20 PLANKTON OF WISCONSIN LAKES 



CHAPTER II 



THE NET PLANKTON OF LAKE MENDOTA 



The material for the investigations relating to the quantity and 

 chemical composition of the net plankton was obtained from four lakes, 

 namely, Mendota, Monona, Waubesa, and Kegonsa. These lakes are 

 situated in the Yahara river basin ; they occupy local enlargements of 

 this valley and they are named in order beginning with the one nearest 

 the headwaters of this stream. (See map, fig. 6, p. 21). They have 

 a northwest-southeast trend, the general course of the Yahara river 

 being southeast. Detailed descriptions of these lakes are given in 

 bulletins No. VIII (Second Edition) and No. XXVII of the Wisconsin 

 Survey and they need not be repeated here. 



The areas, volumes, and the maximum and mean depths of these 

 lakes are given in table 1 (p. 181). Lake Mendota is by far the largest 

 and the deepest member of the group. While the maximum depth of 

 Lake Monona is only about 12 per cent less than that of Lake Mendota, 

 the mean depth of the former is only two-thirds as great, thus showing 

 that the basin occupied by the former is relatively much shallower. 

 Lakes Waubesa and Kegonsa occupy very shallow basins ; their maxi- 

 mum depths are distinctly less than half that of Lake Mendota, while 

 their mean depths are but little more than a third as great. 



Thermal Stratification 



Lakes Mendota and Monona have sufficient depth to become thermally 

 stratified in summer ; that is, they become separated into three distinct 

 strata for a period of three to three and a half months each season, or 

 from late June or early July to late September or early October. The 

 upper stratum comprises the warm water and is known as the epilim- 

 nion. It is kept in circulation by the wind and is thus freely exposed 

 to the air so that its supply of dissolved oxygen may be replenished 

 should this gas fall below the saturation point at any time. This 

 stratum also receives by far the greater portion of the sun 's energy that 

 is delivered to the surface of the lake, and it is, therefore, the most 

 favorable region for chlorophyl-bearing organisms. These two factors, 

 an abundance of oxygen and light, make this the most favorable region 

 for the major portion of the net plankton, and it is here that the great 



