&& Conservation Department 



In order to show graphically the relative abundance of the con- 

 stituent genera and groups fig. 6 was constructed for the net 

 plankton. An average for each dominant genus or group for the 

 lake as a whole was computed from all the stations and the num- 

 ber of organisms or colonies per liter was plotted for each of the 

 four trips. 



The graphical method of expressing results is that used by Loh- 

 mann and also by Birge and Juday.^ Time is plotted along the 

 abscissa and the quantity of organisms as ordinates. The trips 

 and dates are as follows : 01, July 30-August 1 ; 03, August 13- 

 17; 05, August 28-September 1 ; 06, September 12-14. The width 

 of the black band for any genus or group at any time represents 

 the diameter of a sphere whose volume is the number of organ- 



3/ V 



isms per liter. K=: w — ^jg- where R is the radius of of the sphere 



whose volume is V. V in this case is the number of organisms 

 per liter. The width of the black band in the chart is equal to 2R, 

 i. e., the diameter of the sphere. The scale employed in making 

 the graphs is R=l=:.25 cm. To solve the graph for the average 

 number of organisms present in the lake at any given time the fol- 

 lowing formula may be applied: 



V=4.19(5D)^ Avhere V^number of organisms per liter; 

 D= width of the band in centimeters. 



It must be borne in mind that the width of the bands is a func- 

 tion of the cube root of the actual number per liter and hence the 

 increase in width becomes proportionately less with the increase 

 in number of organisms represented. 



Upon examining the chart, several things appear outstanding. 

 With the exception of the rotifers, there is an increase in the 

 number of organisms in autumn. This is especially true of the 

 diatoms, which in September are rapidly approaching their 

 autumnal maximum. A concept of the relative abundance of the 

 constituent genera at any time during the summer may be formed 

 by looking at the chart of the crop. It might be pointed out that 

 any given organism is not always present in constant numbers nor 

 is it necessarily true that one or several organisms maintain a 

 dominant position in the environment at all times. This is due to 

 fluctuations in the physical and chemical factors of the lake 

 environment and to the action of predatory rotifers and crusta- 

 ceans. These latter are in large measure dependent upon the 

 smallest micro-organisms for food and in turn are themselves eaten 

 by fish. 



In view of these various factors operative in the formation of 

 plankton as basic fish food, it is very evident that further study 

 is necessary in interpreting the none too simple problem of lake 

 production. It is hoped, however that these studies may con- 

 tribute a little more knowledge as to the kinds of microplankton 

 in Lake Erie and something of their significance in the economy of 

 the lake. 



1 Wisconsin Cool. & Nat. Hist. Siirvov Bull. No. 04. ScicMitific Series No. 13. 



