FRESH WATER ECOLOGY; SOIL ALGAE 363 



being represented in Fig. 199. The fact that periodicity of the 

 different species is of widespread occurrence is evident from work 

 in various parts of the world. Thus a study of a pond near Har- 

 penden 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 di- 

 atoms, whilst Ranunculus aquatilis and Callitriche were the 

 dominant phanerogams. 

 (h) Spring phase dominated by Conjugatae, Oedogonium and 

 Conferva^ with Ranunculus aquatilis as the most important 

 phanerogam. 



(c) Summer phase v^th Euglena, desmids and Anahaena as- 

 sociated with a phanerogamic vegetation of Lemna, Glyceria 

 and Bidens. 



(d) Sparse autumn phase with Lyngbya 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 Cladophor a 

 and Melosira, and in their place there is a greater development of 

 Xanthophyceae. The two types of flora could be regarded as dis- 

 tinct 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 venter phase 

 characterized by a hardy filamentous form {Cladophora or Micro- 

 spora) and diatoms, a spring phase vdth Zygnemaceae 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 climatic con- 

 ditions, such as rainfall and insolation, but also upon what might 

 be termed irregular microclimatic factors, e.g. aeration in the body 

 of water itself. In the case of many of the species there is a profound 

 relationship between the meteorological data and the frequency of 

 the flora, e.g. Microspora and the Protococcales with temperature, 

 Oedogonium and Hormidium with sunshine. The factors influencing 

 the growth of aquatic algae are (i) seasonal, (2) irregular, (3) cor- 

 related. The first group, which are very obvious and need not be 

 detailed, are principally of importance for large bodies of water, 



