INTRODUCTION 



The following bulletin is based upon data obtained in an extensive 

 quantitative and chemical study of the plankton of four lakes situated 

 in the vicinity of Madison, Wisconsin. (See map, fig*. 6.) The in- 

 vestigation dealt chiefly with the plankton of Lake Mendota, while the 

 observations on the other three lakes were made for purposes of com- 

 parison. 



The investigation was undertaken for the purpose of securing definite 

 gravimetric data relating to the size of the plankton crop at different 

 seasons of the year and also for the purpose of obtaining some notion 

 of the food value of this plankton material. Such data were desired 

 not only for the larger organisms which can be readily secured with 

 a plankton net but also for the small organisms which easily escape 

 through the meshes of the net. These two groups of organisms con- 

 stitute the total plankton and it is very important to know just what 

 part of the total each group contributes ; some of the data presented in 

 this report show this relation very clearly. 



The annual cycle of physico-chemical changes which take place in 

 the waters of these lakes has a very important bearing upon the crop 

 of plankton. This cycle is separated into four phases which correspond 

 closely to the four seasons of the year. The disappearance of the ice 

 in the spring is followed by an overturning and complete circulation 

 of the water. The temperature of the water rises as the season ad- 

 vances and the vernal period of complete circulation is finally termin- 

 ated by the rapid warming of the upper water; that is, the upper 

 stratum becomes so much warmer, hence lighter, than the lower water 

 that the wind can no longer force the former down and mix it with the 

 latter. This results in a thermal stratification of the water; evidences 

 of this phenomenon appear in the latter part of May, but the strata 

 are not very clearly defined until late June or early July. 



Two of these lakes, namely Waubesa and Kegonsa, are so shallow 

 that the water is not permanently stratified during the summer. Dur- 

 ing the summer period of stratification in Lakes Mendota and Monona 

 there is a warm upper stratum, the epilimnion, which is kept in cir- 

 culation by the wind and a cool lower stratum, the hypolimnion; be- 

 tween these two there is a relatively thin stratum, the mesolimnion or 

 thermocline, in which the temperature of the water changes rapidly 

 from that of the warm epilimnion to that of the cool hypolimnion. As 

 the temperature of the epilimnion falls in the autumn the mesolimnion 

 descends to greater and greater depths and this finally results in the 



