GENERAL CHARACTERISTICS OF THE NORTHERN SEAS 51 



animal part preponderates over the vegetable one. With the advent of 'bio- 

 logical spring' phytoplankton begins rapidly to predominate over the zoo- 

 plankton, and the total amount of plankton rises to 2,000 mg/m 3 . 'Biological 

 summer' is characterized by a decrease of the plankton biomass (about 

 1,000 mg/m 3 ) with an increase of zooplankton ; moreover the phyto- and zoo- 

 plankton components become almost equal. This change is illustrated by the 

 data given in Table 23. 



Thus the zooplankton biomass is doubled between 'winter' and 'spring', 

 and is increased almost five times by the 'summer', while phytoplankton 

 increases 60 times between 'winter' and 'spring', and decreases five times by 

 ' summer '. The composition of plankton also changes. In ' winter ' zooplankton 

 consists mainly of adult wintering stages, in the spring the plankton teems with 

 eggs and larvae of the pelagic forms, infusoria, rotifers and fritillaries. More 

 adult stages of Copepoda and the larvae of bottom forms are predominant 

 in the 'summer'. Under Arctic conditions, 'biological spring' arrives at the 

 time of melting of the ice and the appearance of open water ; it develops at the 

 edge of floating ice where the abundant bloom of phytoplankton is always 

 encountered. ' Biological summer ' is observed in plankton in places which have 

 been free of ice for some time. 



Thus different phases of plankton development may be observed at the 

 same time in different regions of the sea or, on the other hand, the very same 

 phases of its development at different times. The microclimate causing the 

 transition from one phase of plankton development to another is determined 

 by the ice conditions. It is possible that, depending upon the ice, an approach- 

 ing phase may be broken off and started again with the recurrence of better 

 conditions. 



V. Wiese (1943) notes that as it were the 'continuous' temperature and 

 saline front of the Arctic waters gets broken near the fringe of polar ice and 

 certain special conditions set in there, determined in winter by the formation 

 of a mass of floating ice and in summer by its melting. 



These special conditions created near the ice fringe are reflected in hydro- 

 logical, hydrochemical and biological phenomena. In summer a reduction of 

 salinity, an increase of specific alkalinity, a fall in carbon dioxide pressure and 

 a rise in the hydrogen ion concentration and in oxygen content are observed 

 at the ice fringe. The phosphate and nitrate contents of the surface layer de- 

 crease. Almost all these characteristics are connected with a vigorous develop- 

 ment of phytoplankton. 



