productivity and--directly or indirectly--the composition of the population 

 (C. W. Beklemishev, 1969; Bogorov et al . , 1958; Koblentz-Mishke et al . , 

 1970: Semina, 1974). 



5,1 Structure of Tropical Communities 



Peculiarities of species composition and boundaries between communi - 

 ties. Significance of expatriated species . The tropical oceanic communi- 

 ties include a significant number of widespread species. In the oceanic 

 communities of the Pacific Ocean, among the euphausiids, chaetognaths 

 and foraminifers, taken together, widespread tropical species amount 

 to some 50% of the total number (Heinrich, 1975a). In all communities 

 there are also groups of more narrowly distributed species, which have 

 bases of habitats in one or two large-scale gyres only. These are 

 specific species for the communities. Within the communities of the cen- 

 tral waters, these include the central (southern, northern and bicentral) 

 species, in the equatorial communities--equatorial species, in the 

 distant neritic communities--transient, peripheral and distant neritic 

 species (C. W. Beklemishev, 1959). Among the euphausiids, chaetognaths and 

 foraminifers, these species represent 19% of those found in the north 

 central community of the Pacific Ocean, 16% of those found in the equa- 

 torial community. 



Specific species may be absent around the edge of a gyre, so that 

 all species are encountered only in a certain portion of the area. 

 As a result of this, some heterogeneity develops in the composition of 

 the population, even within a single gyre. Furthermore, organisms are 

 transferred from one gyre to another with turbulence and individual 

 streams of water. Turbulent mixing allows plankton to be carried even 

 against the constant current (C. W. Beklemishev, 1959). Many species have 

 broad areas of expatriation. The areas of expatriation of species with 

 various types of ranges cover significant portions of the central and 

 equatorial communities. Due to this, when communities are crossed in the 

 meridional direction, the appearance of certain species and disappearance 

 of others is a normal phenomenon, and the boundaries of the community are 

 not clearly defined (Heinrich, 1975a, b). 



Nevertheless, as the use of methods of numerical taxonomy has shown, 

 the differences between the communities are quite genuine, and the zones 

 with the most homogeneous population are generally located within the 

 boundaries shown in Fig. 21 (Heinrich, 1977). 



The expatriated species create dependent lower-rank communities 

 within the equatorial and central communities. Their existence results 

 from the influx of water from neighboring communities. Into the equatorial 

 communities central species penetrate from both central communities 

 and the distant neritic species, while into the central communities 



110 



