PLANKTON OK THE GULF OF MAINE 



475 



suppose to have been present originally in the sea water employed in his tests, and 

 second, because distilled water exercises much less solvent action than do the land 

 waters with their load of dissolved organic- compounds, humus acids, and Co 2 , which 

 actually do the work of erosion on their way to the rivers and so to the sea; but, 

 however slowly rock silicates are degraded in the sea, they are so degraded in the 

 end. Indeed, all minerals, given tune, finally succumb to the combined action of 

 water, oxygen, and carbonic acid. Where a constant and rapid interchange of water 

 between the bottom and the upper layers is kept up by r vertical circulation (water, 

 too, of low alkalinity — that is, of comparatively high carbonic acid tension — as is 

 the case on Georges Bank (p. 481) and in the Bay of Fundy) degradation of silicates 

 will be more rapid than in the deeps, where, as Murray (1912, p. 187) points out, 

 "the soluble by-products are removed and the supply of oxygen and carbonic acid 

 maintained by diffusion only." 



Furthermore, we must bear in mind that in the case of the degree of concentra- 

 tion of silica we are dealing with solutions so attenuated that although the destructive 

 action of sea water on felspathic rock fragments is almost inconceivably" slow, it 

 may be sufficient under hydrographic conditions as favorable as Georges Bank 

 offers to yield the very small extra amount of silica needed to favor the active growth 

 and multiplication of diatoms when added to what is in all sea water. Finahy, the 

 frustules of dead diatoms are themselves a potential store of this element and in one 

 of its less insoluble forms. 



It is still to be proved that there is not always a sufficient supply of silica at all 

 times and in all parts of the sea for the growtli and multiplication of diatoms. But 

 stress has often been laid on the apparent parallelism between the seasonal fluctua- 

 tions in the concentration of dissolved silica which Raben (1905) reported for the 

 waters of the North Sea and of the Baltic (Murray and Irvine's earlier analyses 

 are open to criticism) and the ebb and flow of the diatoms. Indeed, the corre- 

 spondence between the two sets of phenomena, as it appears on Johnstone's dia- 

 gram (1908, fig. 30), is striking enough. Subsequent analyses made by Raben 

 himself during the years 1904 to 1912 (Raben, 1905a to 1914), both for the central 

 and eastern North Sea and for the western part of the Baltic, show that the seasonal 

 fluctuations in the amount of silica are less regular than his earlier work suggested. 

 But he again found maxima in February and November over the periods of y^ears 

 covered by the tests, the silica (Si0 2 ) content varying in the Baltic from 0.53 to 1.76 

 milligrams per liter in February, to 0.40 to 0.93 in May, 0.20 to 1.49 in August, and 

 0.93 to 1.36 in November, averaging as follows: 



Average silica (SiOi) content in the western Baltic, 1902 to 1912 



Month 



February 

 March... 



April 



May 



Silica, 

 milli- 

 grams 

 per liter 



0.87 

 .83 



Num- 



ber of 



analyses 



Month 



June 



An 'HI 



November . 



Silica, 

 milli- 

 grams 

 per liter 



0.80 

 .86 



1. 17 



Num- 

 ber of 

 analyses 



2 

 14 

 23 



