SUMMARY AND DISCUSSION 155 



to be depleted of its population in order to furnish some of these forms 

 with their own weight of organic matter for food. The average dry 

 weight of some of the constituents of the plankton of Lake Mendota has 

 been determined and the results are as follows: (1) A large Asplanchna 

 weighs 0.000834 milligram, (2) a mature Cyclops 0.0041 milligram, (3) 

 a Diaptomus 0.00858 milligram, (4) an adult Daphnia longispina hy- 

 alina 0.02172 milligram. Taking the average quantity of organic mat- 

 ter in the nannoplankton of Lake Mendota as a basis for the calculation, 

 namely, 1,630 milligrams per cubic meter of water (table 25), each of 

 these animals would have to remove all of the nannoplankton from the 

 following quantity of water in order to obtain its own weight of dry 

 organic matter for food; (1) An Asplanchna 0.5 cubic centimeter, (2) 

 a Cyclops 2.5 cubic centimeters, (3) a Diaptomus 5.2 cubic centimeters, 

 and (4) an adult Daphnia hyalina 13.3 cubic centimeters. These quan- 

 tities of water seem very small, but when compared with the size of the 

 organisms concerned they are very large. Disregarding temperature 

 and assuming that one cubic centimeter of water weighs one gram, the 

 above organisms would have to filter about 600,000 times their own 

 dry weight of water in order to secure their own weight of dry organic 

 matter in the form of nannoplankton. These animals may also feed 

 upon some of the organisms in the net plankton and thereby reduce 

 the above quantities of water proportionately. 



Computations based on the numerical data indicate that the Crus- 

 tacea and the rotifers contribute from 25.0 per cent to 75.0 per cent of 

 the organic matter in the net plankton; the maximum percentage is 

 found in late winter and in early spring when the algae and protozoa 

 reach their lowest points in the net material ; the minimum percentage 

 is found in the early summer and in the autumn when the protista 

 flourish most abundantly. Since the maximum percentage of Crustacea 

 and rotifers is correlated in time with one of the minimum periods of 

 the net plankton, it seems probable that these two groups or organisms 

 furnish something like 30.0 per cent to 40.0 per cent, or about one-third, 

 of the mean quantity of organic matter in the net plankton ; that is, an 

 average of about 115 milligrams out of 343 milligrams per cubic meter 

 of water (table 25). If the protista of the net plankton are included 

 in the computation, therefore, the quantity of water that a rotifer or a 

 crustacean would have to strain to obtain its own weight of organic 

 matter would be reduced about 12.0 per cent. 



These quantities of water are based on the mean quantity of organic 

 matter in the nannoplankton and in the protista of the net plankton. 

 Whenever the quantity of organic matter in these two groups of organ- 

 isms is above the mean, the quantities of water would be smaller than 

 the amounts indicated and whenever it is below the mean, these amounts 

 of water would be larger than indicated above. 



