124 American Fisheries Society. 



animal life there may come an annual supply of potential food 

 for fish which we can not estimate at less than 1,500 pounds per 

 acre and which is probably much larger. How far the fish use 

 this supply is a question to be determined by you who raise them. 



I mentioned the insect larvae, and while these do not belong 

 to the plankton I must say something about them, or at least 

 about those insect larvae which live on the bottom of the open 

 lake in the deeper water. You are all familiar with these larvae, 

 may-flies, bloodworms (Chironomus), phantom larvae (Corethra) 

 and many others. These constitute a second and smaller stream 

 of animal life that issues from the vegetable plankton, aided by 

 that part of the shore plants which sinks to the bottom of the 

 deep water. For as the planktonts die they gradually sink in a 

 sort of gentle rain to the bottom and there they accumulate in 

 an ooze which supplies nutrition to these larvae and as well to 

 worms, to clams, and to Crustacea. All of these animals furnish 

 food to bottom feeding fishes and the quantity of animal matter 

 thus maintained is often very considerable. In the depths of 

 Green Lake there lives a crustacean — Pontoporeia — a relative of 

 the scud or "shrimp" familiar in every fish hatchery pond. We 

 have found this animal in quantities as large as 74 pounds dry 

 weight per acre, over large areas of the bottom. This is much 

 larger than the standing crop of the plankton Crustacea, but re- 

 production is much slower and the annual crop is therefore 

 smaller. Lake Mendota has no Pontoporeia but has almost in- 

 numerable insect larvae, chiefly midge larvae — Chironomus — and 

 especially Corethra. There are also many worms and small 

 clams (Pisidium). The total annual crop from all of these may 

 aggregate some 112 pounds of dry organic matter per acre in 

 the deep water, an amount which is much less than the annual 

 crop from the open water itself. 



In order to give you some visible idea of the amount and dis- 

 tribution of the plankton I have made a diagram showing a 

 set of observations made this summer on Green Lake, Wisconsin. 

 On the right side you will see the temperature indicated. There 

 are some ten meters of warm water on top ; then comes the ther- 

 mocline, five meters thick, in which the temperature falls from 

 21° C. to 9° C. (70° to 48° F.). Then follows a very slow fall of 

 temperature through the cold water to the bottom, reaching 

 4.7° C. (about 40° F.) at a depth of 65 m. (about 215 ft.). The 



