PELAGIC PLANT LIFE 



Z7?, 



figures, denoting the number of living cells in every litre of 

 surface-water near Drobak : — 



A quite satisfactory explanation presented itself, however, 

 for the variations turned out to be closely connected with the 

 direction of the winds and currents. The outflowing current 

 in the surface-layers might reduce the quantity of plankton to 

 a mere fraction of the normal amount in the course of a day 

 or two, while the inflowing current might perhaps double the 

 quantity in a few hours. The current exerts so great an 

 influence because the abundant plant life is limited to a thin 

 surface-layer which is sharply differentiated both in salinity and 

 temperature from the water-masses below. On 28th March 

 1907, for instance, the temperature from the surface down to 

 20 metres was 2.6'^-3.6° C, and the quantity of chlorine worked 

 out at 16.74-17.62 per thousand ; from 40 metres down to the 

 bottom at 80 metres the temperature was 6.2^ C, and the 

 quantity of chlorine was 18.73 P^^ thousand. The outflowing 

 current carries the surface-layers with their algse out of the fjord, 

 and the infertile deep water may be sucked up to perhaps 

 5 metres below the surface. The inflowing current, on the 

 other hand, heaps up the fertile surface-waters. We found, on 

 examining the plankton at different depths, that the bulk of the 

 plants was limited to a very thin surface layer, say 5 metres in 

 depth, after the current had set outwards, whereas subsequent 

 to the inflow of the current they were as abundant down to 

 30 or 35 metres as at the surface. 



At a place like this it was difficult to trace any regular 

 connection between the local conditions of existence and the 

 development of plankton-algae, in view of the fact that currents 

 caused variations of even greater extent than those actually due 

 to^conditions of existence. We had therefore to conduct our 

 investigations on other lines. Supposing it were possible to 

 determine the rate of growth of the algae we should get a better 

 measure of production, and probably also of the influence due 

 to vital conditions, than variations in the total amount could 

 give us. The number of individuals at any given moment 

 depends not merely upon the rate at which production has 



