Scott: [Lakes of Tippecanoe Basin 



19 



dioxide as it sinks. In the cooler denser water of the thermocHne 

 the rate of sinking would be decreased. This may account for the 

 total disappearance of carbon dioxide at that level. It must be 

 remembered also that the decreased temperature would also 

 reduce the rate of the chemical reaction. 



At the west end of Plew lake there are considerable marl 

 deposits so that the conditions in it may have a similar explana- 

 tion. 



Plankton 



Quantitative plankton collections have been made in connec- 

 tion with each series of gas samples. As I have already mentioned, 

 Mr. G. N. Hoffer made series of bacterial determinations in 

 several of the lakes (see table p. 26). Some correlations between 

 the dissolved oxygen and carbonates, and the plankton are 

 apparent, but these relations are not constant or general. For 

 instance, in Pike lake the decrease in oxygen between 2 M. and 

 4 M. is accompanied by a reduction in Microcystis and Ceratium 

 but Melosira reaches its maximum at 4 M. and is three times as 

 numerous at 8 M., where there is no oxygen, as at the surface. 

 In Hammon (Big Barbee) lake no marked change accompanies 

 the sudden decrease in oxygen between 4 M. and 6 M. 



The bacterial counts indicate that there are always more 

 bacteria at the surface than at 2 meters. There is generally an 

 increase in or near the thermocline and often one at the bottom 

 altho there may be a marked decrease in the number of bacteria 

 at the bottom as in Chapman lake. 



It has been demonstrated that all of these lakes are hard 

 water lakes and have at all levels a surplus of carbon dioxide 

 available for photosynthesis. Notwithstanding this fact, it is 

 certain that some of our lakes produce much more plankton 

 than others. Yellow Creek lake is the poorest in plankton of 

 any so far examined. 



This variation in the amount of phytoplankton in lakes 

 of approximately equal hardness and the lack of close correlation 

 between the physical and biological elements of the lake complex 

 which we have considered indicate that there are other potent 

 factors influencing the life of a lake. The relation of the littoral 

 to the pelagic region of a lake should certainly be given more de- 

 tailed analysis than it has received in the past. For instance. 

 Yellow Creek lake has associated with its paucity of plankton 

 a very narrow littoral region (13 per cent of its area above 2 M.). 



