372 DEPTHS OF THE OCEAN chap. 



March and May, and that they occur in rather smaller quantities 

 from June to August. From September to October there is 

 again a maximum, but from then onwards they decrease rapidly 

 and reach their minimum between December and January. It 

 is not surprising that the plankton is scanty during the dark 

 period of the year, but the unmistakable secondary minimum 

 in the summer months must be due to some special cause, 

 which it should be possible to discover by studying carefully 

 the whole year round the variations in quantity and the 

 fluctuations in the outward conditions of existence. It struck 

 me that the factors at work might be analogous to those which 

 cause the differences in production met with in different regions 

 of the great oceans. 

 Method of To ascertain the quantity of plankton present we employed 



^uant?"ff'^^ the method introduced by Sedgwick and Rafter for drinking- 

 piankton. water tests in North America, which has been described by 

 Whipple. A litre of water is filtered through a fine grade of 

 sand, and the algse that collect on its surface are rinsed off". 

 To the rinsed-ofT water containing the algae, filtered water is 

 added until the whole comes to exactly lo c.c. We then transfer 

 I c.c. of this with a pipette to a counting-chamber 5 cm. 

 long, 2 cm. broad, and i mm. high, which exactly holds it. For 

 examination we use a microscope which magnifies to 40 or 50 

 times the natural size. A thorough knowledge of the species 

 is requisite to enable us to enumerate them correctly. When 

 counting species represented by many individuals we require 

 a micrometer, with a larger or smaller number of millimetre 

 squares marked off by lines, placed in the eyepiece of the micro- 

 scope. 



We soon found that our task was more difficult than we 

 had at first imagined. The quantity of plankton fluctuated 

 greatly in the course of short periods of time, yet the variations 

 could not be ascribed directly to conditions of existence, since 

 these remained fairly constant. The temperature in the surface- 

 layers rose steadily during March to May from 1.5° C. to 6.3° C, 

 the quantity of chlorine was about 16 per thousand, and according 

 to Nathansohn the quantity of free ammonia in filtered samples 

 of sea-water was between 0.0175 mg. and 0.031 mg. per litre, 

 and of ammonia in organic combined form between 0.105 ^S- 

 and 0.217 mg. per litre. Of nitrates and nitrites he only found 

 infinitesimal quantities up to 0.009 ^ig-. set down as ammonia. 

 Chcetoceras constrictum, one of the commonest diatoms in the 

 spring plankton of the Christiania fjord, furnished the following 



