28 Twenty-sixth Annual Meeting 



does not change except in connection with the zone where the 

 warmth is \vorking its way down very slowly as the season ad- 

 vances through the late sunnner and early fall. During July 

 the zone of rapid descent of temperature — the bottom of the 

 upper and warm lake — lies from 25 to 30 feet below the sur- 

 face. In August the warm water may become as much as 40 

 or more feet thick and in late September the entire mass of water 

 becomes mingled and uniform in temperature. 



During the sunnner season, tlien, we have a warm lake on top 

 subjected to the action of the winds, and a cold lake on which 

 the wind has no influence. As a result we find that the bot- 

 tom of the lake during the hot months of tlie year and during 

 the months when most vegetation is found in all our lakes, is 

 entirely cut off from immediate access to the air, and, further- 

 more, everything that goes down tliere stays there. In Lake 

 Mendota the water down in the lake l^ecomes decidedly foul, 

 not as foul as in some lakes on record, l)ut as the minute plants 

 and animals of the upper waters die and sink, there results an 

 accumulation of decomposing matter in the lower water, and 

 the deeper or cold water becomes distinctly foul. It smells like 

 rotten eggs, to put it plainly, and it tastes like sulphur water, 

 evidently from compounds arising from the decomposition of 

 these small plants and animals. As a result of this accunudation 

 of decomposing matter the plants and animals of the lake which 

 are the ultimate food supply of the open water, are unable 

 to live in the lower water of the lake, and during the 

 months of July and August and the greater part of September 

 all of the i)lankton life of the lake is confined to the upper 

 water. You may say that 95 per cent, and more of the Crus- 

 tacea, and the proportion of plants would not be essentially 

 different from that, are found in the warm water above, and less 

 than S P^'i" "-^^^'it are found in the cold waterin the lower part 

 of the' lake. It makes no difference whether you go to the shal- 

 lower part of the lake or the deeper part. Where the lake is 

 say 85 feet in depth there may be 50 feet of this water with 

 practically nothing in it with the exception of a very few small 

 animals and many of these are in a weak and dying condition. 

 Apparently you get none of the smaller forms, except those that 

 have become weak or are dying or have got stuck in moulting 

 their shells, and in one manner or another become incapacitated 

 and sink down there toward the bottom. 



The bottom of the warm water forms the lower limit of the 

 plankton life and this life closely follows that limit as the warm 



