BIOLOGICAL SURVEY OF THE UPPER MISSISSIPPI RIVER 



151 



of Tubificidae here, as compared with the number occurring at station No. 7, the 

 presence of dragon fly nymphs, Caddis fly larvae, Asellus, and the large number of 

 Hyalella, may all be considered as marking a stage of transition in the conditions of 

 the Mississippi River. Table 5 shows that dragon fly nymphs, Caddis fly larvae, 

 and Hyalella occurred at station No. 1 on the Mississippi River, and that these forms 

 have not been taken at any of the stations on the Mississippi River between stations 

 1 and 9. In Europe, the presence of Asellus is taken as the first sign of improve- 

 ment in a polluted stream (Wundsch, 1926). 



The bottom sample taken at station No. 10 consisted of coarse sand. No ani- 

 mals were taken in this sample, but some May fly nymphs were seen clinging to the 

 undersurface of stones. This suggests that the Cannon River is not polluted at 

 this station. 



Bottom samples from station No. 11, on the Mississippi River, indicate further 

 improvement in the conditions of the river. Tubificidae are absent entirely from the 

 samples taken along the left shore. In August the shore sample gave 18 dragon fly 

 nymphs, 54 damsel fly nymphs, 54 larvae of Chironomus, 3,600 Hyalella, 36 Planaria, 

 and 90 Pleurocera acuta per square yard. In September there were, near shore, 12 

 dragon fly nymphs, 12 damsel fly nymphs, 36 larvae of Chironomus, 3,200 individuals 

 of Hyalella, 18 Pleurocera acuta, and 12 leeches per square yard. In August the bot- 

 tom fauna of the channel consisted of 18 Tubificidae, 36 larvae of Chironomus, 18 Hya- 

 lella, 18 individuals of Asellus, 36 Planaria, 90 Pleurocera acuta, and 180 leeches; 

 and in September, of 900 Tubificidae and 18 P. acuta per square yard. 



Due to a lack of proper equipment, no bottom samples could be obtained at 

 station No. 12. 



Station No. 13, on the Zumbro River, was not visited. 



The bottom samples of station No. 14 were all taken near shore. The samples 

 for August showed that each yard of bottom contained 36 damsel fly nymphs, 144 

 individuals of Hyalella, 144 May fly nymphs, 684 larvae of Chironomus, 36 indi- 

 viduals of Campeloma integrum, and 36 leeches. In the September samples there 

 were 342 May fly nymphs, 180 Tubificidae, and 18 specimens of Campeloma integrum 

 per square yard. The May fly nymphs are the most significant element in the bottom 

 fauna at station No. 14. They have not been found at any other station in the 

 Mississippi River except at station No. 1, which is above. Minneapolis. May fly 

 nymphs also were taken in the St. Croix and Cannon Rivers. The presence of May 

 fly nymphs at station No. 14 may be taken as an indication that conditions in the 

 river above Winona are fairly good and probably are comparable with conditions as 

 they exist at station No. 1 above Minneapolis. 



A study of the bottom samples (Table 5) shows that the clean-water animals were 

 taken at stations 1, 8, 9, 10, 11, and 14 only, and that at these stations and at No. 4 the 

 tolerant forms, in general, were least abundant. The study shows, further, that all 

 the bottom samples taken from the Mississippi River between stations 1 and 9, 

 except one of those taken between stations 2 and 3 in the metropolitan area, which 

 took no animals at all, contained relatively large numbers of typically tolerant forms 

 but not a single individual of a clean-water form. Clean- water forms were expected 

 at stations 1, 4, 8, and 10, inasmuch as they are situated outside the area polluted by 

 72090—28 3 



