144 



BULLETIN OF THE BUREAU OF FISHERIES 



DISSOLVED OXYGEN 



All organisms require oxygen for the maintenance of life. Aquatic animals, 

 with the exception of air breathers, depend for their oxygen supply upon the oxygen 

 dissolved in the water. The amount of dissolved oxygen, therefore, furnishes one 

 good index as to the suitability of a body of water to support life. It is possible, 

 of course, to find waters with a high dissolved oxygen content that are unsuitable 

 for living organisms. This is often the case when waters are polluted by mineral 

 acids, bases, and salts, or by other chemical ingredients that act as specific poisons. 

 However, in a body of flowing water such as the Mississippi River, which does not 

 stratify and stagnate, the presence of oxygen at normal temperatures in minimal 

 quantities is an indication of pollution. 



Tables 3 and 4 give the results of dissolved oxygen determinations made by 

 Mr. Crohurst for the months of August and September, 1926. These tables also 

 give the average monthly temperatures of the water at the various field stations. 

 From these data the average percentage of saturation for each month was calculated. 

 For some purposes it is more expedient to express oxygen content in terms of the 

 degree of saturation than in terms of the absolute amount. 



Table 3 shows that during August the amount of dissolved oxygen was far 

 greater at some stations than at others. Dissolved oxygen was present in fairly 

 large amounts at all the stations on the tributaries (the average ranged from 5.70 

 to 8.08 parts per million, or 64 to 87 per cent of saturation) as well as at stations 

 1, 2, 11, and 14 (average ranged from 5.37 to 6.59 parts per million, or 60 to 74 per 

 cent of saturation) on the Mississippi River. The waters at stations 3, 5, 6, and 7 

 on the Mississippi River (average ranged from 0.39 to 0.87 parts per million, or 4 

 to 9 per cent of saturation) contain very small amounts of dissolved oxygen, some- 

 times a trace only, or none at all. At station No. 9 (average = 2.25 parts per million, 

 or 25 per cent of saturation) conditions with respect to dissolved oxygen are much 

 better than they are at stations 3, 5, 6, and 7, but are not nearly as good as they 

 are at stations 1, 2, 11, and 14. The monthly average at No. 9 probably has been 

 raised through the heavy rains that fell during the latter part of the month. Table 

 4 shows that a marked improvement with respect to dissolved oxygen occurred at 

 stations 3, 5, 6, and 7 after the first week in September. This improvement un- 

 doubtedly is due to the cooler weather and to the large increase in the volume of 

 water in the river, the results of heavy rains. 



The data in Tables 3 and 4, then, show the following facts: 



1. During August and the first week of September, 1926, the dissolved oxygen 

 content is decidedly less in that section of the Mississippi River that extends from 

 station No. 2, at the beginning of the metropolitan area, to station No. 9, at the 

 head of Lake Pepin (a distance of approximately 64 miles), than it is above or below 

 this section or in the tributary waters. The tributaries and the Mississippi River 

 above the Twin Cities obviously are not polluted by the sewage of Minneapolis and 

 St. Paul. As stated above (p. 142), many of the suspended materials in the Missis- 

 sippi waters settle at the head of Lake Pepin, which virtually is a settling basin. 

 The water at stations in and below this lake (Nos. 11 and 14), then, should be com- 

 paratively free from any sewage that may be carried down to station No. 9; and 



