424 



fishermen, in certain places, that when a hole is cut in the ice the fish 

 will come to the place "to get air" and are very easily taken. It seems 

 quite clear that the reason fish die under ice or come to holes in the ice is 

 because there is a deficiency of dissolved oxygen in the water. 



A layer of ice over a body of water very effectually prevents the 

 interchange of gases between the water and the air. The ice itself does 

 not decrease the amount of oxygen, but it stops the aeration of the water. 

 The decrease in amount of dissolved oxygen is brought about by the oxi- 

 dation of organic matter in the water. This organic matter does not 

 oxidize directly, but it forms the food of the bacteria and of larger plank- 

 ton — the immense numbers of minute plants and animals which utilize 

 the oxygen in their life processes. As a result, the amount of dissolved 

 oxygen that is removed from the water depends on the amount of organic 

 matter present and on the rate at which this is consumed by aquatic organ- 

 isms. 



The life processes of these plants and animals which consume the 

 oxygen go on very slowly in water that is cold enough to be covered with 

 ice. It would seem that for this reason those lakes and very sluggish 

 streams which are frozen over early and thaw out late would sutler 

 most from the lack of oxygen. However, due to their sluggishness the 

 solid organic matter settles to the bottom and can not quickly reduce the 

 dissolved oxygen content of the whole body of water. In the deeper 

 lakes fish do not die under the ice, probably because of the relative scanti- 

 ness of organic matter and the greater amount of dissolved oxygen in the 

 deeper water. In most streams large amounts of organic matter are 

 constantly being added by drainage from the land and by wastes from 

 cities. The stirring action of the current is usually sufficient to keep 

 much of the solid organic matter in suspension and thus it exposes all 

 parts of the stream to loss of oxygen. Some streams are so swift that 

 they do not freeze over completely, and enough open water i.> left to keep 

 the dissolved oxygen content of the water high enough for the life of 

 fishes. 



Recent Observations on the Relation of Ice and 

 Dissolved Oxygen in the Illinois River 



The Illinois River is not often covered with ice long enough for the 

 oxygen dissolved in the water to be reduced to a point where the fish 

 show signs of distress. The river froze over in December, 1924, and did 

 not thaw out until the first week in February, 1925. It was completely 

 covered with ice except for small patches of open water where the cur- 

 rent was swiftest, notably in Peoria Narrows and below all bridges. 

 Fishermen say that this is the first lime the river has been frozen over 

 for any considerable time since the winter of 1917-1918. Kofoid* states 

 that during the winter of 1894-95 the river froze over the latter part of 

 December and the ice did not go out until the last of February.. Thb 



• EuL in. state Lab. Nat. Hist., 6:176. 1901. 



