FRESH WATER ECOLOGY; SOIL ALGAE 361 



nants. In the summer Chlorophyceae and desmids predominate 

 and in the winter Ulothrix and Hormidium reappear. The effect of 

 bank vegetation (overhanging trees, reeds, etc.) may be profound, 

 especially in the degree of light reduction that it causes. 



The most important controlling factor is apparently temperature 

 but the chemistry of the water may also be significant. The absence 

 or near absence of calcium, for example, may induce a myxo- 

 phycean-rich flora. Light intensity variations and oxygen concen- 

 tration changes may bring about local modifications of the water. 

 In respect of pollution^ it has been pointed out that algal species are 

 not necessarily unerring indicator organisms. 



It is possible to classify the communities of rivers and streams in 

 the following way : 



(a) Communities of vertical substrates : 



(i) Those attached to stones, mud banks or roots. 



(2) Epiphytic communities on phanerogams. 



(3) Spray communities. 



{h) Communities of horizontal substrates : 



(i) Those attached to stones, sand or embedded in mud. 

 (2) Epiphytic communities. 



(c) Free living communities. 



Certain algae appear to demand specific habitats, some of them 

 unusual. Among such algae are species of Basicladia that grow on 

 turtles, and species of Char actum and Characiopsis that grow on the 

 Crustacean Branchipus. There is still considerable scope for further 

 work on river ecology, not only descriptive, but also studies that, 

 for example, could link algal vertical distribution with depth of 

 water. 



2. Associations of bogs and swamps 



These are very mixed associations with Httle or no periodicity, 

 probably because of the relatively uniform conditions. Zygnema- 

 ceae, desmids and diatoms are most frequent, the desmid element 

 changing considerably with altitude and type of substrate, whilst 

 the presence of Utricularia apparendy also increases the number 

 and variety of the desmid species. 



1 In the SaUne river of Michigan the entry of certain metallic ions, as a result 

 of poUution, causes the normal flora to be replaced by Stigeoclomum tenue and 

 Tetraspora (Blum, 1957)- 



2 A 



