344 POPULAR SCIENCE MONTHLY 



lakes. Those lakes whose food supply is such that they are capable of 

 supporting large quantities of animal life — I may say for our purposes, 

 large numbers of fish — are likely from that very fact to exhaust the 

 stock of oxygen in the lower water, which thus becomes uninhabitable ; 

 while those lakes whose lower water is fully habitable are likely to be so 

 poor in organic life that they can support only a limited number of 

 fish. It may be that further study will show that this relation is 

 not so unfavorable as it now appears, but at present we must face the 

 probability that it exists. 



A noteworthy exception to this statement should be made in the 

 case of very deep lakes — lakes two hundred or more feet in depth — in 

 which the quantity of the lower water is so great and the consequent 

 amount of dissolved oxygen is so considerable that no ordinary amount 

 of decomposing material can exhaust it or materially reduce it. This 

 is the case, for example, with Green Lake (237 feet in depth) in Wis- 

 consin, and the same statement would doubtless hold for the deep lakes 

 of New York and similar bodies of water. Such lakes may support an 

 abundant population of fish both in the warmer and in the cooler 

 water. If they do not do so, the fault does not lie with the oxygen 

 supply. 



Thus we see that if we desire to determine the capacity of a lake 

 for the development of higher life, we must consider not only its 

 capacity for food production, but also its respiratory conditions. It may 

 be that an imperfect respiratory mechanism renders a very large share 

 of the bottom of the lake wholly uninhabitable for animal life during 

 the warmer part of the year; that while, for instance, mud-living in- 

 sect larvae may be found in the mud around the lake to a depth of 

 twenty or thirty feet, they are excluded by the absence of oxygen from 

 the entire bottom of the lake beyond this depth, an area of perhaps 

 many square miles. The supply of food which the lake offers to the 

 higher animals may thus be greatly limited by the lack of oxygen. It 

 may be true also that the greater part of the volume of the water of the 

 lake is uninhabitable for similar reasons, and that a lake whose surface 

 appearance would indicate that it is capable of supporting enormous 

 quantities of fish may be very considerably restricted in this respect 

 by its respiratory capacity. Each lake should be studied as to both 

 food and oxygen if an intelligent economic use is to be made of its 

 waters; and when this is done, the possibilities of use will often be 

 found to depend on the respiratory mechanism. 



I have said nothing on another side of the methods of absorbing 

 and transporting gases in a lake. The same processes which take 

 oxygen from the surface bring waste gases to it and they are as efficient, 

 or as inefficient, in the latter operation as in the former. Processes of 

 absorption and transportation have much to do with the story of the 



