As mentioned above, I am not qualified to 

 judge whether the saproblen system which uses 

 indicator organisms will prove to be useful and 

 workable. Dominant species and indicator organ- 

 isms have been widely used in other branches of 

 ecology . 



Separate studies of individual species might 

 go a long way toward solving some of the problems 

 of the effects of water quality. 



One habitat factor which is of considerable 

 importance in stream ecology seems to be mis- 

 understood by many llmnologists and phycologists . 

 This is the effect of current. For a long time it has 

 been known that certain species grow only in a 

 current of water or grow better there . This fact 

 was at first explained by assuming that running 

 water has a higher content of dissolved gases and 

 a lower temperature than still water. There is now 

 much evidence that these assumptions are incor- 

 rect, but there is evidence that some rapids 

 species have a higher respiratory rate than cor- 

 responding lenltlc species. Many investigators 

 recognize this "inherent current demand" of such 

 species but offer no explanation. Ruttner (1926) 

 seems to have offered the first partial explanation . 

 His statement as translated by Frey and Fry 

 (Ruttner, 195 3) is as follows: "In quiet or in 

 weakly agitated water the organisms are surrounded 

 by a closely adhering film of liquid, which speedily 

 forms around the animal or plant, a cloak Impover- 

 ished of substances Important for life. In a rapid 

 current, however, the formation of such exchange- 

 hindering Investitures Is prevented, and the ab- 

 sorbing surfaces are continually brought into con- 

 tact with new portions of water as yet unutilized. " 

 Moving water, he continues, "is not absolutely 

 but rather physiologically richer In oxygen and 

 nutrients . " 



The reason for this "physiological richness" 

 can be explained, I believe, by the laws of co- 

 hesion and diffusion which are familiar to most 

 llmnologists and phycologists. It Is a simple mat- 

 ter of the diffusion gradient being steeper around 

 plants in rapid water. If all other factors are 

 equal, diffusion will occur twice as fast at half the 

 distance. There is formed around a cell In still or 

 slowly moving water a gradient of concentration of 

 diffusing materials. For a material diffusing In- 

 ward the concentration In the medium is least at 

 the cell wall and the material Increases in concen- 

 tration outward a certain distance until it reaches 

 100% of that In the surrounding water . For the 

 smaller Inorganic molecules this distance seems to 

 be about 1/4 mm. Even in a current cohesion holds 

 a film of relatively still water against the surface. 

 In a rapid current at least part of this film Is swept 

 away bringing the region of high solute concentra- 

 tion closer to the surface — In other words making a 

 steeper diffusion gradient and therefore increasing 

 the rate of diffusion. Ferrell, Beatty, and Richard- 



son, (1955) have shown that the speed of current 

 necessary to displace this film is of the order of 

 one-half foot (15 cm.) per second. 



In case of oxygen and carbon dioxide, which 

 are more soluble In cold water, low temperature 

 may reduce or eliminate the "current demand . " 

 Cedergren (1938) reports that certain algae which 

 grow in still water during the cooler seasons are 

 found only in rapids in summer. I discovered this 

 to be true for Stigeoclonlum and Draparnaldla in 

 North Carolina, many years before seeing a ref- 

 erence to Cedergren's work. One species of Sti- 

 geoclonlum grows in summer in very swift water at 

 a temperature averaging close to 25°C. 



We^ have data which indicate that some fif- 

 teen or more species of algae require a rapid cur- 

 rent at least at temperatures above 15°C. These 

 include species of Stlgeoclomum , Chaetophora , 

 Gongosira , Oedogonlum , and Spirogyra as well as 

 most of the species of red algae we have collected. 

 A few crust-forming species of blue-green algae 

 almost certainly belong In this group also. 



One of our most Important rapids species is 

 Oedogonlum kurzii . It Is a perennial In rocky 

 rapids especially in the Piedmont of the southeast. 

 It seems to occupy the same place In soft water 

 streams as does Cladophora In hard-water regions . 

 In late spring It forms great masses and skeins 

 over a meter in length, but by late summer it is re- 

 duced to short tufts attached to rock in the swiftest 

 water. During long sunny autumns it may again 

 become fairly abundant and it never completely 

 disappears all winter. 



The problem of communities has received the 

 attention of numerous llmnologists and phycolo- 

 gists. Everyone who does ecological work must 

 consider It. Since there has been more work done 

 on the plankton than on other communities there 

 have been more attempts to classify the phyto- 

 plankton than other communities . Several hundred 

 papers deal at least in part with community rela- 

 tionships . Perhaps the best summaries and lists 

 of literature are those of Str^m (1924), Fritsch 

 (1931), Symoens (1951), Tiffany (1951) and more 

 recently Prescott (1956), and Blum (1956). In his 

 summary of the ecology of fresh-water algae. 

 Tiffany says, " . . .the ecological factors are iden- 

 tical with those affecting the larger land plants, 

 but the degree of intensity, the availability, and 

 distribution of such factors are different;. . .atten- 

 tion must be directed more and more to the micro- 

 environments of algae. Algal communities, though 

 quite distinct In many habitats , are more difficult 

 to define and delimit than associations of many 

 terrestrial seed plants . Successlonal phenom- 

 ena in the algae are often matters of seasonal 



Schumacher and Whltford , unpublished data. 



