THE RESPIRATION OF AN INLAND LAKE 345 



complex relations of carbon dioxide gas in the lake. These matters, 

 however, can better be spoken of under internal respiration. I need 

 only say here that the accumulation of waste gases in the lower water 

 does not seem to affect life unfavorably if there is plenty of oxygen 

 present also. Respiratory inefficiency limits life in a lake because of 

 lack of oxygen rather than because it allows poisonous gases to collect 

 in large quantities. 



The subject of internal respiration deals with the changes of gases 

 within the lake itself and with the manufacture of gases by the organ- 

 isms which inhabit it. No branch of physiology is more intricate and 

 none less understood than is that of internal respiration. This is true 

 also of the internal respiration of the lake. The gaseous exchanges and 

 the manufacturing operations in the interior of a lake are far more 

 complex than those of any animal. From the water living beings are 

 drawing supplies of gas, each after its kind, and to the water each is 

 contributing gases differing in amount and composition. Animals are 

 withdrawing oxygen from the water and giving carbon dioxide to it. 

 Algae are repeating this process by night and exactly reversing it by day. 

 Fungi and bacteria are using oxygen in the course of their internal 

 vital activities ; they are employing far larger quantities in the fermen- 

 tative processes which they maintain. The innumerable chemical 

 changes included in decomposition and fermentation, going on under 

 all sorts of conditions, involving numerous kinds of materials, and 

 operated by various organisms, are adding to the water gases of dif- 

 ferent kinds and in varying proportions. The upper water, the lower 

 water and the mud present very dissimilar fields of work to the organ- 

 isms which inhabit them. It is, therefore, impossible even to attempt 

 a picture of the internal respiration, with its countless operations, each 

 adding to or subtracting from the sum of gases in the lake; in an in- 

 tricate network of processes, consecutive, correlative and antagonistic; 

 connected by relations which cross and interlock at a thousand points. 

 I shall speak of only a few detached topics. 



I have said that the oxygen of the lake is absorbed from the air. 

 This is true so far as the main stock of oxygen is concerned ; but a lake 

 has a second source of oxygen which is always considerable and which 

 in certain places and relations may become important. The green 

 plants which inhabit the lake are able to take up carbon dioxide from 

 the water, and under the influence of light they can use it in the manu- 

 facture of starch, setting free oxygen in the process. In lakes which 

 contain an abundance of alga?, considerable quantities of oxygen may 

 arise from this source and this manufactured oxygen may play an 

 important part in the vital history of the lake. 



Consider the effect of the addition of this power of the alga? to the 

 numerous factors which are affecting the supply of oxygen in the upper 



