8 INTRODUCTION 



varying degrees of hardness are due to the combination of the carbon 

 dioxide with calcium and magnesium to form carbonates, the fixed 

 (CaC0 3 and MgC0 3 ) and the half -bound (CaC0 3 .H 2 C0 3 and 

 MgC0 3 .H 2 C0 3 ). In neutral and acid lakes the two types of carbon 

 dioxide are present in equal amounts but in alkaline lakes there is an 

 excess of fixed carbon dioxide. This is due to the ability of the chloro- 

 phyll bearing organisms to draw upon the half-bound carbon dioxide 

 as a source of supply after the disappearance of the free carbon diox- 

 ide. The amounts of magnesium and calcium vary from lake to lake 

 and there is an almost continuous series from those having the softest 

 waters to those having the hardest. They consider those lakes whose 

 average fixed carbon dioxide does not exceed 5 cc. per liter as soft 

 water lakes, while medium waters have 6-22 cc. of carbon dioxide per 

 liter and the hard waters from 23-50 cc. per liter. The carbon dioxide 

 content varies with the depth at which the sample is taken and also 

 with the season of the year; some of the medium class lakes, for ex- 

 ample, have a sufficiently low content at times to qualify as soft water 

 lakes. All of the soft and medium waters, with the exception of Devils 

 lake in Sauk county, are located in the northeastern and northwestern 

 lake areas and many of them have sufficient free carbon dioxide to 

 give an acid reaction with phenolphthalein. The hardwater lakes are 

 all in the southeastern area and the upper stratum in them is alkaline 

 to phenolphthalein for the greater part of the year. 



Aside from the question of the hardness of the water complete min- 

 eral analyses of a few lakes are also available. These results are em- 

 bodied in Table 1 and represent average conditions. They are taken 

 from the data given by Birge and Juday and their data show that 

 there is a seasonal variation and a vertical variation in the amounts of 

 certain elements present. These variations in vertical distribution are 

 well shown in their Fig. 7, p. 106 (I. c.) The lakes of southeastern 

 Wisconsin contain a rich flora of Chlorophyceae, Myxophyceae and 

 Phaeophyceae, both in quantity and quality. In the northern lake 

 areas the total volume of the plankton is, generally speaking, consider- 

 ably smaller and quite different in character. These lakes have a con- 

 siderable desmid flora in many instances, while with the exception of 

 one or two species desmids are not found in the hard waters of the 

 southeastern region. The volume of phytoplankton that can be pro- 

 duced in a lake is primarily dependent upon the available supply of 

 carbon dioxide for photosynthesis. In the northern lakes this supply 

 is chiefly the carbon dioxide dissolved in the water but in the southern 

 lakes there is in addition considerable quantities of half -bound carbon 

 dioxide which may be drawn upon after the free carbon dioxide is ex- 

 hausted. It is not surprising, therefore, that the southern lakes with 

 greater supplies of potential food materials have a greater production 



