Muttkowski—The Fauna of Lake Mendota. 
435 
II. THE SEASONAL CYCLE. 
Probably the most characteristic feature of the annual cycle 
is the brief ascendancy of plant life during the sumiher months, 
and the synchronous maxima of many forms of animal life. So 
closely are these two connected, so interdependent, that the plant 
ascendancy may be assigned as the direct cause of the faunal 
ascendancy. The seasonal succession is not continuous, but 
really composed of a series of maxima. 
Vernal Succession—Period of Elimination and Transforma¬ 
tion. —After the ice has gone out, a very small number of plants 
will have survived; most of them are but a few inches in length. 
G-rowth is rapid, and by middle June most of them are able to 
send their floral heads to the water surface. But in the mean¬ 
time the filamentous algae of the lake, especially Cladophora 
and various Spirogyra, have increased to a prodigious extent. 
If molar agents have not been too severe, the great waving 
plumes of Cladophora will form a miniature wilderness along 
the shores, while Spirogyra, Anabaena, Hydrodictyon and other 
algae wdll cover the plants and the bottom of the vegetation zone 
with a green fllm, and also form immense mats of scum in the 
shallow cut-offs. Among higher plants Castalia, Nymphaea, 
Lemna, and Myriophyllum, dominate behind the bars; in the lake 
Myriophyllum, Ceratophyllum, and Potamogeton species, 
though short, send forth long spiral stems with floral heads 
protruding above the surface. 
This period lasts till the water temperature rises above 16° C., 
after which Cladophora decays very rapidly and disappears; 
by the time a temperature of 18° is reached only the younger 
and more resistant growth will be found. 
The animal life of this algal period is characterized chiefly 
by a series of periodic eliminations rather than by any distinct 
dominance. In April large series of the less common Chirono- 
mid species leave the lake, including Cricotopus exilis, Chirono- 
mus modestus, and a varying number of C. tentans and C. 
plumosus. Early in May the annual shore migration of Sialis 
larvae takes place; in certain parts of the shore armies composed 
of many thousands may be observed climbing up some small 
drain or ravine. Two weeks later the adults may be seen laying 
their eggs everywhere on branches overhanging the water ’s edge. 
