126 American Fisheries Society. 



plankton constitutes about one-tenth. Probably more than half 

 of the net plankton consists of animals, chiefly Crustacea. 



(Dr. Birge had brought with him the portable centrifuge mentioned in 

 his address and at the close he extracted the plankton from a pint or more 

 of water from Lake Mendota. This water was perfectly clear to the eye, but 

 there collected in the bowl of the centrifuge a considerable quantity of minute 

 organisms which adhered to the wall of the bowl. These were chiefly algse 

 belonging to the genus Aphanocapsa, the dominant form in the centrifuge 

 plankton of most lakes). 



Discussion. 



Mr. J. W. TiTcoMB, Hartford, Conn.: May I ask Dr. Birge whether 

 he gets the same results in his winter observations? 



Dr. Birge: In general, yes, but you cannot make plankton observa- 

 tions in the winter with the same regularity as in the summer. During the 

 open season we expect to make two observations a week on Lake Mendota, 

 but during the winter may go out only four or five times altogether. In 

 one year the maximum amount of plankton was in December ; it ran down 

 during the winter, and then in March it began rising again slowly. It went 

 up very rapidly in April, then fell off in the summer and rose again in the 

 fall. The number of Crustacea in Lake Mendota, the lake which we know 

 most about, is greatest in April, May and early June. I have always had 

 the notion — I give it for what it is worth — that in the lakes these water 

 fleas "get the jump" on the fishes in the spring; they start out earlier than 

 the fish, and then the little fishes come on and run them down. That is my 

 thought so far as Lake Mendota is concerned, but the question would have 

 to be worked out on a good many lakes before we could speak very definitely. 

 I do not know that this sequence of forms has been worked out quan- 

 titatively in streams. 



Mr. TiTcoMB : You have in a scientific way explained why the shallow 

 lake is more productive than the very deep lake. But the point not quite 

 clear to me is about the relations of the higher forms of plants. The plank- 

 ton and the algae are intermixed, are they not? 



Dr. Birge : The algae are part of the plankton. 



Mr. Titcomb : What is the relation of the higher forms of plants to all 

 these valuable plants that we think so much of here? Do they properly 

 enter into the discussion? 



Dr. Birge : A few years ago we published a very elaborate paper on 

 the shore plants, the insects, etc., that live in Lake Mendota. So we have 

 a report on that subject, but we are still very far from knowing much 

 about it. We have just completed also reports on water weeds — showing 

 the amount of these plants per acre in Lake Mendota ; we also worked them 

 up in Green Lake, but the report is not yet published. It can be said that 

 in the most productive parts of the edge of the lake about as much green 

 stuff will grow per acre as will grow upon a meadow. 



The only connection I brought in between these plants and the animals 

 of the open water is the fact that as these plants die parts of them break 



