MOUNT DESERT ISLAND, MAINE 317 



(9) Rhodymenia community. Range +2-0 to —5-0 ft. m.l.w. 



(10) In some years a Spongomorpha arcta society can be found. 

 Range +2-0 to —5-0 ft. m.l.w. 



(11) A Spongomorpha spinescens society appears during the 

 summer months. Range +2-0 to —7-0 ft. m.l.w. 



Only one association is recorded from the subHttoral, but this 

 ought to be subdivided into three communities if it is to be com- 

 pared with British coasts. 



SubHttoral community 



(12) An Alaria-Halosaccion-Lithothamnion community in which 

 the red alga Halosaccion is most abundant on sloping rocks although 

 in some parts of the coast it is replaced by Chondrus. Range 



-6-0 to +2-0 ft. M.L.W. 



Local societies of Saccorhiza may occur between — i -o and 

 + 1-0 ft. M.L.W. The kelps and Halosaccion are usually so dense 

 that they prevent the downward migration of species from the 

 littoral zones above, although where there is any available space 

 such a migration will readily occur. This illustrates the effect of 

 competition in determining zonation. 



ZONATION IN WARM WATERS 



So far we have only described the vegetation of temperate and 

 cold waters. It is, however, very instructive to consider briefly the 

 algal ecology of warm waters and observe how it differs from that 

 of the colder waters. In the Mediterranean, for example, con- 

 siderably more attention has to be paid to the sublittoral region, 

 partly because of the extensive vegetation that persists in such a 

 place, and partly because the small tidal rise, 20-30 cm., renders 

 this region much more important. If an ecological survey were to 

 be carried out in the Caribbean a similar state of affairs would be 

 encountered. Here the very small tidal rise of less than a foot means 

 that there is practically no intertidal vegetation, and indeed, 

 zonation of algal belts is very rare although it may occasionally be 

 encountered on beach rock. The algal vegetation of the Caribbean 

 is almost wholly sublittoral, the associations being determined very 

 largely by the type of substrate. The extent to which the sublittoral 

 in the Mediterranean is of importance is illustrated in Table XV 



