CLIMAX FRESH-WATER COMMUNITIES 305 



Gersbacher (1937) has found that artificial pools are populated 

 by species living in the natural pools of a stream that has been 

 dammed, all the usual dominants being present. The gizzard shad is 

 one of the early fishes to occur in increased numbers. Larvae of 

 Chironomus plumosus (bloodworms) appear within two or three 

 months; these are followed by the larvae of Corethra and Procladius 

 (midge) and after four or five years by Hexagenia nymphs. The ap- 

 pearance of these is correlated with large amounts of plankton and its 

 detritus. Some may await a certain accumulation of the latter in the 

 mud, and some the presence of other animals. Coincident with the 

 Hexagenia nymphs, sphaerids and mussels usually have appeared and 

 are nearly half-grown by the time the bottom communities have de- 

 veloped this far. (The development is illustrated in Fig. 71.) The 

 work of Behning (1928) indicates similar communities in the Volga, 

 but the taxonomic composition is different. 



Faciations of the Climax. The communities of medium and large 

 rivers differ somewhat from those of the small ones, which for con- 

 venience are considered as typical. There has been no extended study 

 of this type in rivers of various size, but a gradual change takes place 

 between essentially equivalent species, especially in regard to fish 

 dominants and bottom subdominants. 



DoMiN.\NT Fishes 



a. Occurring in dominant abundance only in medium and large rivers: 

 Polyodon spathula (Walb.) Spoon-bill cat 



Food, plankton; reaction, plows 



bottom 

 Scaphirhynchus platorhynchus Shovel-nosed sturgeon 



(Raf.) 

 Ictiohus urus (Ag.) Mongrel buffalo 



Ictiobiis bubalus (Raf.) Small-mouthed buffalo 



b. Increasing in abundance as the larger streams are approached: 

 (For equivalent species see list for small-river climaxes) 



Dorosom,a cepedianum, (Le S.) Gizzard shad 



Ictiobus urus (Ag.) Mongrel buffalo 



Ictiobus bubalus (Raf.) Small-mouthed buffalo 



Opladelus olivaris (Raf.) (60 lb.) Mud cat 



Ictalurus jurcatus (C. & V.) Blue cat 

 (150 lb.) 



Lake Climaxes 



These climaxes are found in the largest lakes, such as the Great 

 Lakes of North America, including Lake Superior, Michigan, Erie, 



