a zoogeographic region of the world’s ocean occupying an intermediate position between the arctic and tropical regions. In contrast to the tropical and circumpolar regions where the water has almost a constant temperature year-round, the boreal region is characterized by sharp seasonal fluctuations in the water temperature from 3° to 15° C and more. Accordingly, the organisms which inhabit the boreal region are able to withstand significant temperature changes (eurythermal organisms). For example, during the winter in the coastal zone, mussels are subject to strong cooling twice a day during the ebb tide.
In terms of lighting conditions, the boreal region differs from the adjacent arctic region in the year-round diurnal alternation of night and day. This makes it possible for the plant organisms to vegetate a larger part of the year. The limits of the boreal region differ for the organisms which inhabit open water (plankton) and the bottom (benthos) and the organisms which live at various depths (littoral, sublittoral, and bathyal). If the distribution of the sublittoral fauna is used as the criterion, then the southern boundary of the region runs between 30° and 40° N lat. and almost coincides with the annual mean 15° C isotherm. The region’s northern limit in the Atlantic would be a line almost coinciding with the annual mean isotherm of 4°-5° C. In both the Atlantic and the Pacific, the boreal region tapers from east to west as a result of the system of warm and cold currents which meet off the eastern shores of North America (to the north of Cape Hatteras) and along the eastern shores of Asia (off Japan) and then diverge widely in the western regions of the Atlantic and Pacific. The boreal region gradually turns into the arctic region to the north and into the tropical region to the south. The intermediate zone populated by both arctic and boreal organisms is sometimes considered separately as the boreoarctic or subarctic region, and the region inhabited by tropical and boreal life is called the subtropical region.
The fauna of the boreal region, in terms of specific diversity, is significantly less abundant than the tropical fauna but more abundant than the arctic fauna. In terms of the degree of the quantitative development of life and in terms of the standing crop, the boreal region is in first place in the world. In the regions where the cold and warm currents meet (the “polar front”), an intensive mixing of all the waters occurs, and good conditions are created for the mass development of the phytoplankton (single-celled planktonic algae)—that is, the primary producers which serve as the food for the zoo-planktonic and benthic organisms. Fish are concentrated here, too. The most important marine commercial fishing grounds are located in these areas—for example, the Grand Banks off Newfoundland, the waters of Iceland, the southern part of the Barents Sea, and the coasts of Kamchatka.
The boreal region is divided into the Atlantic boreal sub-region and the Pacific boreal subregion. There is a marked difference in the specific composition of the fauna of the European and American coasts of the North Atlantic. Along with the species which are common to the entire North Atlantic, there are species which are found only on the eastern or western coast—for example, the European and American lobsters. Certain species which are encountered along the eastern and western shores are not found in the north. The boreal fauna of the seas bordering Europe consists of several groups: the Mediterranean boreal (centered in the Mediterranean Sea), the southern boreal, and the boreal per se.
The fauna of the Pacific boreal subregion is much richer and more distinct than the fauna of the Atlantic boreal sub-region because a major locus of speciation and migration for many groups of marine plants and animals is located in the North Pacific. For example, the greatest number of mollusk, crustacean, and echinoderm species and of the fishes, gobies, armed bullheads, snailfishes, lumpsuckers, and many others are concentrated here. The faunas of the Asiatic and American shores of the North Pacific differ in their specific composition. However, there are also common forms. The Atlantic boreal and Pacific boreal subregions of the boreal region possess a rather large number of common species, the range of which is broken in the north and in the south—that is, they have an amphiboreal distribution.
IA. A. BIRSHTEIN