Birge. — -Plankton of the Lakes. 121 



it are still to seek. For there is a far greater amount of organic 

 substance dissolved in the water of a lake than is contained in 

 the living organisms. There may be four or five times as much, 

 and the sum may rise as high as ten times. We do not know 

 why plants that are not green and can not manufacture organic 

 substances with the aid of the sun, do not utilize this material 

 and develop in large numbers. This indeed they sometimes do 

 and growths of plants like Oscillatoria may occasionally be found 

 in the deeper water of lakes, evidently supported by these dis- 

 solved substances. But such growths are rare and so far as our 

 experience goes they contribute little to the general food supply. 



The same may be said of bacteria. These are always 

 present in the water and often in large numbers. Our studies 

 show that there was an average of about 30,000 bacteria per cubic 

 centimeter of water during the summer of 1920, while the aver- 

 age for 1919 was only about 3,000. But the quantity of organic 

 matter yielded by these numbers is small. Even 30,000 bacteria 

 per cubic centimeter at the average size of those in Lake Mendota 

 'would weigh less than one-six hundredth part of the weight of 

 the other plankton organisms, and the weight of bacteria in 

 1919 averaged only one-tenth as much as in 1920. We are still 

 quite ignorant of the agencies that limit the number of bacteria. 



Thus the fundamental food supply of the open water comes 

 back to the algae and to the creatures that feed upon them. And 

 we must conclude that the total amount of this supply of food 

 is mainly a function of the surface of the lake and not of its 

 depth. Depth dilutes the food supply but does not add to it, 

 and if this is true, then the total amount of living material in 

 the form of fish that can be supported by an acre of open water 

 is rather decreased than increased by addition to the depth of 

 water. 



In this assemblage of plankton plants and animals one group 

 is of especial interest to us — that of the entomostraca, or water- 

 fleas — which convert the algae into a form available as food for 

 fish. These little Crustacea — Cyclops, Diaptomus, Daphnia, and 

 their relatives — constitute one of the most important sources 

 of fish food. 



There is a second similar group in the plankton, that of the 

 rotifers. But these "wheel animalcules" are small individually 

 and they are rarely present in numbers sufficient to make a sub- 



