342 THE ALGAE 



In the belts and zones that have been described so far, only what 

 may be termed the normal has been considered. Variations in any 

 region may occur as a result of changes in protection and exposure, 

 e.g. in Europe Ascophyllum disappears in more exposed places and 

 its niche is taken by Himanthalia. In New Zealand and Tasmania, 

 Hormosira, Ecklonia^ Cystophora, Carpophyllum, disappear on ex- 

 posed coasts and are replaced by Pachymenia and Durvillea or 

 Sarcophycus. Normally substrate does not have a very profound 

 effect on the main belts but chalk or limestone rocks do produce a 

 profound change in the flora. This is very strikingly illustrated by 

 the zonation on the chalk cliffs of Dover. Fresh water, either in the 

 form of substantial streams or rivulets induces a change in the 

 flora. In the case of rivulets a dense growth of Ulva and species of 

 Enteromorpha marks their course through nearly all the normal 

 belts on the shore. In the case of streams the dominants of the belts 

 may change, i.e. in Europe Fucus ceranoides replaces F. spiralis in 

 such places. 



It should be evident that any really adequate study of marine 

 coastal zonation must involve an appreciation of the part played 

 not only by the algae but also by the animals. The ecologist must 

 indeed be a biologist and the communities he recognizes must be 

 biotic communities in the strictest sense of the term. The other 

 alternative is for teams of botanists and zoologists to work together. 



Phytoplankton 



Apart from the attached fauna and flora to be found on the lit- 

 toral shores there is a vast fauna and flora floating primarily in the 

 surface waters of the earth. This is composed of phyto- and zoo- 

 plankton, and is the principal food offish. The phytoplankton con- 

 sists primarily of diatoms and dinoflagellates, and the numbers are 

 so enormous that it is in the waters of the globe that the maximum 

 carbon fixation takes place. The extent of the crop depends prim- 

 arily on five factors: {a) rate of reproduction, {h) rate of removal of 

 individuals by death or consumption by larger organisms, (c) light 

 intensity in relation to photosynthesis, {d) availability of nutrients, 

 {e) temperature. To these it has recently been suggested should be 

 added the eff'ect of excreted or ectocrine substances since they may 

 partly determine the annual sequence of the organisms. 



In nearly all the waters where studies have been carried out it 



