PELAGIC PLANT LIFE 



79 



Stat. 4 a 



106 55-50 



found abundance of plankton in April 1910, algae being present 

 in large quantities as deep down as they have been known to 

 occur, that is to say as far down as sufficient light penetrates. 

 We can appreciate the difference between these conditions and 

 the conditions in coastal areas like the Christiania fjord, if we 

 remember that the nutritive substances in the first case may rise 

 up from the deep water, while in the second they are derived 

 from the surface through the admixture of fresh water. 



Vertical circulation is regulated by differences in tempera- 

 ture at the surface, due to summer and winter, which are 

 sufficient to in- 

 crease the density 

 of the upper 

 layers till it equals 

 the density lower 

 down, and if cir- 

 culation is to have 

 any effect in the 

 open sea, the sur- 

 face-layers must 

 be able to sink to 

 a depth of at least 

 200 to 300 metres. 

 The greater the 

 difference in tem- ^^° 

 perature between 

 summer and win- 

 ter, the more 

 effective will ver- 

 tical circulation 

 generally be. 



Assuming, then, that our view is correct, namely that plant 

 production in the sea is mainly regulated by the amount of 

 dissolved nutritive substances, we must expect to find plankton 

 produced in abundance in coastal areas to which large rivers 

 convey nourishment from the land, and in oceanic areas where 

 vertical circulation takes place on a large scale, or where 

 ascending currents bring up the deeper water-masses. Where 

 vertical circulation is the controlling influence, the greatest 

 profusion will be at seasons when the temperature of the 

 surface reaches its minimum ; that is to say, generally in 

 winter, or in higher latitudes in the early months of spring. It 

 would be possible to test the truth of this theory if we could 



Hydrographical Section off the Irish Coast 

 (April 1910). 

 Temperature and salinity nearly uniform from the surface down 

 to a depth of 250 metres. 



