CLIMAX FRESH-WATER COMMUNITIES 307 



shifting sandy shores and toward deeper water. A similar assemblage 

 occurs in Lakes Erie, Ontario, and Nipigon, but with nymphs of 

 Hexagenia added (Adamstone, 1924; Adamstone and Harkness, 1923). 

 Influents. In the pelagic layer which includes plankton, the fol- 

 lowing fishes are important because of their destruction of other fishes 

 and invertebrates: 



Lota maculosa (Le S.) Burbot or ling 



Cristivomcr namaycush (Walb.) Great Lake trout 



Leucichthys artedi (Le S.) Cisco 



The waters of the Great Lakes support a luxuriant plankton. Eddy 

 (1927) has recently studied that of Lake Michigan and found a num- 

 ber of the important species to be the same as in mature rivers. The 

 large lakes contain several species of fishes, such as the whitefish, 

 which are pelagic in their young stages but bottom feeders as adults. 



Dominance of Lake Fishes. The phenomena are similar to those of 

 a river. Evidence of the dominant character of the sucker is given 

 by Ricker (1932), wlio stated that this species {Catostomus commer- 

 soni) reduced and removed Chara beds from a certain Ontario pond. 

 The number of trout were diminished in the pond by the destruction 

 of the Chara, which supported trout food (see page 301). Chara has 

 some tendency to grow locally in depressions at less than 25 meters 

 in Lake Michigan (Ward, 1897). Its extensive growth would elim- 

 inate the sphaerid community, destroy much of the fish-breeding 

 grounds, and thus change the character of the community (see also 

 Baker, 1916, 1918, 1922, and 1928). 



Stream Habitats and Their Communities 



Rapids usually alternate with pools and may be intermittent, espe- 

 cially in the earlier stages and in climates with dry seasons. "Wherever 

 the habitat is stable for a considerable period, definite communities 

 tend to develop. As a stream ages, a series of communities pass a 

 given point, much as a series of terrestrial biomes passed a point near 

 the Ohio River with the retreat of the icesheet. In the case of the 

 stream, invasion and succession take place, but the larger phenomenon 

 is the migration of a series of communities over the same ground. In 

 the process described as physiographic succession, the habitats are fre- 

 quently denuded by flood before the baselevel is reached (Shclford, 

 1911, a-e). In each one of these denuded stages, a community de- 

 velops to a point that appears to have some permanency, only to be 

 swept away again (cf. Moffett, 1936). 



