GENERAL CHARACTERISTICS OF THE NORTHERN SEAS 69 



(2) the vertical characteristics of the body of water (deep-water and epi- 

 continental bodies of water) and 



(5) the general character of the hydrological conditions, and in particular, 

 the formation of an ice cover in winter. 



In fact the character of the hydrological conditions (salinity, temperature, 

 the presence of gas, etc.), water circulation, the supply of nutritive substances 

 and other factors influencing biological production differ greatly in each of 

 the above-mentioned types of bodies of water. 



In high latitudes, in closed and semi-closed bodies of water, a loss of salinity 

 inevitably takes place, leading to the disappearance of a number of typical 

 sea forms, and sometimes of whole groups of organisms. In connection with 

 this either a lowering of the biomass is observed or, in the presence of favour- 

 able feeding conditions, quantitatively rich communities of either meso- or 

 oligo-mixed type are developed; whereas polymixed communities are char- 

 acteristic of the fully saline open sea. 



It is likewise easily shown that the course of the hydrological processes and 

 also a whole series of factors directly determining the character of the biolo- 

 gical productivity — most important being the supply of nutritive substances — 

 differ greatly in near-bottom bodies of water on the one hand and epicon- 

 tinental ones on the other. 



The pre-polar parts of the Arctic basin, approximately within the limits of 

 the high Arctic sub-region, with Novaya Zemlya, the northern part of the 

 Barents Sea and the shores of Spitsbergen and Greenland as its boundaries on 

 the Atlantic side, and to the east the parts of the Bering Sea adjacent to 

 the Bering Strait, have four main characteristic features : (J) lowered salinity 

 in their upper 200 m layer and a considerably greater salinity of the deep 

 waters (saline stratification), (2) a vertical circulation rendered difficult in 

 consequence, (3) a very low (usually below— 1° C) temperature, except for a 

 short and slight summer heating of the surface layer, and (4) a cover of float- 

 ing ice usually throughout most of the year and sometimes during the whole 

 of it. As regards its fauna and palaeoclimatic conditions this region has the 

 following characteristics : (a) a preceding much colder phase connected with 

 the Ice Age and post-glacial period, (b) a comparatively short phase of higher 

 temperature during the Atlantic period, (c) a notable increase now of pene- 

 tration of forms more adapted to warm waters, and (d) the saturation of the 

 region of lower salinity by brackish relict fauna. 



All the factors mentioned explain the low indices of biomass usually ob- 

 tained for the high Arctic sub-region (less than 50 g/m 2 ) while in the circum- 

 polar zone the productivity rate is low. The poor quality of the population 

 for an undoubtedly mesomixed community, and a tendency of passing over at 

 some points to the oligomixed one, are also characteristic. 



In the summer season plankton biomass in the surface layer comprises 100 

 to 3,200 mg/m 3 , but the amount of zooplankton is usually about 50 to 230 

 mg/m 3 ; zooplankton biomass rises to 400 mg/m 3 only in inlets and river 

 mouths. Rotatoria, Cladocera and among the Copepoda, Pseudocalanus 

 elongatus become significant in the plankton as a result of a considerable loss 



