Muttkowski—The Fauna of Lake Mendota. 
439 
The same storms bring about the death, no doubt, of many 
other aquatic animals. Dead crayfish, Hyalellae, Ephemerid 
larvae of various kinds, and other forms are a common sight 
along the shores after a vigorous storm, not only in the autumn, 
but also at other times of the year. But the autumn storms bring 
about a rapid stripping and cleansing of the entire littoral area, 
and by late November only a very short young growth is left in 
the different parts of the lake. 
The Winter Period—Period of Torpor, Suppressed Activities. 
There remains the winter life of the fauna. The removal of 
the greater part of the flora has destroyed the clinging surfaces 
for most of the species, and they are obliged to become bottom 
dv/ellers for the winter months. Plant growth goes on during 
the winter, even under the ice, but it is so slight that it cannot 
be regarded as a direct factor. 
In general, the winter period can be regarded as beginning 
at the time of the greatest density of the water,—at 4° C. This 
water sinks to the bottom, and hence there is the reversal of 
the summer condition,—namely, warm water below and cold 
v/ater above. The low^ered temperature inhibits the activities 
of many species and these may become more or less dormant. 
As a whole, physiological activities are lowered, and hence there 
is a corresponding lowering of food requirements. 
That there is no complete cessation is evidenced by the activity 
of fish, plankton Crustacea, Corethra, Ceratopogoninae, and 
other groups. There is, however, a general diminution or 
deferment of activities. Once the ice covers the lake, many 
species become entirely inactive. The amount of light pene¬ 
trating the ice depends on its quality—whether smooth, rough, 
or stratified,—and the amount of snow upon it. The problem 
of continued darkness makes the feeding of animals precarious 
and, in general, the discriminating feeders become dormant. 
For the surface-breathers of the lake shallows, ponds, streams, 
and other waters, a new era sets in,—the period of dormancy. 
For a while, indeed, many of the surface-breathers are very 
active under the ice (Wesenberg-Lund), hunting to and fro in 
quest of food. Their oxygen is obtained from the bubbles se¬ 
creted by the plants under the ice. Soon, however, the ice will 
be too thick and the light for photosynthesis will be insufficient, 
—hence no more secretion of oxygen,—^and the animals will dig 
into the mud and become dormant for the winter. 
