ANAEROBIC HABITATS 



27 



There is some biological activity, nevertheless, in the 

 lower strata, and it brings about a more or less pro- 

 nounced depletion of oxygen. The oxygen content fre- 

 quently sinks so low that the usual methods of determin- 

 ation indicate a total absence of it for weeks or even 

 months. In winter a thick layer of ice often prevents 

 the diffusion of oxygen from the atmosphere into the 

 water. Ice also inhibits photosynthesis in aquatic plants, 

 hence no oxygen becomes available from this source 

 (cf. Birge and Juday, 1911 and Thienemann, 1927). 



Occasionally lakes are found which exhibit a perma- 

 nent stratification {cf. Yoshimura, 1937 and Smith, 1940). 

 The lower water strata are then permanently anaerobic. 

 In the case described by Smith (1940) at least some 

 zooplankton organisms were caught in these layers. 



Figures 1 and 2 show the annual oxygen variations in 

 different layers of two lakes as illustrations of the tran- 

 sitory and of the permanent oxygen depletion character- 

 istic of the deeper water masses. Many more data will 



7 8 9 10 1112 



Oxygen content in cc. per liter 



Fig. 1. Seasonal variations in oxygen content of a lake showing 

 temporary oxygen deficiency in the lower strata. (Drawn from Berg's 

 data, 1938.) 



