354 Annals New York Academy of Sciences 



microhabitats whose existence and individuality is attested by striking differ- 

 ences in the algal populations which colonize them at certain seasons. 



A primary influence of current on algae, therefore, is the exclusion of certain 

 species from pool areas or other places where current is minimal, or the en- 

 hancement of growth of such species in the most favorable, frequently the 

 fastest current. The fact that algae colonize so dangerous a habitat as flowing 

 water suggests that they can be provided some unique service by this habitat. 

 The relationship of algal photosynthesis and respiration to water movement 

 has been discussed by various investigators including Gessner (1937) and 

 Steeman-Nielsen (1944). Oxygen consumption in the dark and the photo- 

 synthetic rate are increased in moving water above the respective values for 

 standing water. More recently, respiratory rate and P uptake by Oedogonium 

 kurzii Zeller have been studied by Whitford (1961). Radioactive P uptake 

 in water moving at 18 cm. per second was found to be over 10 times that in 

 still water. He concludes that the cause for "inherent current demand" by 

 lotic organisms is the need for rapid exchange of materials with the water and 

 that the steep diffusion gradient in a current satisfies this demand. 



This inherent current demand and the gradients involved may be of sig- 

 nificance to algae in two ways: for materials which are brought to the algae 

 by the current and for removal downstream of substances which might be 

 harmful. At least some algae are known to excrete substances which eventu- 

 ally retard their own growth rate. That such materials would be flushed 

 away from an alga growing in a current is evident, and may explain the limita- 

 tion of at least certain species to rapid water. It may also explain the high 

 cell density achieved by many current algae. 



EJfecls of current on algal size or shape. Precisely how current influences the 

 structure of an individual algal cell or thallus has received relatively little 

 attention. Many benthic stream algae are so flexible that the current con- 

 tinually bends and twists them without visible damage or effect. Unlike a 

 tree which bends permanently under the influence of prevailing winds, there 

 is nothing about their structure which would even betray the usual direction 

 of the current if by some means the current were suddenly averted or brought 

 to a stop. The same is true of certain less flexible bottom-inhabiting forms. 

 The Phormidium-Audouinella-Schizothrix community which is known from 

 streams of the North Temperate Zone (Blum, 1956) does not, in the surface 

 topography of its crust, show any very evident polarity with respect to the 

 current. Others — and relatively few cases are known — show by the form or 

 orientation of their thallus the effects of unidirectional current as in the Phor- 

 midium community described by Wehrle (1942), a composite community of 

 Vaucheria and Plectonema described by WaUner (1934), or in the colonies of 

 Cocconeis growing on a vertical cylindric stake as described by Gessner (1955). 



How the current controls the size of certain benthic algae is shown by work 

 done by Picken on the alga Rivularia. In regions of relatively rapid flow 

 thallus size was found to be proportional to the size of the stones to which the 

 thallus was attached. In slower water, however, thallus size was independent 

 of stone size. The bulk of this alga increases more rapidly than the area of 

 its attachment, and the current limits the maximal size of the thallus, either 



