424 Ecology 



such habitats. Most desmids, for instance, are perennial and, although fewer 

 in numbers in the winter months, a percentage of individuals invariably 

 survives that period in the vegetative state. The same is true of freshwater 

 diatoms, but to a more marked degree, since these organisms are mostly 

 cold-water types. 



Where there is a succession of dominant types the cycle is controlled 

 by many diverse factors. There are seasonal factors, such as temperature, 

 intensity of light, amount of dissolved oxygen, etc. ; and various other deter- 

 mining factors, among which may be mentioned the geological formation 

 (and its effect upon the chemical composition of the water), altitude, and, in 

 the case of lakes, the nature of the banks, whether marshy or rocky. 



A. Algal Associations of Swiftly Running Water. 



In this category are all those Algse which inhabit swift rivers, cataracts 

 and waterfalls. Among the most characteristic are certain of the freshwater 

 Rhodophycese, the genera Leinanea, Sacheria and Chantransia being especially 

 noteworthy. The two first-named genera occur only in the most rapid 

 torrents and in waterfalls, always where the force of the water is greatest. 

 The most abundant species in temperate countries is Sacheria mamillosa, an 

 Alga which often occurs in artificial torrents such as mill-sluices. Species 

 of Batrachospermum may occur in running water, but are found abundantly 

 in pools and lakes, and even in bogs. 



Of the Chlorophycese, several species of Cladophora are abundant, more 

 especially Cl. glomerata and Cl. fracta. Vaucheria geminata is frequently 

 abundant where running water overflows rocks and boulders, and also where 

 the waters of a spring irrigate mossy ground. That most extraordinary of 

 all desmids Oocardium stratum occurs in swift mountain streams in 

 limestone areas, forming small opaque white, encrusted pilules attached by 

 much mucus to submerged rocks and stones. Microspora amoena is also a 

 frequent Alga in swiftly running streams, more especially in the early spring 

 or late autumn. 



Many attached diatoms occur in running water especially some of the 

 stalked species of Cymbella and Gomphonema. The most notable is Gompho- 

 nema geminatum which often forms greyish felt-like masses attached to 

 the rocks of mountain streams and cataracts. 



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Here should be mentioned the ALG^E OF HOT SPRINGS, since they occur 

 in running water, but the water is at a comparatively high temperature 

 and in many cases sulphurous. They are mostly Blue-green AlgaB and have 

 been found to occur in water at over 80 C. (vide p. 34). In the warm 

 streams flowing from these springs several species of Rhizoclonium occur and 

 also Zyynema ericetorum. 



