TEMPORARY POND COMMUNITIES 173 



vegetation above the water including the birds are about the same as 

 in the preceding sub-formations. 



Permanent and temporary swamps are covered with trees. The 

 most important permanent swamps are the tamarack swamps. The 

 aquatic phase of these will be discussed in connection with the tamarack 

 swamp itself (p. 193). Temporary swamps will be discussed under the 

 head of temporary forest ponds. 



2. TEMPORARY POND OR TEMPORARY SWAMP AND MARSH 

 FORMATIONS 



The situations known as temporary ponds, temporary marshes or 

 swamps, or summer dry ponds, are common about Chicago and usually 

 contain water in early spring, drying before the first of June. At some 

 points at the south end of Lake Michigan much sand has been removed 

 for commercial purposes and frequently the workmen remove it to points 

 below the ground-water level of the spring months and accordingly make 

 temporary ponds which have pure white sand bottoms. A few of these 

 have been studied, one when it was one year old, another when about 

 twelve years old. These were compared with ponds of the horizontal 

 series which are much older. 



a) Bare-bottom association. — Twelve-months-old pond association 

 (Station 40; Table XXXVII): In April, 1910, we found this full of 

 filamentous algae, and containing rotifers, copepods, and ostracods, the 

 eggs of all of which will probably withstand drying and may have 

 blown into the pond during the preceding dry seasons. There was a 

 single full-grown snail (Physa gyrina), a small individual (probably 

 Physa heterostropha), and a small long snail, Lymnaea (probably exigua). 

 These snails may have been carried into the pond, from other ponds a 

 few rods away, on the feet of turtles or frogs. 



Twelve-year-old pond association (Station^; Table XXXVII) : As 

 such a pond as we have just described grows older, the algae continue 

 and the reed {J uncus balticus) comes in, together with some sedgelike 

 plants. In such ponds the number of species is usually greater than 

 at an earlier period. 



In addition to the species found in the twelve-months pond, we 

 obtained water-beetles, which are, however, not particularly signifi- 

 cant because they may occur in rain pools. Cladocera, the flat snails 

 (Planorbis sp.), and the nymphs of damsel-flies and dragon-flies are 

 also found. The difference between this pond and the preceding one 

 is not great. Indeed, it is only when the bottom of the pond becomes 



