152 



BULLETIN OF THE BUREAU OF FISHERIES 



the Twin Cities. That no clean-water forms were taken from the Mississippi River 

 below the Twin Cities above station No. 9 indicates that somewhere between station 

 No. 7 (at Hastings, about 39 miles below St. Paul) and station No. 9 (at Red Wing, 

 about 50 miles below St. Paul) the Mississippi River is recovering from its grossly 

 polluted condition. The bottom fauna at station No. 11 (at the lower end of Lake 

 Pepin) suggests a still greater improvement in the Mississippi River, while that at 

 station No. 14 (situated at Winona about 110 miles below St. Paul) indicates that 

 the conditions in the river here are probably as good as they are at station No. 1 

 (situated above the Twin Cities). The few data shown on Table 6 support the above 

 conclusions. 



PLANKTON 



RELATIONSHIP OF PLANKTON ORGANISMS AND POLLUTION 



A quantitative study of the plankton — the ultimate source of the food of probably 

 all fishes — gives some information as to the abundance of the food supply. Plankton 

 studies, however, may do more than that. Some plankton organisms are suspected of 

 being tolerant forms — that is, they seem to thrive best in a situation where large 

 quantities of organic matter are in a state of decomposition. Therefore, the presence 

 of such an organism in large numbers in a plankton sample may be taken as a sign of 

 pollution. Plankton organisms known or suspected to be tolerant are Nitzschia 

 amphioxys, Synedra ulna, Pleurosigma acuminatum and attenuatum, among the Diato- 

 macese; Spirulina oscillarioides and jenneri and five species of Oscillatoria, among the 

 Cyanophyceae; and Closterium acerosum, moniliferum, and parvulum and three species 

 of Cosmarium, among the Chlorophyceae. Among the zooplankton organisms, one 

 species of Paramecium, one species of Euglena, and at least one species of Rotifer are 

 considered tolerant. (This list of organisms is taken from Fair's list in the revised 

 edition of "The Microscopy of Drinking Water," by Whipple, in press, 1927.) 



There are three dangers that must be guarded against in drawing conclusions from 

 plankton studies: 



1. The mere presence of a tolerant organism does not necessarily indicate polluted 

 conditions. Only when the organism occurs in comparatively large numbers may it 

 be taken as a criterion. 



2. The absence of tolerant organisms from certain waters may not be proof, neces- 

 sarily, that these waters are unpolluted. This is especially true when the study, as in 

 the present case, is continued for a limited period of time only. Every student of 

 the plankton knows that there are two types of plankton cycles — (a) the total amount 

 of the plankton in any body of water varies with the seasons of a year and may vary 

 even with the years, (b) certain species may dominate the plankton population in one 

 season and disappear entirely in another. 



3. In a river, especially after a rise in the level of the water, organisms are carried 

 downstream. This often makes it very difficult to tell whether an organism was pro- 

 duced where it was taken or whether it was carried there by the current. 



METHODS 



Plankton samples were taken by means of a plankton pump and plankton net 

 made of No. 20 silk bolting cloth. While taking the samples, the net was suspended 

 over a vessel of known capacity, to measure exactly the volume of water strained for 



