CH. X] 



THE CONDITIONS Ob' LIFE IN THE SEA 



237 



curve is much the same for the whole series of years covered, and 

 we may take it that it is a general one. All organisms taken 

 in the quantitative nets are included in the results of these catches, 



1903 



Fig. 30. Variation of plankton and silicic acid in Kiel Bay 1902 4. 



but the diatoms form so overwhelming a proportion of the total 

 volume of plankton during the maximal months that we may 

 regard the seasonal variations as due almost entirely to the 

 variations in abundance of these organisms. The thin line then 

 represents the change in the abundance of the plants from month 

 to month, and the thick line represents the variation in the 

 amount of silica in solution in the sea from quarter to quarter. 

 Note that there are two maxima in the abundance of silica as 

 w^ell as in the abundance of diatoms. At the beginning of the 

 year the proportion of silica contained in the sea is small, and 

 so also are the numbers of diatoms. But the silica gradually 

 increases in amount until the beginning of February, and then, 

 almost immediately afterwards, follows the spring maximum in 

 the abundance of diatoms. Thereupon the proportion of silica 

 rapidly falls, indicating that it has been used up by the plants ; 

 and wdth the fall in abundance of this food-stuff, there occurs 

 a rapid drop in the number of diatoms present per unit volume 



