Chapter V 



— 71 — 



Distribution in the Sea 



are summarized in Table XIX, demonstrate that sunlight does have a 

 lethal effect on bacteria suspended in shallow layers of sea water, but the 

 effect does not extend to depths exceeding 20 cm. In their experiments 

 the temperature of the water was maintained at 25° to 26° C. in a bath of 

 running water. Controls were kept in the dark. 



Table XIX. — Average number of bacteria per vil. of sea water of dijfercnt depths in battery 

 jars after beint^ exposed to sunlight for two hours from ii:oo A.M. to 1:00 P.M.: — 



It has already been pointed out on page 12 that the penetration of 

 sunlight decreases rapidly with depth. Ultraviolet radiations are far less 

 penetrative than visible radiations, the penetration of the former decreas- 

 ing with wave length. This is graphically illustrated by Figure^ 8 which 



PERCENT OF RADIATION 

 25 50 75 100 



0.5 



w 



a. 

 iJ 1.0 



H 

 U 



5 



Z 1.5 



I 



a 2.0 



u 



o 



2.5 



3.0 



FiG. 8. — Percentage of incident radiations of different 

 wave lengths (expressed as Angstrom units) which pen- 

 etrate clear sea to different depths {adapted from 

 HULBURT, 1928). 



gives the absorption curves for ultraviolet radiations of different wave 

 lengths in the abiotic range in clear sea water. According to the literature 

 reviewed by Ellis et al. (1925) and by Buchanan and Fulmer (1930), 

 the bactericidal range of solar radiations is from 2100 to 2960 A, the max- 

 imum being between 2500 and 2800 A. Wave lengths from 3000 to 3660 A 

 are only feebly abiotic. 



From Figure 8 it will be observed that the penetrative power of wave 



