FRESH-WATER ECOLOGY 



335 



In America Transeau (191 3) concluded that fresh- water pond 

 algae can be divided into seven classes based on abundance, dura- 

 tion and reproductive season, these classes and their periodicity 

 being represented in fig. 198. 



I 



X±n . Feb. n^r. /^/>r Hay Tune July Aug. ^t Oct Nou. Dec. 



Fig. 198. Chart showing the estimated relative importance of the different t>'pes 

 of algal periodicity throughout the year in the waters of E. Illinois. The irregulars 

 are not depicted. (After Transeau from West.) 



(4) Associations of pools and lakes. 



West was the first investigator of lake and pool algae who 

 appreciated the fact that the geology of the substrate was of pro- 

 found importance. He showed that the desmid flora is richest 

 W'here the substrate is precarboniferous, w^hilst diatoms become 

 abundant in younger areas or where there has been much silting 

 with consequent solution of mineral salts. Later workers have 

 greatly extended this important study, and the present treatment of 

 the problem is more or less summarized in the schema on p. 336. 



A third type is the Dystrophic lake or pool, w^hich 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. 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 



b 



