BIOLOGICAL SURVEY OF THE UPPER MISSISSIPPI RIVER 



153 



each sample. As a rule, 25 liters were strained for each sample, but in a few cases 

 larger volumes of water were strained. The samples were concentrated in the net to 

 from 15 to 20 cubic centimeters and then transferred to a small vial. The materials 

 were preserved in 4 per cent formaldehyde. 



As some of the plankton organisms are too small to be retained by the net, a 

 liter of the strained water was preserved in the field in 1 per cent formaldehyde and 

 shipped to the laboratory for centrifuging. The centrifuging was done at the Uni- 

 versity of Minnesota with a Foerst continuous-acting centrifuge loaned by the 

 Geology and Natural History Survey of Wisconsin. The organisms that are removed 

 by the centrifuge constitute the nannoplankton. In the discussion of the plankton, 

 the net plankton and nannoplankton are considered together for each station. 



The method used in enumerating the phytoplankton, or plant organisms, was 

 as follows: The sample was made up to definite volume and stirred, so that the 

 organisms were well distributed; 1 cubic centimeter of this sample was then placed 

 in a counting cell having an area of 1,000 square millimeters and a depth of 1 milli- 

 meter, and the number of each kind of organism in 20, 30, or 40 fields, as counted 

 under a compound microscope, was determined. The area of the field of the micro- 

 scope was known. From the concentration of the sample, the number of organisms 

 in the various fields counted, and the area of the counting cell the number of individ- 

 uals per liter was calculated. In practice, it is not necessary to make this calculation 

 for each organism separately. All that is necessary is to make one calculation for 

 a factor. The factor is that number which, when multiplied by the number of organ- 

 isms found in the different squares counted, gives the number of organisms per 

 liter or whatever other unit may be chosen. This factor will remain the same as 

 long as the concentration and the number of fields counted remain constant. The 



factor may be calculated according to the formula * '-t^- cx V x = X. Where 1 ,000 is 



~V~ 



the area of the counting cell, a = the total area of the fields counted in mm. 2 ; V x = 

 the volume of the sample in cubic centimeters; c = the total number of organisms in 

 the fields counted; F=the volume of water strained through the net or centrifuge; 

 and A = the factor. It is most convenient, in the calculations of the factor, to let 

 c = unity. 



In the case of the zooplankton, or animal organisms, all the individuals occurring 

 in 2 cubic centimeters of the sample, well stirred, were counted directly under a 

 binocular. From these counts, and the concentration of the sample, the number per 

 liter was then calculated. 



Inasmuch as it is quite generally accepted that no vertical stratification of 

 plankton organisms exists in a river with a fairly rapid flow, it was not always 

 attempted to take samples from all levels by raising the hose of the pump at a uni- 

 form rate. As a rule, in the shallow places half the sample was taken near the surface 

 and the other half close to the bottom. However, at stations where the water 

 reaches considerable depth (20 to 25 feet) and where the current is slack, the pump 

 hose was raised and lowered at a uniform rate so as to get approximately the same 

 amount of water from all levels. Whenever expedient, two samples were taken at 

 each station — one from the channel of the river, the other from the shallower water 

 near shore. (See Table 7.) 



