384 THE ALGAE 



capacity, and one must further suppose that attachment of the 

 zygote to the rock surface must take place rapidly in such an en- 

 vironment. This factor is absent on a salt marsh coast since salt 

 marshes and mangrove swamps only develop in protected areas. 

 On the other hand it can be operative on fresh-water lakes where 

 quite sizeable waves can be engendered. Such waves may determine 

 the type of lake edge algal community. 



(b) Wave action 



By this is meant the actual height of the waves and the degree of 

 splash and spray that results from them. It has already been noted 

 (see p. 332) that this is essentially a modifying factor, elevating the 

 zones, especially the upper ones, often to a considerable degree. 

 Persistent wave action on a lake shore may presumably also elevate 

 an algal zone though this appears to have been but little studied. 



(c) Tidal range 



This also is a modifying factor since the range of the tide is 

 primarily responsible for the actual width of the algal zones that 

 are to be observed. Even with a very small tidal range, e.g. Carib- 

 bean, it is possible to observe a distinct zonation of the organisms. 

 This factor also determines the extent to which rock pools may 

 exist, the greater the tidal range the greater the variety among the 

 rock pools. Up to the present, workers have generally paid insuffi- 

 cient attention to the different types of tidal variation found in 

 various parts of the world. These are of great importance (Doty, 

 1957) and may indeed alter the position of an organism on the sea 

 shore (see p. 330). 



(i) Tidal currents 



In certain places there may be very pronounced currents close 

 inshore where algae may grow, e.g. between Orkney Is. and the 

 north of Scotland, at the mouth of the Bay of Fimdy, etc. Such 

 currents affect the growth of the plants but they can also operate as 

 a presence or absence factor. Thus on the east coast of Scotland the 

 strength of the current determines whether Laminaria saccharina 

 or L. clous font is the dominant. The strength of the airrent flowing 

 up salt marsh and mangrove swamp creeks determines the degree 

 of erosion, and upon this rests the extent to which creek bank algal 

 communities can develop (see p. 350). 



