Barney and Anson. — The Top-Minnow 271 



By careful approximation and averaging of the volume of 

 the different organisms found in the stomachs and intestines 

 of 105 Gambusia collected at Mound, La., during the summer, 

 fall, and early winter of 1916, there were found the following: 

 Crustaceans, mostly entomostracans, 23.9% ; insects, mostly 

 dipterous larvse and pupae, 7.2%; rotifers and protozoa, 

 6.1%; algas, mostly blue-green filamentous, 47.7%; and un- 

 recognizable debris, 14.4%. These examinations were made 

 at the U. S. Biological Station, Fairport, Iowa, by H. Walton 

 Clark. 



Referring to the most important considerations of pond 

 culture outlined on a previous page, we may now well take up 

 the matter of food production in these ponds. Gambusia is 

 a plankton feeder. 



For this study, then, the production of plankton and the 

 factors that influence its abundance must be given especial 

 consideration. The extent of plankton production in fresh 

 water, as is the case in plant production on land, depends pri- 

 marily on the amount of nitrogenous material available for 

 the metabolic processes of the plankton organisms. Needham 

 and Lloyd* point out that: 



The supply of nitrogen for aquatic organisms is derived from the 

 soluble simple nitrates (KNO3, NaN03, etc.). Green plants feed on 

 these and build proteins out of them. And when the plants die, their 

 dissolution yields two sorts of products, ammonia and nitrates, that 

 become again available for plant food. 



Kofoid has indicated the effect of temperature on plank- 

 ton production in a planktograph in his work on the plankton 

 of the Illinois River.f In the four ponds herein considered, 

 abundant plankton production was guaranteed, before the vege- 

 tative features of the habitats were added, by the presence on 



*Needham, James G., and J. T. Lloyd: The life of inland waters. Ithaca, N. Y. 

 1916. P. 48. 



tKofoid, C. A.: The plankton of the Illinois River, 1894-1899, with introductory 

 notes upon the hydrography of the Illinois River and its basin. Part I. Quantitative 

 investigations and general results. Bulletin, Illinois State Laboratory of Natural 

 History, Vol. VI, Art. II, November, 1903, p. 626. PI. VIII. Champaign, 111. 



