FRESH WATER ECOLOGY; SOIL ALGAE 367 



members of the Myxophyceae, Dinophyceae, Bacillariophyceae 

 and Chlorophyceae, and according to the nature of the constituents 

 it may exhibit maxima in spring (very commonly), spring and 

 autumn or summer and autumn. 



Evolutionary Trend — Lake Waters 



Hard ancient rocks Softer more recent rocks Soft rocks or much 



or some silting 

 Decreasing depth 



C/3 

 U 



o 



U 



u 



unchanged 

 Generally deep 

 No Oo decrease with 



depth at thermo- 



cline 



TT- , Alkali 

 High 



Ca +Mg 



Poor in dissolved 



minerals 

 Rich in number of 



species 

 Poor in actual 



numbers of 



individuals 

 Desmids abundant 



Minerals in solution 



increasing 

 Decreasing number of 



species 

 Increasing number of 



individuals 



Diatoms and Diatoms and 

 desmids Eudorina 

 Oligotrophic waters > Over many years 



silting 

 Generally shallow 

 O2 decreases with 



depth at thermo- 



cline 



AlkaU 



Low -^ -rj- 



Ca +Mg 

 Rich in dissolved 



minerals 

 Poor in number of 



species 

 Rich in actual 



numbers of 



individuals 

 Asterionella and 

 Myxophyceae 

 • EuTROPHic waters 



Only one example of the algal flora of lakes v^ill be discussed in 

 detail in these pages, and so the student must remember that lakes 

 from other parts of the world may exhibit differences not only in 

 species but also in the normal periodicity of the communities. A 

 study by Godward in 1937 of the littoral algal flora of Windermere 

 in Cumberland brought out a number of interesting facts. In the 

 Continental lakes, some of w^hich are of a considerable depth, many 

 of the algal communities are markedly hmited in the depth to which 

 they can descend. In Windermere, however, any species of the 

 deeper waters is also able to exist in the surface layers, but as only a 

 shallow depth of water is occupied by the various communities, 

 depth per se can only be employed on a broad basis as a means of 

 distinguishing the communities. 



Three different groups of communities were recognized : 

 (i) Communities growing on stones and rocks : 



(a) Spray zone dominated by Pleurocapsa (May-September), 



Tolypothrix and Phortnidiimi (April-September). 

 {b) Zone 0-0-5 ^i- Dominated by Ulothrix, diatoms and 

 Myxophyceae. 



