28 



A'S AEROBIC HA BIT. 1 7'*s' 



c 10 



Q 



15 



20 



12 3 4 5 6 7 



Oxygen content in cc. per liter 



Fig. 2. Seasonal variations in oxygen content of a lake showing 

 permanent oxygen deficiency in the lower strata. (Drawn from Smith's 

 data, 1940.) 



be found in the limnological literature and the reader 

 is referred to standard textbooks of limnology (Thiene- 

 mann, 1925; Welch 1935) for further information. 



Fresh water streams will rarely be truly anaerobic. In 

 heavily polluted streams, however, the oxygen may 

 occasionally disappear more or less completely. Ex- 

 amples are given in Whipple's *' Microscopy of drinking 

 water" (1927). 



The best-known anaerobic or near anaerobic salt wa- 

 ter habitats are the lower strata of the Black Sea (below 

 200 m.), the Sea of Azov and the Caspian Sea. The 

 former are entirely oxygen-free, the two latter are at 

 least very poor in oxygen ( Knipowitsch, 1922 and 1925 ; 

 Nikitin, 1931). Extremely low oxygen values have also 

 been reported from certain parts of the Baltic Sea (Rup- 

 pin, 1912) and from oyster basins in Norwegian waters 

 (Helland-Hansen, 1923). 



