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 obvious, 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 2 0 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 86 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- 
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Mar. 
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Sep 
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Nov. 
Dec. 
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Fio. 134.— The broken curve shows the concentration of dissolved silica (as Si02) near Gloucester, monthly, for the year 
1921, from determinations by Dr. R. C. Wells (see p. 476). The solid black line indicates the flowering seasons o 
diatoms for that neighborhood 
daily 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. 718) 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, and the vernal flowerings of diatoms, but by 
experimental evidence, for he had earlier (1923) found that in a culture of the diatom 
NitscJiia closterium 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 
89 In review of these see Mathews (1916) and Atkins (1923a and 1925) 
87 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 
8951—28 31 
