PLANKTON OF THE GULF OF MAINE 



477 



preceding those of the latter, and with the dependence of flowerings of diatoms on 

 an adequate supply of silica ohvious, the parallelism between the curves for this 

 substance and for abundance of diatoms can not reasonably be regarded as accidental. 



PHOSPHORIC ACID 



Recent analyses of seasonal fluctuations in the amount of phosphoric acid in north 

 European seas make it probable that exhaustion of the supply of this essential food- 

 stuff operates, widespread, to check the vernal flowerings of diatoms. Phosphoric 

 acid (P 3 5 ) exists in such weak solution in sea water (usually less than one part per 

 million), and its analysis is attended with such difficulty that none of the earlier 

 determinations can be depended on; but recent tests 88 have shown a definite seasonal 

 periodicity in the silica content of the English Channel, the North Sea, and the 

 Baltic. Atkins's (1923a and 1925a) data for the neighborhood of Plymouth (espe- 



Monrhs, 1921 



Fig. 134.— The broken curve shows the concentration of dissolved silica (as S1O2) near Gloucester, monthly, for the year 

 1921, from determinations by Dr. R. C. Wells (sec p. 476). The solid black line indicates the flowering seasons of 

 diatoms for that neighborhood 



cially significant, as they extend over two years) show maximum values in winter 

 and minimal in summer, when the water may be almost phosphate free. Atkins's 

 (1925a, p. 71S) conclusion that "where illumination is adequate the phytoplankton 

 increases until the phosphate is almost absolutely used up" is supported not only by 

 the parallelism between the increase in phosphoric acid in northern seas in winter, 

 followed by its depletion in late spring, ami the vernal flowerings of diatoms, but by 

 experimental evidence, for he had earlier (1923) found that in a culture of the diatom 

 Nitschia closteriurn a great increase in the number of diatoms reduced the phosphoric 

 acid from 2.38 parts per million (milligrams to the liter) to 0.006. 87 



A supply of phosphoric acid being essential for plant growth, it is obvious 

 enough that whenever and wherever this substance is entirely used up the lack of it 



«* In review of these see Mathews (1916) and Atkins (1923a and 1925) 



" In one of Moore and Webster's (1920) experiments on photosynthesis on a unicellular fresh-water alga a lack of phosphate 

 was demonstrated as the growth-limiting factor. 



