158 



BULLETIN OP THE BUREAU OF FISHERIES 



(p. 152); for it may be noted in Table 8 that the number of Pleurosigma svencerii 

 and of Lyngbya sp.? is greatly reduced between stations 7 and 9. Table 9 shows 

 that, to a large extent, at least, the genera that are most abundant in the unpolluted 

 waters are also most abundant in the polluted waters. 



Table 9. — Nine most abundant genera for each group of stations 



Genus 



Rank of 

 genera in 

 Group I 

 (stations 1, 

 8, 10, and 

 12) 



Rank of 

 genera in 



Group II 

 (stations 2, 

 11, and 14) 



Rank of 

 genera in 

 Group III 

 (stations 3, 

 5, 6, 7, and 

 9) 



Genus 



Rank of 

 genera in 

 Group I 

 (stations 1, 

 8, 10, and 

 12) 



Rank of 

 genera in 

 Group II 

 (stations 2, 

 11, and 14) 



Rank of 

 genera in 

 Group III 

 (stations 3, 

 5, 6, 7, and 

 9) 



Melosira _ 



1 



2 



2 



Synedra _ 



7 



5 



1 



Nitzschia 



2 



4 



7 



Chlorella (colony) 



8 





9 



Scenedesmus 



3 



3 



3 



Amphora.- 



9 



9 





Navicula 



4 



7 





Ceratium 





6 





Chlorella (single) 



5 



1 



8 



4 



Pleurosigma 







5 



Stephanodiscus.- 



6 



6 



Lyngbya 







8 

















To recapitulate, a study of the phytoplankton of Table 8 indicates (1) that 

 those species of plants listed by Fair as tolerant forms, and taken by me, are valueless 

 in the present survey as criteria of conditions of pollution, and (2) that none of the 

 other well-represented species of plants taken by me (except, possibly, Pleurosigma 

 spencerii and Lyngbya) show a distinct preference for polluted waters and may be 

 employed as criteria of the presence of pollution. Table 9 showed that, in so far as 

 my material is concerned, the character of the phytoplankton changes little with 

 the degree of pollution in the river, and that the plankton organisms that are most 

 abundant in the unpolluted waters are, in general, also most abundant in the grossly 

 polluted waters. 



Table 10 shows for each date of collection the abundance per liter of water for 

 each genus of zooplankton taken at each field station. It may be seen that the genus 

 Rotifer, listed by Fair as including tolerant species, occurred in my samples. Rotifer 

 sp? occurred regularly in samples from the polluted waters and was decidedly more 

 abundant there than in the unpolluted waters. The average number per liter for 

 the polluted stations (Group III, Table 9) is 70.4; that for the unpolluted stations 

 (Group I, Table 9) is 2.5. The rotifer, therefore, seems to be a tolerant form. Table 

 10 shows also that Nauplii are about four times as abundant in the grossly polluted 

 waters (average per liter, 9.5) as in the unpolluted waters (average, 2.3 per liter), 

 but that they are twice as abundant in the slightly polluted waters (17 per liter) 

 as in the grossly polluted waters. A nauplius, therefore, can not be employed as a 

 criterion of polluted waters. For Cyclops the average numbers of individuals per 

 liter are 0.3 for unpolluted, 6.8 for slightly polluted, and 11.5 for grossly polluted 

 waters. The average numbers per liter for Anuraea are 0.2 for unpolluted, 1.36 for 

 slightly polluted, and 6.8 for grossly polluted waters. It is believed that the numbers 

 given here for the most abundant forms (the genus Rotifer excepted), are too small 

 to enable one to ascertain the degree of tolerance of the various species of zooplankton. 

 It is to be noted from Table 10 that the tolerant Rotifer is very abundant at stations 

 5, 6, and 7, on the Mississippi River, and declines suddenly at station No. 9. This, 

 as in the case of bottom fauna (p. 151) and the phytoplankton (p. 157), suggests 

 a change in the condition of the river between Hastings and Red Wing. 



