334 



THE ALGAE 



exist alone, but where there is more shelter they are usually 

 associated with a succession of fucoid belts : 



(a) A belt of either Fucus spiralis or var. platy carpus. 



(b) A belt of Ascophyllum nodosum with one of F. vesiculosus im- 

 mediately below or vice versa with an intervening middle 

 belt where both species occur. 



(c) The lowest belt of the mid-littoral is commonly dominated 

 by F. serratus^ or by species of Laurencia and Rhodymenia 

 where there is shelter, or Gigartina or Chondrus crispus where 

 there is more exposure. 



One feature of the belts in this part of the world is the important 

 part played by the fucoids. A comparable covering of algae is com- 

 monly absent in the corresponding cold waters of the Pacific (see 

 p. 336). 



Where the substrate is suitable, the sub-littoral fringe is charac- 

 terized by species of Laminaria^ especially L. digitata. During the 

 last war these algae became of interest commercially (see p. 451), 

 and surveys of the Laminaria beds were made by the present writer 

 and more extensive ones later by members of the Scottish Seaweed 

 Research Association. It is evident that the relative proportions of 

 the different species vary with depth and locaHty and there may 

 also be considerable variations. In the sub-littoral when rock gives 

 way to stones and then shingle so the algal dominants change from 

 Laminaria digitata to L. saccharina to Halidrys and finally to Chorda 

 and small red and brown algae. 



On the rocky shores throughout the world communities of 



DIATOM COMMUNITIES— SOUTH ENGLAND 



ON CONCRETE ON CHALK 



Supra-littoral Achnanthes-Myxophyceae Amphora-Nitzschia 



Amphipleura rutilans 



