BIOLOGICAL SURVEY OF THE UPPER MISSISSIPPI RIVER 



155 



cance. The differences in the number of plankton organisms in August and Sep- 

 tember can hardly be attributed to the fact that the rains had improved conditions 

 in the river, for the plankton at station No. 1 decreased, while that at station No. 8 

 increased. Both of these stations are in unpolluted waters. Stations No. 5 and 

 No. 9 are both in polluted waters, yet the plankton at No. 5 decreased, while it 

 increased at No. 9. (Table 7.) 



In the foregoing comparison station No. 4 on the Minnesota River has been 

 grouped with the unpolluted stations; but the data on dissolved oxygen show that 

 its waters may be slightly polluted by the two cities some 15 miles above. (See 

 Tables 3 and 4 and location of sampling stations, p. 141.) If we omit station No. 4 

 from the group of unpolluted stations, the average number of organisms for that 

 group is reduced to 50,054, or 47,996 below the average number of organisms in 

 the grossly polluted stations. The average number per liter for the presumably 

 slightly polluted waters (stations 2, 11, and 14) is 35,028, or 63,022 less than for 

 the grossly polluted waters. The variations in the abundance of the plankton 

 may very well be explained on a basis of seasonal variations, as the samples were 

 taken on different days and in different months. (It is a common experience that 

 two samples of plankton taken at one place on two consecutive days may show 

 a great difference in the number of organisms.) 



The seasonal variation is well illustrated in my samples by Melosira. This 

 plankton form was present in very small numbers at station No. 8 in August (Table 

 8), while nearly a month later, in September, it was very abundant (176,960 fila- 

 ments per liter). I therefore believe that the data of Table 7 warrant the conclu- 

 sion that no correlation exists between the total number of plankton individuals 

 and the degree of pollution in the upper Mississippi River system, and, therefore, 

 the abundance of plankton can not be employed as a criterion of the degree of pol- 

 lution in the river. 



Table 8 shows, for each genus of phytoplankton, the number per liter of water 

 taken on the various dates at each station. It may be seen that this table includes 

 four of the genera of algae (Nitzschia, Synedra, Pleurosigma, and Closterium) listed 

 by Fair as including species that are known to be tolerant (p. 152). However, most 

 of my species of these forms are not those listed by Fair as tolerant. The excep- 

 tions are Closterium acerosum and Synedra ulna. My data (Table 8) show that 

 the former species occurred in very small numbers at stations 2, 7, 9, and 11 (waters 

 at stations 2 and 11 are not grossly polluted), while the latter occurred in small 

 numbers at station No. 14 only, the waters at which, as I have already shown, 

 were at most only slightly polluted. Obviously, G. acerosum and S. ulna, the 

 so-called "tolerant" algae, are, in the present survey, valueless as criteria of con- 

 ditions of pollution. The Synedra found at all the other stations were identified 

 as S. delicatissima, a form whose degree of tolerance has not been established. 



