BIOLOGICAL SURVEY OF THE UPPER MISSISSIPPI RIVER 



165 



SUMMARY OF RESULTS 



1. This biological survey of the upper Mississippi River system was undertaken 

 by the Bureau of Fisheries at the request of the States of Minnesota and Wisconsin. 

 The field work was done during the period August 12 to September 27, inclusive, 

 and covered approximately the same territory now under investigation by the 

 United States Public Health Service — viz, the Mississippi River from above Minne- 

 apolis (Camden Bridge) to Winona, Minn., a distance, by water, of approximately 

 120 miles, and the following tributaries: The Minnesota, St. Croix, Cannon, and 

 Chippewa Rivers. 



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

 content was decidedly less in that section of the Mississippi River that extends 

 from station No. 2, at the beginning of the metropolitan area of the Twin Cities, 

 to station No. 9, at Red Wing, at the head of Lake Pepin (a distance of approximately 

 64 miles), than it was above or below this section or in the tributary waters. It 

 was concluded that this decrease or depletion of dissolved oxygen was due primarily 

 to the pollution of the river by the cities of Minneapolis and St. Paul. 



3. All the bottom samples taken from the Mississippi River between station 

 No. 1, above the metropolitan area, and station No. 9, at Red Wing, at the head of 

 Lake Pepin (except in one of the samples taken between stations 2 and 3 in the 

 metropolitan area, which took no aninals at all), took relatively large numbers of 

 typically tolerant (pollution) forms, but not a single individual of a clean-water 

 form. Clean-water forms first appeared in the bottom samples of the Mississippi 

 below Minneapolis, at station No. 9, at Red Wing. 



4. The major portion of the organic residue of the strained bottom samples 

 taken from the Mississippi River between stations 1 and 9 consisted of coarse organic 

 debris and garbage, whereas the greater part of the residue of the samples taken 

 outside of this area and examined consisted of cleaner materials such as aquatic 

 plants, empty mollusk shells, sand, chips of wood, etc. 



5. A study of the plankton showed that only three of the species taken by me 

 may be considered tolerant forms — that is, forms that may be employed as rough 

 criteria of the degree of pollution. Two of the three species (Pleurosigma spencerii 

 and Lyngbya sp.?) are plants (phytoplankton), while the third (Rotifer sp.?) is an 

 animal (zooplankton). These forms usually were found to be comparatively very 

 abundant when taken at stations situated in the more polluted waters but relatively 

 sparse when taken from less polluted waters. All three forms showed a marked 

 decline in abundance at station No. 9, at Red Wing. The species of phytoplankton 

 listed by Fair as tolerant forms and taken by me could not be employed in this 

 survey as criteria of conditions of pollution. 



6. The data on dissolved oxygen, the organic composition of the residue of 

 strained bottom samples, the bottom fauna, and the tolerant plankton forms all 

 show (1) that the waters of the Mississippi River were badly polluted from Minne- 

 apolis to Hastings, a distance of about 49 miles; (2) that somewhere between Hastings 

 and Red Wing (situated about 15 miles farther downstream) the river recovered 

 somewhat from its grossly polluted condition; (3) that at station No. 11, at the 

 lower end of Lake Pepin, conditions with respect to pollution were much improved; 



