Of the abiotic factors acting on the composition and distribution 

 of littoral communities, the most important, essentially determining the 

 specifics of the intertidal zone as an independent zone, are the tidal 

 fluctuations in sea level. The relationships between the durations of 

 periods of immersion and exposure to air in different parts of the 

 intertidal zone determines, basically, the vertical stratification of 

 organisms and their communities in this zone, which is manifested quite 

 clearly in all climatic zones. The principle of relating the vertical 

 boundaries within the intertidal zone to the levels of the neap and 

 spring tides, as well as the mean sea level for European shores with 

 true semidiurnal tides, was first clearly formulated by L. Vaillant 

 (1891), then later successfully used by a number of researchers (Pruvot, 

 1897; Beauchamp, 19U; Gurjanova et al . , 1925. 1930a, b). According to 

 this principle, the intertidal zone is subdivided into three horizons, 

 of which horizon II, or the middle horizon, is bounded by the mean low 

 water stages and mean high water stages, while horizon I, the upper 

 horizon, and horizon III, the lower horizons are determined by the 

 boundaries of the littoral itself. Each horizon is divided into two 

 stages, the boundaries of the stages being the mean high water and the 

 mean low water springs for the upper and lower horizons, while the 

 middle horizon is divided by mean sea horizon. The use of the principle 

 of Vaillant, developed for an intertidal zone with true semidiurnal 

 tides, is quite difficult in those coastal areas where other types of 

 tides occur. P. V. Ushakov (1951) has shown that the principle of 

 Vaillant is applicable to conditions of irregular semidiurnal tides, but 

 the boundaries of the stages are defined differently: In horizons I and 

 III, these boundaries are the mean high water and mean low water springs. 

 We consider that the principle of Vaillant can also be applied to inter- 

 tidal zones with diurnal and irregular diurnal tides, although in these 

 cases it is not the levels of the springs and neaps which are characteristic, 

 but rather the levels of the tropical and equinoctial tides (Kussakhin, 

 1958a, 1951). It is interesting in any case to subdivide the middle 

 horizon into two stages with a boundary at mean sea level. 



With this expansion of the principle of Vaillant in all cases the 

 upper horizone of the intertidal zone remains above water most of the 

 time, exposed to the air for several days in a row (during the equinoctial 

 or neap tides). Conversely, the lower horizon spends most of its time 

 beneath the water, and is exposed only during the lowest (spring or 

 tropical tides; the middle horizon, with all types of tides, is character- 

 ized by periodic daily immersion and drying out. Similar conditions will 

 be observed in corresponding stages with all types of tides. 



In addition to the tidal fluctuations in sea level, the vertical 

 stratification is greatly influenced by the force and constancy of the 

 surf. An increase in strength of the surf usually results in shifting of 

 the littoral communities upward. Sometimes, this displacement may be as 

 great as several meters vertically; in these cases, the entire intertidal 

 zone may be occupied by purely sublittoral communities, with the true 

 littoral community shifted far upward. Of the biotic factors, competitive 

 relationships may greatly influence the vertical distribution of organisms. 

 In many cases, there may also be seasonal changes in stratification of 

 communities, with this type of displacement observed in all climatic zones. 



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