366 THE ALGAE 



insistence upon the importance of external conditions in this respect 

 has also been emphasized by Fritsch and Rich in their study on the 

 Harpenden and Bristol pools. 



4. Associations of pools and lakes 



West was the first investigator of lake and pool algae who ap- 

 preciated the fact that the geology of the substrate was of profound 

 importance. He showed that the desmid flora is richest where the 

 substrate is precarboniferous {Oligotrophic waters), whilst diatoms 

 become abundant in younger areas or where there has been much 

 silting with consequent solution of mineral salts {Eutrophic waters). 

 Later workers have greatly extended this important study, and the 

 present treatment of the problem is more or less summarized in the 



scheme on p. 367. 



A third type is the Dystrophic lake or pool, which is to be found 

 on moorlands, where desmids form the most abundant part of the 

 flora in a water that is often highly coloured. In the course of years 

 Oligotrophic waters may also change into Dystrophic waters. 



Originally there was supposed to be a fairly sharp distinction 

 between oligotrophic, eutrophic and dystrophic waters, but with 

 the reaHzation that one type can gradually change into another it is 

 evident that any such distinctions cannot be sharp. A considerable 

 body of data is now available on the physical and chemical environ- 

 ment in pools and lakes. This has enabled workers to standardize 

 the waters from the purely physical point of view, but it still re- 

 mains to be seen whether this information will be of any great 

 assistance in understanding their ecology. Among these physical 

 factors is oxygen concentration. In sheltered lakes as compared 

 with open lakes there is an oxygen stratification which closely 

 follows the bottom contours, whilst the influence of any rivers 

 entering the lake together with the problem of periodic floods is yet 

 a further factor. 



Where there is a shallow littoral shore the communities are 

 difficult to recognize unless there is a rocky substrate, in which case 

 there may then be a zonation that is dependent on changes of water 

 level and wave action: this type of zonation has been observed in 

 several continental lakes. In deeper waters the communities are 

 more distinct because a zonation develops which is primarily main- 

 tained by the Hght intensity factor. The Limnoplankton of lakes 

 is not usually of great bulk and is composed principally of various 



