ECOLOGY OF SALT MARSHES 353 



(a) Species that are littoral on a rocky coast are to be found grow- 

 ing at lower levels, usually sublittoral, on the marsh coast. This 

 must be ascribed to the lack of a soHd substrate at the higher 

 levels where they would normally grow. 



(b) Littoral species of the rocky coast are found growing at higher 

 levels on the marsh coast. This can be understood in the case of 

 those species living in pans or in the streams where they are 

 continually covered by water, and in the case of a few species 

 actually growing on the marshes it is probable that they are en- 

 abled so to do because the phanerogams provide protection 

 from desiccation. On a rocky coast such protection would be 

 lacking at high levels. 



Turning now to the algal vegetation of the marshes proper, 

 Carter (1932, 1933) has suggested that on the Canvey and Dovey 

 marshes light and space relations, rather than factors relating to 

 level, influence the distribution of the various species. Whilst this 

 is imdoubtedly true there is no doubt that the increasing height of a 

 marsh with its consequent greater exposure does nevertheless 

 effectively determine the upper height to which many plants can 

 go. The species to be found on the higher marshes in Norfolk are 

 either fucoids or gelatinous Myxophyceae, both of which have the 

 power of retaining moisture. The more deHcate Chlorophyceae are 

 more or less confined to the lower levels. On the other hand a dense 

 phanerogamic vegetation, such as one finds on the south coast 

 marshes where the tall Spartina townsendii must lower the Hght 

 intensity considerably, does reduce the quantity of algal vegetation. 

 A similar state of affairs has been observed on the grass-covered 

 marshes of New England. 



From data available it is possible to compare the distribution in 

 space (e.g. among the different phanerogamic communities) and 

 time of the marsh communities recorded from Canvey, Dovey and 

 Norfolk. Figs. 195 and 196 show the distribution of the Canvey and 

 Dovey communities and they should be compared with Figs. 197 

 and 198 for similar marsh communities of Norfolk. Some of the 

 smaller communities, e.g. Rivularia-Phaeococcus and Gelatinous 

 Cyanophyceae^, are apt to be overlooked in summer because the 

 constituent species shrivel up so much or else because the colonies 

 become covered by an efflorescence of salt. An examination of the 

 distribution of the various communities on the Norfolk marshes 



^ Myxophyceae is probably preferable: see p. 278. 



