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Страница №295

. ДВУСТВОРЧАТЫЕ МОЛЛЮСКИ БЕЛОГО МОРЯ. Опыт эколого-фаунистического анализа

3) Species which can withstand a short rise of temperature up to 20°C or even higher.

It should be stressed, however, that even the most cold-water species of Arctic origin, e. g., Portlandia arctica, can withstand considerable rise of temperature if it does not continue for too long of a period.

It is well known that the maximum species number is associated in seas with the normal oceanic salinity about 35% (Chlebovich, 1962, 1974; Kinne, 1971). Whereas, the total species number decreases in areas of high salinity in the White Sea in many taxa (Berger et al., 1995). This holds true for bivalve mollusks (Fig. 82). This fact prima facie looks surprising. However, reduction of the total species number under high salinity conditions in the White Sea clams can be easily explained by their temperature and edaphic relations.

There are not so many clam species in the White Sea which need constant negative temperature. Such conditions take place in a single biotope - in the Central White Sea Depression at the depth over 100 m. Similar biotopes with high salinity and temperature close to 0°C can be encountered in some cold-water inlets with ridges in their mouths. The sea bottom, both in Central Depression and in deeper parts of such inlets, is covered with very thin silt. All these biotopes are inhabited by soft bottom fauna of Arctic origin. Less diverse edaphic conditions lead to small species number in cold-water biotopes, which only are filled with water of high salinity (Naumov, 1979, б; Naumov, Oshurkov, 1982; Naumov et a., 19866; Naumov, Fedyakov, 2000, a, b). Species numbers grow versus salinity increasing in Arctic-boreal and Arctic components investigated separately in the White Sea fauna

(Fig. 84).

Hence, the suggestion of the dualistic nature of the White Sea bivalve mollusks fauna proves to be true. One part of this fauna has boreal and Arctic-boreal origin, while the second part - an Arctic one. As a result, the whole sea can be defined as an intermediate one between the Northern Atlantic and the Arctic Ocean (Fedyakov, Naumov, 1987; Naumov, Fedyakov, 1989). It is expressed in a peculiar species composition, in deposit feeder to filter feeder ratio (Naumov, Fedyakov, 1990, 1994) and in the relationship of species number against salinity (Berger et al., 1995).

Investigations of a regional distribution of clam species in the White Sea showed five of its main variants which can be preliminary referred as local distribution areas.

Bivalve mollusks of the first local distribution area type are found mainly in the Gorlo Strait, at the shallows along the Tersky, Kandalaksha and Karelia Shores, and in the northern part of the Onega Bay (Fig. 80, Б)

Bivalve mollusks of the second local distribution area type are found mainly at the shallows along the Tersky, Kandalaksha and Karelia Shores, in the northern part of the Onega Bay, and in the Dvina Bay (Fig. 80, В).

The third local distribution area type includes all the shallows except the Gorlo Strait (Fig. 80, Г).

The fourth local distribution area type embraces practically the entire sea except the Gorlo Strait, the Mezen' Bay, and southern part of the Onega Bay (Fig. 80, Д).

The fifth local distribution area type covers the whole White Sea (Fig. 80, Е).

It should be stressed that species without pelagic larvae are spread more widely in the White Sea. It emphasizes the Arctic visage of this waterbasin according Thorson's principle (Thorson, 1936) once more.

It has been shown earlier (Gontar, Naumov, 1994; Naumov, Gontar, 2004) that the spreading of bottom fauna along the shelf of Arctic seas during Holocene satisfactorily fits Piccoli-Sartori-Francino model (Piccoli et al., 1986). In spite of it, there are two times less bivalve species in the White Sea than theoretically estimated. The species deficiency in some taxa inhabited the White Sea is a well known fact, which is commonly referred as negative faunistic features (Derjugin, 1928). Thus, a kind of barrier can be assumed on the way of colonization of this basin by bivalve mollusks. The isolating hydrodynamic regime

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