THE INFLUENCE OF WATER CURRENTS ON 

 THE LIFE FUNCTIONS OF ALGAE* 



John L. Blum 

 Canisius College, Bn_ffalo, N. Y. 



Selective effects by the current. Of the many habitats on Earth which are 

 colonized and exploited by sessile organisms, those which are in contact with 

 a mass of air or water in relatively rapid movement are likely to constitute a 

 convenience or a necessity to the uptake and excretory systems of the organ- 

 ism but to represent, at the same time, a major threat to the organism's se- 

 curity. Metabolizing organisms as we know them are inhabitants of fluids. 

 These fluids when laden with small cjuantities of nutrients and motionless may 

 or may not be suitable for successful growth and reproduction. When the 

 fluid is in unidirectional or turbulent motion and the organism remains in place, 

 the possibilities for successful growth of many sessile organisms are greatly 

 enhanced, but security is likely to be threatened by factors like evaporation 

 or physical buffeting by the current, and by the molar agents which are flung 

 at the organism. Areas where surface or subsurface currents run in close 

 proximity to the bottom or other stable objects are successfully exploited by 

 numerous sessile marine plants and invertebrates; in fresh water currents ses- 

 sile invertebrates are relatively few and inconspicuous, but the algae have 

 successfully colonized what to most animals is a peculiarly dangerous spot, the 

 rapids of streams both large and small. So unicjue is this habitat that some 

 of the algae which are found in the rapid water habitat are seldom if ever found 

 anywhere else. 



The present paper concerns algae which inhabit and are essentially limited 

 to fresh water currents, that is, algae which have moving water all around them 

 or in very close proximity; but inasmuch as the current has varied influences 

 as well on organisms which are in it only temporarily, I shall make occasional 

 mention of other river algae. The true current-inhabiting species are not 

 adequately described by the term "river algae" because the latter category 

 includes many forms which cannot attach and which are often unable to remain 

 in place in a strong current. Essentially all surface streams are inhabited by 

 some such forms, many of which are found as commonly or more commonly in 

 standing water. 



From source to mouth a freshwater stream consists of alternating shallow 

 (rifHe) areas and pools. These respective habitats differ in many ways and 

 it is usual to find that each is inhabited by a distinctive assemblage of animals 

 and plants. Current rate is influenced by a number of well known variables; 

 in small streams these variables act so as to subject different but adjacent 

 points to quite different pressures. Such pressures are likely to fluctuate 

 greatly from moment to moment but minute differences in depth and pre- 

 sumably in average current rate between points distant by only a few milli- 

 meters on the stream floor make of each shallow area a mosaic of differing 



* This study was aided in part by funds from the National Science Foundation (G-10898). 



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