404 THE ALGAE 



algae. It is, however, a very complex factor, and much further work 

 is necessary before its operation is fully unravelled. 



C. Chemical factors 



(a) SaUnity 



This may operate as a presence or absence factor in places where 

 fresh water runs into the sea. In such places Rhodophyceae and 

 Phaeophyceae are replaced by Chlorophyceae. In estuaries Fucus 

 spiralis is replaced by F. ceranoides. There is also the problem of 

 sahnity changes due to loss of water when the algae are exposed. 

 This results in changes in osmotic pressure of the cell sap. Biebl 

 (1938) found that algae could be placed in three groups according 

 to their behaviour on exposure. 



(i) Deep growing algae which are never exposed to the air; 

 they are resistant to a concentration of 1-4 times that of sea 

 water, 

 (ii) Algae of low water mark and the lower littoral tide pools 

 which never become dry: these are resistant up to a con- 

 centration of 2-2 times that of sea water, 

 (iii) Algae of the littoral belt: these are often completely exposed 

 and they can resist a concentration of 3-0 times that of sea 

 water. 

 The influence of lowered sahnity upon metaboUc processes (see 

 Doty, 1957) requires further study, especially in view of Fritsch's 

 statement that the compensation point of marine algae is reached 

 at shallower depths with lowered sahnity. 



Sahnity changes can also occur in pools and salt pans, expecially 

 those at high levels during neap tidal periods when there may be 

 considerable evaporation. This factor can operate as a presence or 

 absence factor in high level pools. In pools, intertidal algae exhibit 

 a greater tolerance to changes in sahnity than do infra-httoral algae 

 (Table 20), and this may well explain why the latter are restricted 

 to pools in the lower half of the httoral zone. 



{b) Substrate 



Generally, differences in chemical composition have httle effect 

 on the flora. An exception to this is the presence of chalk, the algal 

 flora of chalk chffs (Anand, 1931) differing distinctly from that of 

 metamorphic and igneous rocks. This, however, is only a presence 

 or absence factor. 



