SUMMABY AND DISCUSSION 141 



CHAPTER VII 

 GENERAL SUMMARY AND DISCUSSION 



In previous chapters attention has been called to the fact that there 

 are marked variations in the quantity of net plankton and of nanno- 

 plankton during the course of the year. This fact is shown graphically 

 in figures 8, 9, 28, and 36. There are also more or less marked changes 

 in the chemical composition of the plankton, which is illustrated by the 

 variations in the percentages of nitrogen, ether extract, pentosans, and 

 crude fiber. (Tables 43 to 48.) These variations in quantity and in 

 chemical composition make it difficult to give a summary of the entire 

 series of plankton samples. It is necessary to remember, therefore, in 

 connection with the general data presented in table 25 (p. 196) that 

 this plankton material is subject to such changes in order to guard 

 against a wrong interpretation of these data. Furthermore, it should 

 be noted that the data presented in this table do not represent the total 

 amount of organic matter produced by the plankton; they show only 

 the average standing crop maintained during the period of this investi- 

 gation. 



The mean quantity of dry organic matter indicated for the different 

 lakes in table 25 represents the average amount for all of the plankton 

 samples from each lake ; this mean also represents the average amount 

 for all depths in the deeper part of each lake. The plankton was more 

 abundant in the epilimnion than in the hypolimnion during the summer 

 period of stratification (see p. 101), but the water from all depths 

 was combined into one sample in this investigation. In addition to the 

 average for the complete series of net catches, the averages for the net 

 samples corresponding to the nannoplankton samples are indicated also. 

 The mean percentages of nitrogen, ether extract, pentosans, and crude 

 fiber were ascertained by dividing the mean quantities of these sub- 

 stances in the various samples on which such determinations were made 

 by the average amount of organic matter in the same samples. For 

 example, the general table (No. 43) shows that nitrogen determinations 

 were made on 166 samples of net plankton from Lake Mendota ; these 

 samples yielded an average of 28.6 milligrams of nitrogen per cubic 

 meter of water, excluding the nitrogen of the crude fiber, and an aver- 

 age of 341.8 milligrams of dry organic matter. The nitrogen in this 

 material, therefore, amounted to 8.37 per cent of the organic matter. 



