Chapter XIV 



FRESH-WATER ECOLOGY, 

 SOIL ALGAE 



One of the major problems in this branch of algal ecology appears 

 to be the estabHshment of a successful classification upon which 

 field studies can be based. Up to 193 1 the outline given by West in 

 1 91 6 was in current use, but since then a scheme proposed by 

 Fritsch (1931) has more or less taken its place. It would seem, how- 

 ever, that neither scheme alone is wholly satisfactory, but that a 

 combination of the two provides a very suitable basis for workers 

 in this field. An outline of such a combination of the two schemes 

 is briefly described below. 



A. Suhaerial communities 



These develop at their best in the tropics although they can also 

 be found in temperate regions. In the latter, Protococcales and 

 Trentepohlia form the principal elements, whilst in the tropics the 

 Myxophyceae and other members of the TrentepohHaceae (Cepha- 

 leuros, Phycopeltis, etc.) represent the dominant components. 



B. Communities of dripping rocks 



This can be subdivided into 

 (i) Permanently attached communities : 



(a) On Uving material. 



(b) On dead organic material. 



(c) On the hard rock (EpiHthic). 

 (ii) Temporarily attached communities : 



(a) On Uving material. 



(b) On dead organic material. 



(c) On the hard rock (Epilithic). 



C. Aquatic communities 



These vary from season to season and frequently have a marked 

 periodicity which is controlled by diverse factors. Four subdivi- 

 sions of the aquatic communities can be recognized (see below) and 

 each one of the subdivisions can be treated as follows : 



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