24 



PLANKTON OF WISCONSIN LAKES 



per cubic meter of water. For 47 catches in Lake Monona the yield 

 was 1,499 milligrams; for 18 catches in Lake Waubesa it was 2,182 

 milligrams, while in the one catch in Lake Kegonsa the amount was 

 G,112 milligrams per cubic meter. The sample from Lake Kegonsa ex- 

 ceeded even the maxima of the other lakes. The largest amount found 

 in Lake Mendota was only about 1,700 milligrams per cubic meter of 

 water, while that of Lake Monona was 3,820 milligrams and that of 

 Lake Waubesa 4,600 milligrams. 



Table 3 shows the amount of water that was centrifuged for the 

 nannoplankton, together with the quantity of dry material obtained 

 therefrom. 



Table 4 (p. 182) gives in detail the quantity of water used for the 

 various samples of net plankton and nannoplankton, as well as the 

 amount of dry material obtained in each instance. The quantitative 

 results shown in the general tables (numbers 43 to 48) are based upon 

 the quantities indicated in this table. In computing the amount of dry 

 organic matter per cubic meter of water during the period from July to 

 September, it was necessary to introduce a correction ; that is, during 

 these months the hypolimnion of Lakes Mendota and Monona is almost 

 or quite devoid of free oxygen and, as a result, the net plankton organ- 

 isms are unable to occupy this stratum. From 1911 to 1914, therefore, 

 the net samples covered only the inhabited stratum of the lake so that 

 it was necessary to apportion the total catch to the entire depth in order 

 to get a general average for the whole lake. From 1915 to 1917 the cor- 

 rections were very much smaller because material was secured regularly 

 down to a depth of 20 meters in Lake Mendota and down to 18 meters 

 in Lake Monona. 



Some of the net plankton catches made in 1915 have been omitted 

 because two series of such catches were taken in that year; only those 

 are shown which have been subjected to the most complete analysis. 

 Attention may be called to the fact that the quantity of water is the 

 same for a net plankton catch as for the corresponding sample of nanno- 

 plankton, the same sample of water being used for both. 



The studies on the net plankton were begun on June 1, 1911, and 

 were continued regularly until June 1, 1917, with the exception of the 

 period from July, 1914 to April, 1915. During the investigations nine 

 sets of observations were made on Lake Mendota in the winter season, 

 that is, while the lake was covered with ice. No attempt was made to 

 secure winter catches on the other lakes. Work was begun as promptly 

 as possible after the ice disappeared from the lake in the spring and it 

 was continued until fairly thick ice formed along the shore of the lake 

 in early winter and prevented further use of the launch. The latest 

 date on which observations were made with the launch was December 



