for most of the larger fish cannot or do not feed directly upon the 

 microfauna and flora, ( See Fig. 3. ) 



In addition to serving as food for the microfauna, either directly or 

 indirectly, phytoplankton makes up a large part of the food con- 

 sumed by several species of fish such as the gizzard shad, young 

 suckers, black bass fingerlings, and certain mollusks. Recently 

 I examined the alimentary tracts of several dozen snapping turtles 

 and found that in nearly every instance they were heavily packed 

 with algae and other plant fragments, to the exclusion of almost all 

 other types of food. Spirogyra crassa, Cladophora sp., and Cerato- 

 phyllum demersum L. were the principle plants eaten. Lagler 

 ( 1940 ) has published on the food of the snapping turtle and points 

 out that 36.2 per cent of the ingested material consisted of algae. He 

 found that 70 per cent of all turtle specimens examined had con- 

 sumed plant food. (See Fig. 9.) This is of interest because the 

 snapping turtle had been considered almost entirely carnivorous. 

 (See Lagler 1943.) 



FACTORS THAT DETERMINE THE 

 CHARACTER OF LAKE FLORAS 



A SUMMARY 



1. The geological history of the region and the nature of the soil 

 over which the lake lies or which is drained by inlets. 



2. The depth of the lake and the shape of the bottom — V-shaped or 

 U-shaped; presence or absence of shoals, shallow bays, etc.; extent 

 of the epilimnion, location of the thermocline; completeness of the 

 seasonal overturn. 



3. Latitude; altitude (temperature and temperature range). 



4. Relative amounts of oxygen and available carbon dioxide. 



5. Nutrients in solution; salts; conductivity; pH; nitrogen content; 

 phosphorus. 



6. Nature of the bottom; bacterial flora; rate of overturn of or- 

 ganic matter. 



7. Biological enemies and competitors; parasites. 



8. Chance distribution by agencies such as wind, waterfowl; isola- 

 tion by barriers from other bodies of water. 



9. Light; turbidity; color of the water; rate of diffusion of rays in- 

 volved in photosynthesis. 



Because of the interrelationships and the complete interdepen- 

 dence among some of the factors that determine the lake flora, it is 

 obviously impossible to select any one as of paramount importance 

 in a cause-effect analysis. Major interest here, however, is directed 



[48] 



