22 PLANKTON OF WISCONSIN LAKES 



majority of the organisms are found most abundantly. The epilimnion 

 in these two lakes varies in thickness from five meters to seven meters 

 when it is first formed, but it gradually increases in thickness as the 

 season advances, extending to a depth of 10 meters to 12 meters by 

 the middle of September. 



There is a bottom stratum of cool water which may be regarded as 

 stagnant during this period ; that is, it is not kept in circulation by the 

 wind and is not exposed to the air. Since light does not penetrate to 

 this stratum in sufficient amount to permit much activity on the part 

 of ehlorophyl-bearing organisms, it is cut off from the two sources of 

 oxygen supply, namely, the air and photosynthesis, and the quantity 

 of this gas is limited to the amount held in solution at the time that 

 stratification takes place. This supply of oxygen is drawn upon through 

 respiration of the organisms which occupy this region and through the 

 decomposition which takes place there, so that, by late July, substan- 

 tially the entire hypolimnion of these two lakes is devoid of free oxygen. 

 The dissolved gases bave been fully discussed in Bulletin No. XXII of 

 this Survey and the reader is referred to that publication for a more 

 detailed account. 



The absence of oxygen makes the hypolimnion uninhabitable for the 

 plankton Crustacea, and those which occupy this region when there is a 

 sufficient supply of this gas withdraw when the amount falls below the 

 minimum required by the various forms. But this stratum is by no 

 means entirely deserted during the period that it possesses no free 

 oxygen because various forms of protozoa and some insect larvae thrive 

 here even in the absence of oxygen. In fact, one ciliated protozoan 

 has been noted which appeared in this stratum only when the oxygen 

 was substantially or entirely absent, the largest numbers having been 

 found under complete anaerobic conditions. (Juday, Biol. Bui., Vol. 

 36, 1919, pp. 92-95.) 



Between the two strata mentioned above there is a definitely marked 

 transition zone in which the conditions change from those of the warm 

 water above to those of the cool water below. This is the mesolimnion 

 or thermocline and it is relatively thin, usually not exceeding three or 

 four meters in thickness and frequently not more than two meters. In 

 the mesolimnion the temperature of the water rapidly changes from 

 that of the warm water of the epilimnion above to that of the hypo- 

 limnion below and this decrease in temperature is accompanied by a 

 decrease in the quantity of dissolved oxygen from that of the well 

 aerated epilimnion to only a trace or none at all in the hypolimnion. 

 These changes do not make this stratum uninhabitable, however, be- 

 cause the mesolimnion is generally well populated by various forms, 

 some of which, in fact, are more abundant here than at any other depth. 



