92 PLANKTON OF WISCONSIN LAKES 



CHAPTER IV 



THE TOTAL PLANKTON OF LAKE MENDOTA 



The term total plankton is used here to designate the sum of the net 

 plankton and the nannoplankton. Since the ash of the centrifuge 

 material contains a certain amount of silt, and is therefore abnormally 

 high, the discussion of the results is necessarily limited to the organic 

 matter of the net plankton and of the nannoplankton. Only those 

 catches of the former which correspond to the samples of nannoplank- 

 ton are taken into consideration in this chapter. That is, the results 

 given for net plankton in this chapter cover only those samples which 

 were obtained between April 21, 1915, and June 1, 1917. 



Variations in the Quantity 



The distribution of the dry organic matter of the net plankton and 

 of the nannoplankton of Lake Mendota by months for the different 

 years is shown in table 24, in which the average amount of each is in- 

 dicated for the different months as well as the sum of the two, or the 

 organic matter of the total plankton. The months of January and 

 February are represented by single catches, but the results for the 

 other months are averages of two to eight or more catches per month. 



The large crop of Aphanizomenon which developed late in 1916 held 

 over into January, 1917, and gave a large catch of net plankton in this 

 month. By February the net plankton had decreased to less than a 

 quarter of the January amount, even being below that of February, 

 1916. In both years the March samples were smaller than those of the 

 previous month. The average of April, 1917, was about 10.0 per cent 

 below March, but in 1916 there was an increase in the net plankton 

 during this period. The average for May showed an increase in all 

 three years ; June yielded a distinctly larger average than May in two 

 years, but it was somewhat smaller than May in 1916. July and 

 August were characterized by declines in the amount of net plankton, 

 but September ushered in the autumnal rise both in 1915 and in 1916 ; 

 the maximum point was reached in December of both years. 



The difference in the amount of organic matter in the two February 

 samples of nannoplankton was only a little over 5.0 per cent, the quan- 

 tity being somewhat larger in 1916 than in 1917. The catch in Febru- 

 ary, 1917, showed a decrease of somewhat more than 20.0 per cent over 



