SUMMAEY AND DISCUSSION 151 



terial. This gives an average yield of 102 milligrams of ash per cubic 

 meter of water for the entire series of samples. The 47 net samples 

 from Lake Monona contain an average of 14.6 per cent of ash, or 145 

 milligrams per cubic meter of water. In the 18 net samples from Lake 

 Waubesa the ash content averages 16.4 per cent of the dry weight of 

 the material, which gives a yield of 331 milligrams per cubic meter of 

 water. 



The quantitative data presented in the various tables do not represent 

 the total amount of plankton produced during any given period of time, 

 but they show simply the standing crop of this material that was pres- 

 ent at the time the observations were made. Neither do the mean quan- 

 tities shown in table 25 refer to the problem of production, but they 

 indicate the average amount of organic matter in the standing crop of 

 plankton when the whole series of samples from each lake is considered 

 as a unit. The seasonal or annual production of this material involves 

 the question of the rate of the turnover in the plankton crop and this 

 is a very complex problem. 



The plankton of a lake may be regarded as analogous to a pool in 

 a stream, with a current of water constantly flowing in on one side and 

 a regular outflow on the other. The pool itself represents the standing 

 crop of plankton, while the inflowing stream is analogous to the process 

 of plankton production and the outflowing one typifies the losses of this 

 material from various causes. The stream of water that is continually 

 passing through the pool closely resembles the constant stream of 

 plankton life which exists in a body of water. Since the standing crop 

 of plankton shows marked variations in quantity during the year, it is 

 necessary to regard the pool and stream as variable in size, expanding 

 to several times their normal size in periods of flood and falling con- 

 siderably below normal in periods of drought. In spite of these seasonal 

 variations in quantity, however, a surprisingly close correlation in the 

 size of the standing crop of plankton is found at corresponding periods 

 of the different years. (Figures 34 and 35.) 



In the foregoing illustration it would be a relatively simple problem 

 to ascertain, with some degree of accuracy, the amount of water that 

 passes through the pool annually, and this may be thought of as corre- 

 sponding to the annual production of plankton. It is also a compara- 

 tively simple problem to ascertain the annual productivity of a given 

 area of land because the crop can be limited to a single kind of grain or 

 hay and because there are definite seasons for planting and harvesting 

 the crop. But the problem of ascertaining the plankton productivity 

 of a body of water is far more complex than the determination of the 

 amount of water that passes through the pool in the foregoing illustra- 

 tion, or than determining the productivity of a piece of land. 



