i6] 



VEGETATIONAL TYPES OF SEAS 



525 



4. What may be termed dynamic factors tend to be most effective 

 about shores or in shallow waters and consequently to affect the 

 marine benthos quite markedly. Examples are turbulence, due to 

 waves and currents, and emersion, due to variations in water-level 

 particularly as induced by tides. The importance of wave action 

 has already been implied, and is illustrated by the often very different 

 algal flora and vegetation developed on exposed capes and in sheltered 

 bays. The effects are complex but appear to be primarily mechan- 

 ical, in that the fixation of spores or persistence of fragile Algae is 



Fig. 172. — Postelsia pahnaefoimis. Note the strong but flexible axes which are 

 resistant to surf. ( •: about \.) (Courtesy of the National Museum of Canada.) 



prevented on heavily battered rocks, whereas an absence of turbulence 

 in sheltered situations leads to the deposition of sediment which 

 constitutes an obstacle to the establishment of some Algae. Others, 

 however, are favoured by this muddy substratum ; and calm con- 

 ditions may allow a heating of the water which favours many Algae 

 but leads to the disappearance of some stenothermic species. Some 

 types, such as the Bladder Wrack, when growing in exposed situations 

 may show characteristic features enabling them to resist the tearing 

 action of the waves, while others, such as the surf-loving Postelsia 

 palmaeformis (Fig. 172) of the Pacific coast of North America, 



