112 PLANKTON OF WISCONSIN LAKES 



little more than 22 grams of dry material, was obtained for a chemical 

 analysis. The results of this analysis are given in table 49, p. 215. 

 This material was obtained about 10 a. m. on a bright, calm day and 

 the colonies of Volvox were concentrated in a relatively narrow stratum 

 about three meters below the surface of the water, thus showing a 

 definite negative reaction to the sunlight. Their subsequent behavior 

 was studied by Smith 1 and the results of his investigation have already 

 been published. 



The autumn maximum of organic matter in the net plankton of 1916 

 was noted on November 9 (sample No. 6183) and it was due chiefly to a 

 large crop of Melosira. Stephanodiscus also showed a fairly large 

 increase in numbers at this time. 



The Nannoplankton 



Quantitative studies of the nannoplankton of Lake Monona were 

 made in 1915 and in 1916 ; 8 samples were obtained in the former year 

 between June 23 and October 29, and 13 samples in the latter year be- 

 tween May 12 and November 9. The amount of water centrifuged in 

 1915 was 9,777 liters and 12,510 liters in 1916, making a total of 22,287 

 liters for the two years. (See table 3, p. 182.) 



Organic Matter. The results of the determinations of the organic 

 matter in the nannoplankton material from Lake Monona are shown in 

 table 29, p. 198. In 1915 the largest amount of organic matter was 

 found in sample No. 5166, October 15, and the smallest amount in No. 

 563, July 13 ; the former sample yielded 3,746.5 milligrams per cubic 

 meter of water and the latter 723.0 milligrams, a little more than a 

 fivefold difference. In 1916 the maximum amount was noted in sample 

 No. 6158 on October 12, namely, 5,696.2 milligrams and the minimum 

 amount in No. 648 on June 14, namely, 672.1 milligrams; the former 

 quantity is approximately eight and a half times as much as the latter. 

 (Table 46.) While the maximum was very much higher in 1916 than 

 in 1915, the mean of the two years was substantially the same, namely, 

 2,355.6 milligrams and 2,339.5 milligrams, respectively, the average 

 amount for 1915 being only 16.1 milligrams smaller than that for 1916. 



The maximum amounts of dry organic matter in the nannoplankton 

 of Lake Monona were appreciably higher than those noted for Lake 

 Mendota; in the latter lake, for example, the maximum for 1915 was 

 2,776.3 milligrams per cubic meter of water, 3,151.5 milligrams in 1916, 

 and 2,603.4 milligrams in 1917. In 1915 a summer minimum of 944.2 

 milligrams was noted in August in Lake Mendota, while in the summer 

 of 1916 the smallest amount of organic matter, 1,053.5 milligrams per 



1 Amer. Jour. Botany, Vol. 5, 1917, pp. 178-185. 



