CLIMAX FRESH-WATER COMMUNITIES 299 



lar in taxonomic composition to those of streams of corresponding 

 depth. In the largest lakes the pelagic grouping constitutes a unit 

 with a considerable degree of independence and may have some or all 

 the properties of a major community. 



River Climaxes 



In middle North America these communities are usually composed 

 of plankton organisms, fishes nesting on the bottom and feeding 

 largely on detritus, mussels, sphaerids, mayfly nymphs of the genus 

 Hcxagenia, and the larvae of various midges (Chironomidae), etc. 

 Attached vegetation plays only a minor role because of its slight 

 abundance. This type of climax community occurs in the sluggish 

 pools of most small and large rivers, but in general it exists only as 

 local fragments where the current is slowed to a minimum. In most 

 streams, however, a fragment no sooner becomes established than 

 severe floods bury it or sweep it away, leaving only sterile silt. In 

 those that are at baselevel practically throughout their courses, so 

 that the load of silt is not large and the fall of the streambed so 

 small as to make the greater part of its course a pool in character, 

 the entire stream may have this community on its bottom. The Illi- 

 nois River, a tributary of the IMississippi, originally contained climax 

 communities over various portions of its course, and the same is true 

 of some of the streams affluent to the Great Lakes. The Mississippi 

 is at baselevel only in its lower course, and the development of ex- 

 tensive climax areas is prevented in a measure by the load of silt 

 from the small tributaries and headwater streams. High turbidity is 

 a characteristic feature of its aquatic climate, and there is also an 

 annual rhythm of high and low water. 



There is more or less difference between the communities of small 

 and large rivers, but since the former have been studied more, they 

 will be treated first. 



Small-river Climaxes. Gersbacher (1937) has investigated the de- 

 velopment of these communities as regards the bottom constituents, 

 with some attention to fishes, and Eddy (1934) has made a study of 

 their plankton. Thompson and Hunt (1930) have depicted the con- 

 ditions of the habitat and noted the dominant and less abundant 

 fishes, as well as their food and associates. This account of the fishes 

 is supported by the extensive work of Forbes (1883, a, c; 1888) , Forbes 

 and Richardson (1913, 1919), Richardson (1921-1929), and others. 

 Finally, the work of Cahn (1929) on the introduced European carp 

 as a dominant forms the background for this discussion, since this has 



