Page 133

 NUTRIENT SOURCES FOR LAKE MENDOTA—1 972 133 
William C. Sonzogni 
University Wisconsin— Madison 
and 
G. Fred Lee 
University Texas— Dallas 
INTRODUCTION 
 It has been several years since Lee et al. (1966) published an estimation
of nutrient loadings to Lake Mendota (the paper was revised in 1969, but
only with regard to. the figure for the annual nutrien't input from nitrogen
fixation). Since that time new data and information concerning certain nutrient
sources has become available. In addition, changes in the population and
land use of the watershed have occurred. In view of the recent diversion
of sewage effluent which normally entered tributaries to the lake, it is
imperative that the sources, amounts and types of nutrients be estimated
as well as possible, if the effect of this diversion is to be properly evaluated.
It is the purpose of this paper to update the estimation of the total amounts
of nutrients entering Lake Mendota as well as to provide some quantitative
information on the forms of nutrients annually entering the lake from different
sources. 
LAKE CHARACTERISTICS 
 Lake Mendota, perhaps the most studied lake in the world, is the largest
of the Madison lakes, which form a chain along the Yahara River. Formed as
a result of moranic damming during the most recent ice age, it is classified
as a hard-water, eutrophic lake according to most standards. The drainage
area of Lake Mendota is composed mostly of fertile farm land and urban area.
The hypohimnetic waters become devoid of oxygen during summer stratification
and during late winter oxygen depletion occurs in the bottom water. Excessive
weed growth and periodic algal blooms create offensive conditions during
the summer months. 
 The lake is located in south-central Wisconsin, which has a semi-humid climate.
The monthly average temperatures range between 19 F and 73 F (Dane County
Planning Commission, 1971). Precipitation averages about 30 inches each year,
while evaporation amounts to about 24 inches a year.