3/6 DEPTHS OF THE OCEAN 



The figures in the tables clearly indicate that, though the 

 rate of increase is highest in August, the number of cells of 

 Ce^-atiiini in the fjord makes no great advance before October. 

 Throughout the whole summer the number continues at about 

 the same level, in spite of a comparatively rapid production. 

 This affords a further indication that in the Christiania fjord 

 variations in the current and other factors of loss exert a greater 

 influence than the variations in the conditions of existence 

 which affect rate of increase. 



The fact that we find in the Christiania fjord, and assuredly 

 also in many other places along the coasts of North Europe, 

 that the plankton is less abundant in the summer months than 

 in spring, does not necessarily imply any unfavourable change 

 in the conditions of existence due to summer. It may be caused 

 by the melting of the snow in spring, and by the river water all 

 through the summer driving the surface-water and its plant- 

 life away from the coast, so that the production near land 

 barely replaces the loss. In the autumn it would seem as 

 if the prevalent sea-winds heap the surface -layers together 

 along the coast, and thereby accumulate large quantities of 

 plankton. 



What effect these movements of the surface-water have 

 upon the occurrence of the plankton we are as yet unable to 

 say definitely, but they must be taken into consideration. We 

 were obliged, therefore, to abandon our original intention, 

 which was to ascertain the importance of such conditions of 

 existence as dissolved nutritive substances, and particularly 

 nitrogenous compounds. 



I made a series of cultivation experiments, however, under 

 experiments, conditions of existence resembling the natural conditions as 

 nearly as possible. Stoppered glass bottles holding two and a 

 half litres were kept just floating at the surface, by being filled 

 with about two litres of sea-water ; the amount of plankton 

 present was carefully checked in advance, and then one bottle 

 was left in its original state, while in the other two small 

 quantities of chloride of ammonium or calcium nitrate were 

 placed. After an interval of 3 or 4 days the plankton in all the 

 bottles was once more examined, and it was generally found 

 that most of the species had augmented best when nitrogenous 

 nutriment had been added. The addition had naturally to be 

 made with the utmost care, since anything over 0.5 mg. per 

 litre generally had a poisonous effect. The following table 

 shows the result of one of these experiments : — 



Cultivation 



