SWIFT STREAMS 93 



3. SPRING BROOK COMMUNITIES 



(Stations 10 and 11; Table XIX) 



In glaciated areas many of the streams are fed by springs which 

 have not been produced by erosion, but are the result of porous and 

 impervious layers of till arranged as in regions possessing artesian wells. 

 The presence or absence and numbers of animals in a spring depend 

 largely upon the chemical content of its water. Spring waters commonly 

 have insufficient oxygen to support animals and at the same time may 

 contain sufficient nitrogen and carbon dioxide to be detrimental if not 

 fatal to animals. The mineral matter in solution may be large in 

 quantity and in some cases poisonous also. As the water flows away 

 from the spring it becomes aerated and diluted with surface water so 

 that the animals of the spring brook can live in it. Spring consocies differ 

 in different springs because of variations in the character of the water. 



In an area where there are springs, they are usually numerous. 

 The little brooks unite to form larger streams. Typically, such streams 

 may not be larger than intermittent streams, but a nearly constant 

 flow at all times of the year is one of the characteristic conditions. 

 Pools and riffles are not so well defined, but contain some small fishes. 

 The watercress grows abundantly at the sides of the stream and affords 

 a lodging-place for aquatic animals not furnished so abundantly by 

 young streams of other types. The water is colder in summer and 

 warmer in winter than in other streams. 



Spring brook associations. — Among the watercress are the amphipods 

 (Gammarus fasciatus), the larvae of Simulium attached to the leaves, 

 beetles, dragon-fly nymphs, and young crayfishes. Here are also found 

 occasional snails (Physa gyrina). The species of the cress association 

 are nearly all found under stones or on stones in the riffles. On the 

 stones are Simulium larvae and Hydropsyche (95), the net-building 

 caddis-worm (Figs. 39, 40, p. 96). Under the stones are the nymphs of 

 the May-fly (Baetis and Heptagenia), the larvae of flies and midges 

 (Chironomus, Dixa, and T any pus), the brook beetles (Elmis fastiditus) 

 (Fig. 47, p. 98), and occasional amphipods and crayfishes. 



4. THE SWIET- STREAM COMMUNITIES 



As the spring brooks and the intermittent streams continue to 

 erode their beds, they increase the extent of their drainage systems and 

 become larger streams. Springs tend to disappear in connection with 

 the spring brook and the intermittent stream reaches the ground- water 

 level and becomes permanent. The two sets of conditions converge 



