NET PLANKTON OF LAKE MENDOTA 23 



The area of Lake Kegonsa is so great in proportion to its depth that 

 the wind keeps the entire body of water in circulation during the sum- 

 mer. Lake Waubesa, while only slightly deeper than Lake Kegonsa, 

 is only about a quarter as large, so that the wind is less effective in 

 keeping the water in circulation. As a result, the deeper portion of 

 this lake, which comprises a relatively small part of its total area, 

 possesses a thin bottom stratum of cooler water until about the middle 

 of August ; but after this date the lake is substantially homothermous. 



The maximum temperature of the upper water in these four lakes 

 during the summer ranges between 25° and 30° C. The bottom tem- 

 peratures in Lakes Mendota and Monona in summer vary from about 

 9° to 14°. In winter all of the lakes are covered with ice for a period 

 of three to three and a half months, sometimes even longer. Usually 

 the ice reaches a thickness of about three-quarters of a meter. 



The region in which the lakes are situated is an agricultural district 

 so that the dissolved substances carried by the drainage water of this 

 basin are subject to the modifications usually found in such an area. 

 During the period of these investigations also, Lake Monona received 

 some raw sewage from the city of Madison as well as the eiSuent from 

 its sewage disposal plant. This additional material in suspension and 

 in solution also affected the waters of Lakes Waubesa and Kegonsa to 

 a certain extent, since the Yahara river supplies water from Lake Mo- 

 nona to these two lakes. 



The major portion of the work was done on Lake Mendota, on the 

 south shore of which the laboratory is situated; but for purposes of 

 comparison observations were also made on the other three members of 

 this chain of lakes. Lake Kegonsa was visited only once because the 

 river between it and Lake Waubesa has not been dredged and is not 

 navigable for launches except at an unusually high stage of the water 

 and then only with considerable difficulty. The river between Lake 

 Mendota and Lakes Monona and Waubesa has been canalized so that 

 they can be reached easily by launch from the laboratory. 



Quantity of Water and of Plankton 



Table 2 (p. 181) shows the quantity of water that was strained in 

 each of the lakes and the amount of net plankton obtained therefrom. 

 In all 481 catches were made and a little over two million liters of water 

 were strained. Slightly more than 90 per cent of the total quantity of 

 water was secured from Lake Mendota. The total volume of water 

 yielded a little more than 1,292 grams of dry net plankton, or an aver- 

 age of approximately 600 milligrams per cubic meter. The average 

 yield of net plankton was much smaller in Lake Mendota than in the 

 other lakes ; for 415 catches in this lake the average was 491 milligrams 



