In general, the lakes in the southern sections have the physical- 

 chemical qualities which permit the production of a bulk of vegeta- 

 tion of all kinds greater than that of lakes in the central and northern 

 regions. Hence the ultimate production of animal life is doubtless 

 greater on the whole in the southern lakes, although there are few 

 figures to support this. Crystal and Weber Lakes in Vilas County, 

 and Clear Lake in Oneida County, Wisconsin, are notable examples 

 of soft water lakes which are low in vegetation and are correspond- 

 ingly poor producers of fish. 



Experimental studies have been made, and others are still in pro- 

 gress, on the effect on ultimate faunal production of increasing the 

 plankton by the addition of fertilizers. It has long been considered 

 good practice in Europe to improve fish production in nursery ponds 

 by this means, but few scientific evaluations have been made in this 

 country. Wiebe, Radcliffe, and Ward ( 1929 ) report that in ponds to 

 which fertilizers had been added, especially superphosphate, the 

 microfauna and algae showed a great increase in numbers over con- 

 ti-ol ponds. (See also Ball, 1949.) 



Although the total quantity of plankton in a lake may be large, it 

 is significant that the kind or quality of the plankters may be very 

 important in regulating production of fish. There may be sufficient 

 phytoplankton to support a rich microfauna on which fish feed. But 

 if there is a paucity of intermediate feeders, such as minnows or 

 other small predators, the food chain will thereby be interrupted. 



Figure 9. Diagrams illustrating the percentages of various types of food 

 used by two species of turtles. Left: Percentages by volume of various foods 

 consumed by western painted turtle (Chrysemys picta marginata). Right: 

 Percentages by volume of various foods consumed by the snapping turtle 

 (Chelydra serpentina). (Reproduced courtesy of Karl F. Lagler. ) 



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