Birge. — Plankton of the Lakes. 123 



tion, as yet not exactly determined. The crop doubtless falls as 

 low as 10 pounds per acre at some seasons and rises in spring as 

 high as 50 pounds or even higher. 



In other lakes we have found the crop as small as 7 pounds 

 of dry matter per acre in Canandaigua Lake (150 ft. deep) ; 

 about 10 pounds in Cayuga Lake (450 ft.) ; 24 pounds in Seneca 

 Lake (600 ft.) ; and nearly 30 pounds in Green Lake (237 ft.) 

 These were single observations and I have no doubt that the 

 amount found in any one of these lakes could have been found 

 in any other of them on a different occasion. 



Such a crop of, say, 200 pounds of live Crustacea per acre, 

 seems small, but even so the annual production is great. In Lake 

 Mendota Daphnia produces some three broods per month during 

 the spring. If we estimated the turnover at only twice a month 

 (and such an estimate is doubtless too low) during the period 

 from May 15 to September 15, there would be eight crops during 

 the period and a production of 1,600 pounds of animal food per 

 acre. This is, of course, far in excess of the production of animal 

 food from an acre of land, and the period of production includes 

 spring and fall also, and even winter for some forms. 



Thus if we look at the lake as an enterprise for converting 

 algcC into potential food for fish we must agree that it is by no 

 means inefficient. Little as we know of the details of the pro- 

 cesses, the gross results as we see them today are very creditable. 

 How far the fish are able to utilize this potential food is quite 

 another question, and one of whose answer we are quite ignorant. 

 So far as I am aware, no one has made a study of the subject. 



Here, then, are a few words on one great branch of the story 

 of the plankton of lakes so far as it directly interests us of the 

 American Fisheries Society. We must think of a plant popu- 

 lation numbering millions and often billions of individuals in a 

 cubic meter of water — most of them so small that they add no 

 observable turbidity to the water as seen in a glass vessel, yet 

 present in such numbers as to yield a standing crop of nearly 

 two tons per acre in a moderately deep lake. This standing crop 

 is constantly renewed as its shortlived members reproduce and 

 die. It supplies food which maintains a standing crop of animal 

 life in its higher forms, though still minute, which amounts to 

 200 pounds per acre, more or less; or, say, perhaps one-twentieth 

 of the weight of the plants on which it feeds. And out of this 



