18 



Indiana University Studies 



In so far as the productivity of a lake depends on the materials 

 for photosynthesis^ the more carbonates present in the lake water, 

 the more productive will be the lake. This principle has been 

 established by studies on the lakes of Wisconsin. The soft water 

 lakes produce relatively little plankton while the hard water lakes 

 are very productive. 



The lakes of this basin, so far as they have been examined, 

 are all hard water lakes. In this respect they very closely resemble 

 the lakes of southern Wisconsin. 



Lakes that have a complete vernal circulation begin the 

 summer with aa equal vertical distribution of carbon dioxide 

 and carbonates. However, after the stratification, the carbon 

 dioxide is reduced in the epilimnion by photosynthesis and in- 

 creased in the hypolimnion by the oxidation of carbon compounds. 

 The reduction of the soluble bicarbonates of the epilimnion to 

 the less soluble normal carbonates by photosynthesis resultfe in 

 their precipitation from the epilimnion. In the hypolimnion 

 they pick up free carbon dioxide and again pass into solution as 

 bicarbonates. As a result of this process, the curve for the 

 vertical distribution of free CO2 and carbonates is sigmoid. 

 Most of our gas collections were taken in the afternoon and photo- 

 synthesis had usually been going on actively for several hours 

 just previous. This may in part account for the fact that the 

 upper levels in many of the lakes were alkaline. The lower levels 

 were all acid and usually held in solution rather large amounts 

 of carbonates. The maxima for different lakes ranged from 32 

 cc. per liter in Winona to 60 cc. per liter in Center lake. These 

 maxima, of course, vary with the season and the year. For de- 

 tails, see the appended tables. 



In two of these lakes the distribution of the free CO2 is different 

 from all of the others and different from any of the Wisconsin 

 lakes. The tables for Yellow Creek lake and Plew lake show that 

 free carbon dioxide decreases from the surface to 6 M. where the 

 water is neutral. From this point to the bottom it gradually 

 increases. This decrease in carbon dioxide is not associated 

 with an increase in oxygen as would be the case if it were due to 

 photosynthesis. In Yellow Creek lake the deepest point of the 

 lake is east and a little south of a ridge of marl that projects from 

 the northwest shore of the lake. Most of the water on this ridge 

 is less than a meter in depth. A high wind would stir up the marl 

 on this ridge and carry it over the lake. This marl is largely 

 calcium carbonate and consequently would pick up free carbon 



