338 . FRESH-WATER ECOLOGY 



These facts are important because they serve to indicate that 

 purely mechanical factors may be concerned in the distribution of 

 some algae, and only too often this aspect of an ecological problem 

 is wholly neglected. 



POND ASSOCIATIONS 



The literature on this subject is relatively sparse, but it is evident 

 that periodicity of the different species is of paramount importance, 

 the appearance of the different plants being controlled by a series of 

 factors, only one of which may be limiting for any given species. 

 A study of a pond near Harpenden by Fritsch and Rich (191 3) 

 showed that the general aspect of the flora was dependent upon 

 season and that four phases could be distinguished : 



(a) Winter phase with Microspora, Eunotia and epiphytic 

 diatoms, whilst Ranunculus aquatilis and Callitriche were the domi- 

 nant phanerogams. 



{b) Spring phase dominated by Conjugatae, Oedogonium and 

 Conferva^ with Ranunculus aquatilis as the most important phanero- 

 gam. 



{c) Summer phase with Euglena, desmids and Anahaena 

 associated with a phanerogamic vegetation of Lemna^ Glyceria and 

 Bidens. 



(d) Sparse autumn phase with Lynghya and Trachelomonas but 

 without any dominant phanerogam. 



The algal periodicity is thus more or less associated with a 

 similar periodicity in the phanerogamic vegetation. The flora 

 differs from that of a similar pool near Bristol in the absence of 

 Cladophora and Melosira, and in their place there is a greater 

 development of Xanthophyceae. The two types of flora could be 

 regarded as distinct associations, but the difference is almost 

 certainly due to poor aeration in the Harpenden pool. In spite of 

 this the general trend of periodicity in the two pools is very similar : 

 a winter phase characterized by a hardy filamentous form (Clado- 

 phora or Microspora) and diatoms, a spring phase with Zygne- 

 maceae and an autumn phase with Oscillatoriaceae. The summer 

 phase in the two pools is very different, and this is ascribed to the 

 greater drying up of the Harpenden pool during that period. The 

 flora of pools, therefore, is very dependent not only upon general 



