CHAPTER XIII 



FRESH-WATER ECOLOGY 



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

 to be the establishment of a successful classification upon which 

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

 191 6 was in current use but since then a scheme proposed by 

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

 however, 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. Siibaerial association. 



This develops at its best in the tropics although it can also be 

 found in temperate regions. In the latter, Protococcales and 

 Trentepohlia form the principal elements, whilst in the tropics the 

 Cyanophyceae and other members of the Trentepohleaceae 

 {Cephaleuros, Phycopeltis, etc.) form the dominant components. 



B. Association of dripping rocks. 

 This can be subdivided into 



(i) Permanently attached communities : 



(a) On living material. 



(b) On dead organic material. 



(c) On the hard rock (Epilithic). 



(ii) Temporarily attached communities : 



(a) On living material. 



(b) On dead organic material. 



(c) On the hard rock (Epilithic). 



C. Aquatic associations. 



These vary from season to season and frequently have a marked 

 periodicity which is controlled by diverse factors. Four sub- 



