298 AQUATIC CLIMAX COMMUNITIES 



water. Krogh (1934) has made the suggestion that probably only 

 the very smallest organisms can utilize the dissolved matter. The 

 majority of microscopic animals feed on microscopic plants or minute 

 particles of detritus. The great mass of medium-sized animals live 

 on smaller plants and animals or detritus, while the large motile forms 

 are chiefly carnivorous. Brooks (1893) had earlier called attention to 

 these facts as regards the sea. The importance of copepods as the 

 food of young fishes has been emphasized by Forbes (1880) and Le- 

 bour (1919, a,b, 1920, 1921, 1923, b). 



Another peculiar characteristic of the constituent species of aquatic 

 communities is the short developmental period and short span of life. 

 One to 7 years will cover the life span of the majority of aquatic non- 

 colonial animals. The rapid overturn makes annuation and aspec- 

 tion phenomena stand out sharply (Jensen, 1919; Blegvad, 1925). In 

 general terms, many constituents of marine communities have a span 

 of life of 2-3 years in contrast to 25-50 years for climax grasses and 

 300-500 years or more for forest trees. 



CLIMAX FRESH-WATER COMMUNITIES 



Introduction. The only fresh-water communities possessing the 

 properties of land climaxes to a noteworthy degree are discussed here. 

 These properties are: 



a. The organisms of the community exercise a considerable de- 

 gree of control over the habitat. 



b. The controlling reactions and coactions tend to maintain 

 habitat and community in the climax condition. 



c. The climaxes exist under stable physiographic conditions. 



d. They undergo development on denuded areas, and during 

 this development a series of changes take place both in the habitat 

 and in the composition of the community. 



Climax communities obviously exist in permanent streams under 

 conditions of stable bottoms composed of mud, silt, or very fine sand, 

 accompanied by usually gentle currents, but high water and high tur- 

 bidity in certain seasons. The last condition is productive of an 

 abundant plankton (Eddy, 1934), which further reacts upon the bot- 

 tom and provides one of the conditions of the climax and of its main- 

 tenance. In streams and possibly large lakes, this plankton must be 

 regarded as a layer, because there are no organisms having the prop- 

 erties of dominants which occur exclusively in the pelagic level. 



The connnunities of large lakes with terrigenous bottom are simi- 



