THE RESPIRATION OF AN INLAND LAKE 351 



marsh gas in appreciable quantities, often becoming very great as the 

 bottom is approached. These gases do not seem to have any very 

 definite unfavorable effect on the life of the lake. Diffusion is so slow 

 that they do not reach the upper water and experiments indicate that 

 their presence in the lower water adds little, or nothing, to the unfavor- 

 able conditions brought about by the absence of oxygen. 



It should be noted that these processes involve a loss of material 

 for plant food. Carbon dioxide, produced by aerobic decomposition, 

 is available for plant food in the lake, or, if not there, then elsewhere 

 as part of the general stock of that gas in the atmosphere. But marsh 

 gas has no such relation to plants and all substances converted into it 

 are lost to the cycle of life. Its production means just so much reduc- 

 tion of the food supply of the lake. The same may be said of the 

 carbonized, peat-like substances produced from the partial decomposi- 

 tion of plants under water. So long as these remain under water, they 

 are practically withdrawn from the food supply. Against all these 

 influences which tend to diminish the stock of food for its inhabitants, 

 the lake is contending, but with imperfect means and only partial 

 success. 



I have thus hastily and imperfectly sketched the respiration of an 

 inland lake, not because the story is known with any fullness or com- 

 pleteness, but partly because our present knowledge, imperfect though 

 it is, shows that the subject is one of great scientific interest; partly 

 also because many practical hints regarding the utilization of lakes in 

 fish culture can come from our knowledge of respiratory conditions. 

 We are accustomed to think of the food-producing capacity of the lake 

 as the factor which determines the kind and amount of the crop of 

 fish which it can produce. It is a somewhat new thought to me, and 

 I have no doubt that it is equally new to many of you, that the respira- 

 tory capacity of the lake may have even greater influence in this matter 

 than has the capacity for the production of food. Yet it is plain that 

 such is the case and that a knowledge of the respiratory conditions of 

 the lakes in which our fish are to be planted is necessary if the best 

 results are to be reached. 



