120 American Fisheries Society. 



pounds per acre. The larger planktonts, those caught by the 

 net, consitute only a small part of this total in most lakes and 

 at most seasons. In Lake Mendota the net plankton never quite 

 equalled that extracted by the centrifuge, and at the maximum 

 the centrifuge yielded nearly 25 times as much as the net. On 

 the average there was about five times as much plankton from 

 centrifuge as from net. Thus the more minute organisms, those 

 which have the shortest life and the most rapid reproduction, 

 constitute by far the larger part of the standing crop ; and this 

 is of great significance in estimating the annual turnover. In 

 other lakes the net has yielded an even smaller proportion of the 

 total plankton. In Lake George (164 feet deep) and in Green 

 Lake (237 feet deep) it has been found in mid-summer so low as 

 one-fortieth of the total. 



These studies on large quantities of water have been made 

 on Lake Mendota and the two neighboring lakes which can be 

 reached by a launch. These lakes range in maximum depth from 

 11 meters to 24 meters. During last year and the present season 

 we have been carrying on our studies with smaller and portable 

 apparatus in Green Lake which reaches a depth of 65 meters. 

 We find that in all of these lakes the quantity of plankton per 

 unit of area of the deeper water of the lake is not widely dif- 

 ferent, no matter what the depth of the water may be. Of course, 

 it varies a good deal with years and with season, but, after all, 

 in any of these lakes it is of the same order of magnitude, say 

 about two tons of live plankton per acre of deep water. It is 

 yet doubtful whether this will be found true of the far deeper 

 lakes like the Finger Lakes of New York, and I do not see but 

 that the very deep lakes ought to give a larger amount. But 

 still it seems a fair provisional conclusion from our studies, that 

 the fundamental capacity for production in a lake depends on 

 area and not on depth. While there are plants which might 

 and probably do multiply as saprophytes in the deeper water, 

 they do not seem to add appreciably to the crop of plankton and 

 the bacteria are a very insignificant part of the standing crop. 

 Other things being equal, the production of food in the open 

 water is a function of surface, not one of depth. 



This statement might seem to be a necessary result of the 

 fact that plants depend on sunlight for the manufacture of or- 

 ganic matter. But it is really unexpected and the reasons for 



