360 THE ALGAE 



(a) Eupotamic, thriving in the stream and its backwaters. 



(b) Tychopotamic, thriving only in the backwaters. 



(c) Autopotamic, thriving only in the stream. 



A potamoplankton survey of three rivers in the Goros region 

 showed that there were three maxima for diatoms and Chloro- 

 phyceae, and four for Myxophyceae. The total flora comprised 155 

 species which is an indication of how rich this type of plankton may 

 be. It seems, however, that even though there may be a rich phyto- 

 plankton the component species do not form a distinctive associa- 

 tion. In general the development of potamoplankton is dependent 

 on the age of the water and whether the stream is in flood or drying 

 up. The plankton of small streams appears to arise from the 

 benthos. 



It is convenient also to include here the flora of hot springs and 

 streams. Myxophyceae are the major, and often the only consti- 

 tuents, the various species being capable of secreting carbonate of 

 Hme or siHca to form rock masses such as travertine and sinter, the 

 rate of deposition sometimes being as much as 1-25-1 -5 mm. in 

 three days. The highest temperature recorded for water with living 

 plants {Phormidium (Hapalosiphon) laminosum) is 87-5° C. The 

 number of forms capable of Uving in such an environment is con- 

 siderable, no fewer than 53 genera and 163 species being recorded 

 from the thermal waters of Yellowstone National Park. Recently 

 it has been shown that one species, Mastigocladus laminosus^ is 

 capable of fixing atmospheric nitrogen, though it does not seem 

 that this has any biological significance. 



According to Blum (1956) the dominant benthic alga in much of 

 the North Temperate zone is Cladophora glomerata. In the presence 

 of certain metaUic ions this species is adversely affected and may 

 then be replaced by Stigeoclonium tenue^ Spirogyrafluviatilis, Phor- 

 midium autumnale, etc. In Scandinavian streams it is often replaced 

 by Zygnema and Vaucheria, and this fact has been made a basis for 

 classification. Budde (1928) divided the mountain streams feeding 

 the Ruhr River into two regions, an upper Hildenbrandtia region 

 and a lower Lemanea region. One of the most favourable locations 

 for algal growth is immediately down-stream of large rocks though 

 the exact significance of this fact has not been worked out. As 

 might be expected, seasonal variations occur in the vegetation. 

 Thus in the Ruhr streams the spring period is characterized by 

 dominance of diatoms with Ulothrix and Hormidium as subdomi- 



