IMPROVEMENT OF LAKES AND STREAMS 35 



contain enough oxygen to maintain the fish all winter. Cutting holes 

 in the ice to allow surface aeration has been tried, but the cutting of 

 many small holes does not allow sufficient oxygen to enter the water 

 and diffuse to all parts, largely because the diffusion or spread of 

 oxygen in the water is so slow that the effects of aeration are not found 

 more than a few feet away. The fact that surface aeration is of little 

 value may be seen in summer when, although the entire lake is ex- 

 posed to the air, fish-kills occur because the plants have temporarily 

 ceased to liberate oxygen. In addition, cutting holes causes the fish to 

 crowd into the holes and actually to smother from overcrowding. Fur- 

 thermore, in a winter climate like that of Minnesota the holes quickly 

 freeze over again. The remedies for oxygen depletion as well as the 

 factors involved have recently been discussed in detail by Greenbank 

 (1945). 



A remedy practical for small bodies of water is clearing the snow 

 from large areas of clean ice. Removing the snow allows the sunlight to 

 penetrate to the plants below and enables them to keep up the liberation 

 of oxygen. In several small lakes where this procedure has been tried it 

 has been successful. 



Possible prevention of oxygen depletion may be accomplished through 

 the removal of excessive vegetation along the margin of the lake before 

 the freeze-up. Vegetation, although it usually functions to keep up the 

 oxygen, becomes one of the heaviest consumers when it can no longer 

 liberate oxygen; this is especially true of water weeds, which die and 

 decay during the winter. There are usually enough microscopic plants 

 in the open water to liberate sufficient oxygen so that the lake will not 

 suffer if the larger plants are taken out. The method of removing excess 

 vegetation has been followed in other states, particularly Ohio, to relieve 

 summer-kills as well as winter-kills. It is expensive, for it requires con- 

 siderable labor and special machinery. If Minnesota had only a few lakes 

 in danger of oxygen depletion, vegetation removal would be possible. 

 However, in a state with many hundreds of lakes in this condition, some 

 of which are of enormous extent, this method is far too expensive for 

 general use. 



Of recent years a number of aerators have been placed on the market. 

 These aerators operate on the general principle of pumping air through 

 the water and then circulating by means of propellors or other devices 

 the water thus aerated. This method of aeration is effective over a 

 limited area, determined by the amount of circulation, but it seldom 

 improves the oxygen condition more than 50 feet away. Moreover, some 

 of these pumps are expensive to operate, and many of them require 

 considerable attention lest they freeze. Pumps may be beneficial on a 

 small lake or pond, but as each can aerate only a limited area a hundred 

 or more would be necessary to maintain the oxygen in a lake of con- 

 siderable size. 



