POND ASSOCIATIONS 341 



In another pond somewhat different phases were recorded : 



Phase Dominants 



Autumn Oedogonium, Chaetophora 



Winter Vaucheria 



Late spring Oedogonium, Protococcus 



Summer Chaetophora 



These observations should be compared with those from the 

 Harpenden pool, and it will be seen that although the spring phases 

 are essentially similar with either Spirogyra or Oedogonium, never- 

 theless there are great differences. The two ponds described above 

 also possessed floras that were essentially different and they must 

 therefore be regarded as containing two separate associations. 

 Furthermore, the same worker found that a sudden change in the 

 external conditions checked the growth of an alga and often resulted 

 in the development of a resting stage or else of sexual organs; 

 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. 



LAKE ASSOCIATIONS 



Only one example of the algal flora of lakes will be discussed 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 periodicity of the communities. A recent 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 which are of a considerable depth, many 

 of the algal communities are markedly limited 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 communi- 

 ties, depth ^^r 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 Phormidium (April-September). 

 {h) Zone 0-0-5 "^- Dominated by Ulothrix, diatoms and 



Cyanophyceae. 



