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. ДВУСТВОРЧАТЫЕ МОЛЛЮСКИ БЕЛОГО МОРЯ. Опыт эколого-фаунистического анализа

trated the White Sea from the east, the second one - from the west. Blue mussel, no doubt, could only spread within upper layer of coastal water relatively warm in summer. The fact of simultaneous colonization of the White Sea by these two species is a strong evidence of seawater temperature stratification in the Gorlo Strait during the Preboreal climatic phase. Salinity stratification is also possible. Ecological features of species mentioned allow suggesting of salinity range in the upper White Sea water layer about 13% and in the deeper layer no less than 25%. At the same time, the lower seawater masses should be considered very cold (close to freezing point) all the year round in this period. As a result, we can suggest the White Sea to be double-layer from the very beginning of its existence. The Gorlo Strait water should be also divided into two strata; otherwise the penetration of the cold-water species Portlandia arctica cannot be explained.

The most intensive invasion of boreal and Arctic-boreal species took place at the Atlantic climatic phase, when the mean of air summer temperature exceeded contemporary one by 1-2°C. During the whole phase the threshold of the Gorlo Strait was about 70 m as distinct from 40 m in nowadays. (The depth can be easily calculated taking into consideration isostatic and eustatic variations of the sea level; see Fig. 85). The hydrodynamic regime should be weaker at that time, than presently, due to greater depth of the strait.

Since Subboreal climatic phase, because of the sea level retrogression the hydrody-namic conditions resembling contemporary ones were formed. In spite of rising temperatures, the intensity of colonization of the White Sea by bivalve mollusks significantly dropped as a result in this phase. The difficulties in overcoming competition of previously naturalized species forming stable communities can be the second probable reason of the low rate of contemporary invasion (Naumov, Berger, 2004).

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