LAKE DYNAMICS 19 



water. When dissolved, oxygen diffuses or spreads very slowly in the 

 water. The surface of the water, in actual contact with the air above, 

 absorbs large amounts of oxygen, but owing to the slow diffusion the 

 oxygen seldom travels beyond a thin film next to the surface, unless 

 the water is agitated by strong waves or currents. In rivers the current 

 alone, through its changing contact with the air, will keep the water 

 well aerated, but in lakes currents are usually weak or entirely absent. 

 Thus the principal source of oxygen in lake water is from the aquatic 

 plants. 



The oxygen in a lake is consumed in three principal ways: by fishes 

 and other aquatic animals; by plants that use oxygen as well as liberate 

 it; and last, and perhaps most important, by bacteria and decomposing 

 organic matter. The decomposing organic matter consists partly of 

 dead animals, but mostly of dead plants and the partially rotted ooze 

 that covers the deeper bottoms of our lakes and slowly decays through 

 bacterial activity, creating a heavy demand on the oxygen supply. Con- 

 sequently, lakes that have abundant vegetation and bottoms covered 

 with rich muds draw heavily on their oxygen supply. An excess amount 

 of vegetation becomes a serious oxygen consumer during the winter. 

 The production of oxygen ceases when the ice is covered with snow, 

 and the supply on hand must last throughout the winter. In deep 

 lakes, which have a considerable volume of water, the supply is usually 

 sufficient, but in shallow lakes, 6 to 10 feet deep, the volume of oxygen 

 that can be stored is seldom sufficient to meet all these demands. 



The situation is further aggravated by the fact that these shallow 

 lakes produce the greatest amount of vegetation and consequently 

 have a large amount of dead plant life to decay during the winter. In 

 shallow waters the ice may be so thick that the volume of the water 

 is reduced to a point where it cannot hold sufficient oxygen to last all 

 winter. This critical condition results in a fish-kill. In northern Minne- 

 sota many rocky infertile lakes with depths as shallow as 8 feet do not 

 lose their oxygen, while in southern and central Minnesota mud-bot- 

 tom, fertile lakes of twice that depth may lose their oxygen. 



During the winter many of the fishes are semidormant and require 

 much less oxygen than when fully active, but even so fish-kills as a 

 result of oxygen depletion often occur. The fishes are sealed in the lake 

 throughout the winter by the ice, which is thicker in Minnesota than 

 in most other states. The survival of these fishes depends on the pres- 

 ence of a sufficient amount of oxygen stored under the ice at the time 

 of the freeze-up or the penetration of sufficient sunlight to enable the 

 plants to release enough oxygen to meet the demands of the lake and 

 of the fishes until spring. 



This fact was clearly demonstrated several years ago in Minnesota, 

 when one of the shallow lakes in Anoka County was studied through- 



