standing and flowing eutrophic waters. Both 

 Kolkwitz and Liebmann consider it to be character- 

 istic of the/9-mesosaprobic zone. Growth maxima 

 of this species have been found both In spring and 

 In summer. Nltzschia linearis W. Smith is a true 

 rheobiont, characteristic of springs and flowing 

 water. The related species, Nltzschia palea 

 (Ku'tz.) W. Smith is perhaps the most resistant and 

 most tolerant of all diatoms , a eurytopic, euryhaline 

 eurythermlc form which grows conspicuously in 

 polluted water where it forms a rich brown surface 

 layer on rocks of rapids, as well as in quiet water 

 on shallow silt banks. Both Kolkwitz and Liebmann 

 place it In the <l-mesosaprobic zone of incipient 

 oxidation In a stream, and It is often used as an 

 Indicator of polluted water although Its ubiquitous 

 tendencies permit it to grow throughout the entire 

 gamut of water purity. 



Hildenbrandia rivularis (Liebm.) J. Ag. is a 

 rheobiont, a crust-forming red alga which is found 

 In many hard waters of Europe. It seems to be ab- 

 sent, however, from streams with extremely high 

 calcium content . In a paper reviewing its occur- 

 rence, Luther (1954) has concluded that it is 

 limited to shaded situations. Thus, it is often 

 found in rather deep clear water, being at the ex- 

 treme outpost of macroscopic vegetation due to 

 light limitation. In sunny situations and in clear 

 water it may be found only on the under side of 

 rocks . Whereas, if the water is turbid, it is more 

 likely to be found on the upper side. In the waters 

 of Lough Neagh in Ireland it was not found on 

 basalt or other dark-colored rocks but was found on 

 the under side of quartz rocks where it apparently 

 received most of its light directly through the rock . 

 It seems to have a wide amplitude of tolerance for 

 temperature, as well as for organic pollution. Its 

 absence from otherwise suitable waters has been 

 Interpreted as brought about, in some cases, by 

 competition with other forms; in other cases, by 

 deposition of silt around rocks inhabited by young 

 plants (Luther, 195 4) . 



Ulothrix zonata Kii'tz. is a rheobiont in- 

 habiting cool, well-oxygenated streams. It shows 

 a vegetative growth maximum in late spring and 

 early summer. Fjerdingstad (1950), Kolkwitz 

 (1950), and Liebmann (1951) characterize it as 

 ollgosaprobic in habit, but there is some indica- 

 tion that its growth is enhanced in polluted waters. 



Cladophora glomerata L.) Kiltz . is one of the 

 largest and most widespread of rheoblontic algae. 

 It is known to be highly sensitive to Iron and rela- 

 tively tolerant of high pH values . It seems to re- 

 quire alkaline and calcareous water. It is tolerant 

 of relatively large amounts of sewage in the water 

 and of weak salinity. The length of Its summer 

 growth period varies from a few days to several 

 months . Usually it is Inconspicuous In winter al- 

 though Its full development has been recorded in 

 February and March, as well as in October; and 



Jaag (1938) has observed it throughout the winter. 

 It can apparently produce swarmers throughout the 

 entire year but is probably sensitive to tempera- 

 tures higher than about 25° C . After periods of 

 turbidity, the plants of Cladophora often become 

 heavily loaded with silt and epiphytes, a factor 

 which may be significant in the suppression of 

 growth or in the mechanical detachment of the 

 thallus . 



Stigeoclonium tenue Ku'tz. is a large, com- 

 mon stream alga which thrives in polluted waters 

 of the H- and^-mesosaprobic zones and is sel- 

 dom found growing luxuriantly elsewhere . Budde 

 (1930) concluded that it favored cool waters, but I 

 have found it attaining its best growth in early 

 summer and midsummer. Like Cladophora glom - 

 erata it is likely to be found in the most rapid 

 water of a brook, occupying the crest of a shal- 

 lows. Its preference for polluted water is not 

 limited to organically enriched water, for it can 

 frequently be found downstream from industrial out- 

 falls (which, however, frequently contain sewage 

 wastes) whereas it is apparently absent upstream 

 from the outfall . 



Hydrurus foetidus (Vill.) Trev . , a filamentous 

 member of the Chrysophyceae, is a rheobiont 

 adapted to cold water (approximately 0°-16 C.) . 

 Like Hildenbrandia it appears to favor shaded situ- 

 ations . In Europe, where it has been found much 

 more frequently than in North America, its growth 

 maximum is apparently attained in spring and in 

 autumn . 



Phormidium autumnale (Ag.) Gom. is common 

 in hard-water streams and is frequently associated 

 in polluted water with Sphaerotilus and with Stige - 

 oclonium tenue . With the sheaths which surround 

 its filaments, it forms mats of medium thickness 

 and rubbery consistency which adhere to the sur- 

 faces of rocks in the stream bed . 



The species I have taken up here are only a 

 few of many common and widespread stream forms , 

 and I have tried to give merely general information 

 on the ecological preferences of most of them. 

 Much more precise data on the diatom species as 

 concerns their halobion spectrum, the pH spectrum, 

 and the current spectrum are available in the recent 

 work of Foged (1947-48, 1954) . 



ECOLOGICAL CLASSIFICATION 

 OF RHEOPHIUC ALGAE 



The Naming of Communities. — In streams it 

 is the exception, rather than the rule, to find ex- 

 tensive areas inhabited by recognizable groups of 

 algal species which maintain a definite order of 

 dominance or inferiority with respect to each other 

 throughout all the seasons, although this relation- 

 ship is more commonly found If higher plants are 

 considered. Many algal populations appear to be 



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