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. ДВУСТВОРЧАТЫЕ МОЛЛЮСКИ БЕЛОГО МОРЯ. Опыт эколого-фаунистического анализа

in the Gorlo Strait can play the role of such an obstacle (Derjugin, 1928). The barrier is easily overpassed by species without planktonic larvae. Statistical methods used confirm Derjugin's idea of the species number reduction in the White Sea and estimate it quantitatively.

Twenty of thirty-nine known White Sea bivalve species, a half of the entire species list, were not found in the Gorlo Strait. They were encountered only near its northern or southern boundary. There is no possibility for accumulation of fine-particle sediment fractions in this strait because of strong currents; therefore no soft bottom is present there. Soft deposit is necessary for nutrition of detritophagous species, hence it is not surprising that only one of eight White Sea deposit feeding clams was found in the Gorlo Strait. Nevertheless, in internal parts of the sea deposit feeders are very common. It allows suggesting that in the geological past hydrodynamic conditions in this strait could be different.

Burrowing filter feeders, which need fine-particle mud, were not found in the Gorlo Strait as well. Some non-burrowing filter feeders, mainly Musculus species, also were not encountered there. One can assume that the absence of such forms is somehow connected with their peculiar reproduction properties. To all appearances, they became extinct in the Gorlo Strait after the contemporary hydrodynamic regime was formed. It occurred at the end of Atlantic climatic phase about 4-5 thousand years ago. This tide can be regarded as an isolation term in the White Sea of the species spoken about.

In general, only species connected with hard bottom and strong currents or species less specialized and widely distributed were encountered in the Gorlo Strait.

One can consider that the distribution pattern of bivalve mollusks in the Gorlo may by explained by the geological history of this strait and by biological features of the White Sea clams.

A related problem should be mentioned. There are several very interesting inlets with ridges in their mouths in the White Sea. Such ridges prevent the summer water exchange between deeper part of an inlet and the main White Sea water area. As a result cold water of winter origin remains during the whole year in depressions of such small waterbodies (geological structure and hydrological features are described in Chapter 7). The inlets, as a rule, have a little bit wasted fauna of the Central White Sea Depression (Naumov, 1979, б; Naumov, Oshurkov, 1982; Naumov et al., 1986, б; Naumov, Fedyakov, 2000, a, b). In the case of two depressions within one inlet, the fauna of mouth-part one normally is richer. The vertical distribution of benthos in such inlets closely resembles those in open parts of the White Sea. The inlets mentioned can be considered as natural models of the White Sea itself. Number of bivalve mollusks became isolated in them, and the term of isolation can be calculated with a good accuracy. Explorations of such inlets can lighten the study of colonization of the White Sea by marine bottom fauna in Holocene.

Only process of colonizing the White Sea in Holocene by bivalve mollusks can by reconstructed relatively truthfully, for other taxa are just poorly represented as subfossils.

As for clams, 27 species were found in subfossil state, which makes about 70% of their contemporary fauna. Thus the colonization of the White Sea by clams can be considered as an approximate model of other present-day taxa invasion into this waterbasin.

Colonization of the White Sea by bivalve mollusks was studied by many authors (Govberg, 1968, 1970, 1973, 1975; Nevessky et al., 1977). There were almost no attempts as at the present to reconstruct hydrological regime in this waterbody during Holocene using actualism principle.

Portlandia aestuariorum was the first species which subfossil shells were found in the White Sea deposits of the Young Dryas climatic phase. This allows suggesting a low salinity range (about 10-12%) in this waterbasin during that times. The true marine species Portlandia arctica and Mytilus edulis were found already in the deposits of Preboreal climatic phase however. Probably, the first of them being an Arctic endemic species pene-

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