232 Light 



duced. During the winter in the neighborhood of cities no measur- 

 able radiation shorter than 3100 A can be found, and hence no ap- 

 preciable action of ultraviolet can take place at that season. When 

 birds that ordinarily migrate south were retained in Alberta, they 

 died during the winter apparently from lack of vitamin D. Neverthe- 

 less, we have little evidence that the lack of ultraviolet in winter is 

 generally of crucial ecological importance, but the matter should be 

 investigated further. 



In the aquatic environment ultraviolet is still further reduced by 

 the very rapid rate at which it is absorbed even in the clearest water. 

 Although in some quarters popular imagination has fixed upon the 

 idea that ultraviolet is the most penetrating region of the spectrum, 

 actual measurements have shown that radiation at 3100 A is reduced 

 to 10 per cent of its surface value at a depth of 15 m in the extremely 

 clear water of the Mediterranean and at 1 m in Gullman Fjord on the 

 coast of Sweden. The shorter and biologically more active wave- 

 lengths are reduced even more rapidly by absorption and scattering 

 in the water ( Jerlov, 1951 ) . In most natural waters the effectiveness 

 of ultraviolet diminishes very rapidly with depth. Carefully con- 

 trolled experiments have revealed no bactericidal action below 1 m 

 and probably none at depths greater than 20 cm even in clear ocean 

 water. The ecological effect of the lethal action of ultraviolet in very 

 shallow water awaits further study. It is possible that the bacterial 

 population on the bottom mud in the littoral zone is seriously affected 

 when exposed at low water. 



How can we account for the formation of vitamin D in fish liver 

 oils? Cod and halibut whose oils are particularly rich in vitamin D 

 live far below the depth at which any measurable ultraviolet could 

 penetrate. It has been suggested that the vitamin D might reach 

 the fish livers through a food chain starting with the phytoplankton 

 and the zooplankton that live near the surface of the sea. Since cod 

 and halibut are bottom feeders, a long food chain must be postulated 

 to extend to the forms on which these fish live. Another suggestion 

 is that the vitamin D formed in the Sargassum weed as it floats near 

 the surface of the tropical oceans finds it way via the food chain into 

 such species as the cod. For the most part we know that the animals 

 associated with the Sargassum weed do not eat the weed directly but 

 live on plankton or materials accumulating on the surfaces of the 

 fronds. In addition we know that a sharp temperature barrier exists 

 between the regions where the Sargassum weed is abundant and 

 areas inhabited by such fish as the cod and halibut. The amount of 

 vitamin D in the liver oils of these species is very great. It seems in- 



