ECOLOGY OF ROCKY COASTS 



335 



marine diatoms can be found. These, because of the taxonomic 

 problems involved, have been but little studied. A contribution to 

 this problem has been made by Aleem (1950) working at Brighton 

 and Swanage in South England and may be referred to here as 

 representing European conditions. As might be expected the di- 

 atom communities differ depending on the nature of the substrate. 

 There is also a difference depending on the actual bathymetric 

 level. This is illustrated on page 334 : 



The different diatom communities are not to be found all the 

 year round, but in general are at their best sometime between 

 January to March or in the autumn and winter. 



On the North Atlantic shores of North America belts are found 

 in which Ascophyllum and Fucus still occur but the algal flora is 

 much poorer than in Europe. Examples of the various belts are 

 given below : 



Although fucoids and laminarians do not play so important a 

 part in this region of the world, nevertheless other green and red 



algae replace them. 



The belts in the North Pacific are quite different from those in 

 the North Atlantic. The four lowest belts are dominated by large 

 kelps {Alaria, Egregia, Lessoniopsis, Laminaria, Nereocystis) and it 

 is only in the upper belts (Ralfsia-Prasiola, Endocladia-Gigartina, 

 Halosaccion) that marine invertebrates become conspicuous. On 

 the Japanese coasts the zonation is again quite different. There is 

 commonly an upper belt of the red alga Gloiopeltis, then a belt of 



