THE CHEMICAL COMPOSITION OF LAKE WATEKS 155 
supply of gas in the upper, and very little in the lower, waters ; 
frozen lakes are cut off from the atmosphere and are gradually 
depleted of oxygen until thawing sets in. In this connection wide 
seasonal variations may be expected. In the second place, the gases 
are much shuffled about by the organic life of the lake waters. 
Animals and most bacteria consume oxygen and produce carbon 
dioxide. A defect of oxygen therefore means a scanty fauna, and 
this may involve important economic consequences, e.g. when the 
bottom waters of a lake are unable to harbour fish which require 
coolness in summer. On the other hand, chlorophyll-bearing plants, 
which are, of course, restricted to the photic zone, consume carbon 
dioxide and give in return oxygen. Thus, at springtime the upper 
waters of lakes have frequently been found supersaturated with 
oxygen owing to the luxuriance of algae ; in pond-waters the abnormal 
content of 24 c.c. per litre has even been reported. Here again, 
then, seasonal variations are all-important. Nitrogen is little 
influenced by a^nimal or plant life ; but, in sea-water at any rate, 
some bacteria are known which assimilate nitrogen, and others which 
set it free as gas from nitrogen compounds. 
These superficial considerations will suffice to show that the 
biological economy of lakes is intimately bound up with the dissolved 
gases, and it may be hoped that the study of these gases, experimental 
difficulties notwithstanding, will play a greater part in limnology 
than it has done heretofore. 
