TOTAL PLANKTON OF LAKE MENDOTA 93 



that of January of this year. In 1916 the amount of organic matter in 

 the nannoplankton was almost twice as large in March as in February, 

 but in 1917 the quantity was smaller in March than in February. April 

 showed the maximum average of the -year both in 1915 and in 1916, 

 but the maximum of 1917 was not reached until May. The average 

 amount of organic matter for June showed a variation of only about 

 8.0 per cent in the three years covered by these observations, but there 

 was a much greater difference in July. The summer minimum was 

 reached in August and the averages were substantially the same in 

 1915 and in 1916. The September averages were higher than those of 

 August and the amount remained at this level or somewhat higher 

 during the last three months of the year. 



In the total plankton, that is the net plankton plus the nannoplank- 

 ton, the smallest amount of organic matter was found in February or 

 March and the largest average in April or May. The average for De- 

 cember, 1916, was higher than that of May, 1917. From June until 

 September the monthly averages of organic matter in the total plankton 

 ranged from a little more than 1,350 milligrams to about 2,000 milli- 

 grams per cubic meter of water; October fell within these limits in 

 1916, but for October, 1917, and for the last two months of both years 

 the amount varied from about 2,200 milligrams to substantially 2,500 

 milligrams. 



The smallest amount of organic matter in the total plankton was 

 found in March, 1917, and the largest amount for the entire series was 

 noted in April, 1916 ; the latter was approximately three times as large 

 as the former. 



Figures 34 and 35 show graphically, in more detail, the relations be- 

 tween the quantity of organic matter in the net plankton and that in 

 the nannoplankton; they also show the total organic matter or the 

 sum of the two. The amounts are indicated in milligrams per cubic 

 meter of water. The curve marked A in the diagrams represents the 

 organic matter of the total plankton ; the one marked B indicates that 

 of the nannoplankton, while C represents the organic matter of the net 

 plankton. The nannoplankton curve, B, is distinctly higher than the 

 one for net plankton, C, for the entire series of observations, thus show- 

 ing clearly that the organic matter in the former always exceeds that 

 in the latter. The two curves are most widely separated in April, 

 1915, and 1916, and in May, 1917; they approach each other most 

 closely in December, 1916. 



The fact that the curve for total plankton, A, is, in general, very 

 similar in form to that for the nannoplankton, B, is further evidence of 

 the predominance of the latter over the net plankton. The most prom- 

 inent difference between these two curves is shown in October, 1915, 



