Juday—Quantitative Studies of Fauna in Lake Mendota. 465 
annual range of temperature at this depth varies from a winter 
minimum of about 1°C., or slightly less, to a maximum of approxi¬ 
mately 14° in late summer or early autumn. In some years the 
summer temperature is somewhat less than 10°, which gives a 
fairly low mean annual temperature, but the animals living here 
do not seem to be affected unfavorably by it. At a depth of three 
meters, for example, the variation in temperature is much greater, 
ranging from a minimum only a little above freezing in early win¬ 
ter to a maximum of 25° or more in summer. 
This area lies well below the lower limit of wave action so that 
the bottom here is not disturbed by the waves. The deposition of 
material is relatively slow in this region also because, in general, 
only the finer silt and the smaller particles of the larger acquatic 
plants are transported so far from shore. 
At certain periods of the year there is no dissolved oxygen in 
the lower water of Lake Mendota, and these organisms, therefore, 
must be able to withstand such a condition in order to survive; in 
other words they must be facultative anaerobes. At depths of 20 
meters or more the dissolved oxygen usually disappears before the 
middle of July, and it is not found in this stratum again until 
early October, so that the summer anaerobic period covers an in¬ 
terval of about two and a half months. Also during the month of 
March in certain years some of the bottom water in the deeper 
strata possesses no dissolved oxygen, so that a second anaerobic 
period prevails for two or three weeks, perhaps longer, over more 
or less of the area within the 20-meter contour line. 
Limnodrilus and Tubifex 
These two forms were enumerated in 213 samples taken from the 
five regular stations located in the deepest water. In 68 of these 
samples the two were counted separately, and the ratio of Limnod¬ 
rilus to Tubifex in them was 4:1. No attempt was made to keep 
the two forms separate in the other catches. 
The results of the enumerations are summarized in table 1, which 
shows the number of individuals per square meter of bottom at the 
different stations. A distinctly larger number was found at the 
two stations situated in the western half of the deep-water area; 
that is, at those designated as West Bay and North of Second 
Point. The maximum, the minimum, and the mean numbers were 
larger at the former than at any other station. The difference be- 
30— S. A. L. 
