relatively haphazard mingling s of two to many un- 

 attached species. These groupings may exhibit no 

 constant composition from place to place within the 

 stream. Frequently , too , their composition changes 

 rapidly from week to week. In the composition of 

 any community of aquatic algae, chance probably 

 plays a major role throughout the formative period , 

 which may last for many years, perhaps centuries. 

 Thus, it is only in large, old, aquatic situations 

 that one may hope to find associations so stable 

 that the absence of one form or another would be of 

 real biological significance (Symoens , 1951). The 

 availability of numerous diatom and other species, 

 any one or any combination of which is able to take 

 over a site and quickly achieve local dominance, 

 makes it particularly difficult and unrewarding to 

 apply formal names to such groupings. Only when 

 there is (l) a permanent, distinctive community 

 structure which can be found repeated again and 

 again in a series of streams with essentially the 

 same species composition of algal dominants 

 throughout the series, or (2) a consistent reap- 

 pearance of a ;pecific algal vegetation in a given 

 season of the year, does there seem to be suffi- 

 cient grounds for applying a formal ecological name 

 to the vegetation. It seems preferable to deny for- 

 mal status to the less frequently observed combin- 

 ations . Just as in land vegetation, the application 

 of formal names to all types of variant transition 

 combinations found along ecotones is liable to 

 create confusion without serving any redeeming 

 practical purpose. It is futile to name, aquatic 

 communities without evidence that they are of more 

 than purely local occurrence. Panknin (1945) 

 points out that phytoplank* on can, in this way, be 

 divided up into an endless series of associations . 

 In the benthos, extensive stands of one or more 

 species of dominant algae are relatively more com- 

 mon than among land communities . Often these 

 stands are unialgal, or nearly so, to the exclusion 

 of other green plants below them. Often they grow 

 with great rapidity , carpeting the stream floor in 

 ten days or less when they experience favorable 

 environmental conditions . Many small species are 

 epiphytes (or endophytes) on larger algae which 

 support, and to a certain extent protect, them. The 

 ephemeral nature of attached algae is, furthermore, 

 one of the unique features of the stream communi- 

 ties . With the detachment of a large thallus, it, 

 and its entire collection of epiphytes, is immedi- 

 ately taken downstream. All these organisms with 

 the nutrients they contain are effectively lost to 

 the local ecocosm. 



These are rather deep-seated differences 

 from the conditions on land and from the conditions 

 in standing water. It has been suggested that we 

 may be basically in error in attempting to apply to 

 the hydrosere terms such as "climate" and "climax" 

 which were first used for land vegetation and which 

 may, upon scrutiny, carry with them too many im- 



plications of no validity in hydrobiology . For one 

 thing, a plant community which takes over a site 

 and reproduces there, maintaining dominance and 

 successfully resisting invasion so long as en- 

 vironmental conditions remain substantially un- 

 modified, has fulfilled the conditions for a climax 

 community . But , with reference to river algae , 

 this community may not have been preceded by any 

 other green plants whatever — colonization and dom- 

 inance are thus synchronous . With no previous 

 community occupying this site, succession has not 

 occurred and use of the term "climax" seems in- 

 appropriate (Blum, 1956a) . 



Eddy has concluded that permanent fresh- 

 water communities exist, reach maturity and show 

 aspects comparable to terrestrial communities, and 

 he points out that the maintenance of given cli- 

 matic conditions necessary for the establishment 

 of a "climatic" climax is not confined to land com- 

 munities, but can also be found in permanent 

 streams (Eddy, 1934). The generation and the life 

 span of dominant algae are so much shorter than 

 those of most dominant vascular plants that "perm- 

 anent" climatic conditions can be achieved in a 

 relatively shorter time and more rapid succession 

 is to be expected. For extremely short-lived mic- 

 rophytes, a single growing season in temperate 

 regions may be sufficiently long to represent a 

 "permanent" climate (Blum, 1956a). 



Panknin (1941) has discussed the classifica- 

 tion of seasonal communities which assume tem- 

 porary dominance upon a given site and has con- 

 cluded that such algae should not be regarded as 

 constituting seasonal associations but rather as 

 making up seasonal aspects of the entire associa- 

 tion. He considers that community names based 

 on only two diagnostic species are inadequate if 

 only one season has been studied, and the aspect 

 of the vegetation in other seasons is unknown. 

 The "true" algal community recognized by Panknin 

 includes no higher plants and occurs only on the 

 seacoast, in deep mountain lakes, in the phyto- 

 plankton, and in streams. All other algal com- 

 munities are "dependent, " the algae being merely 

 an undergrowth in an assemblage of higher plants . 

 (Panknin, 1945) . This refusal to recognize many 

 common algal communities separate from those of 

 higher plants around them led, as Symoens (1951) 

 has pointed out, to lumping them with plants 

 mostly larger than themselves and to designating 

 such heterogeneous communities as the Micraster- 

 ieto-Sparganietum. Various communities have been 

 named exclusively on the basis of desmids present 

 or on diatoms present while, at the other extreme, 

 Margalef (1947, 1949) understandably introduces 

 even Protozoa, Gladocera, Copepods, and insect 

 larvae into associations made up partly of algae 

 (Symoens, 1951) . 



The Classification of Stream Algae According 

 to Structural Adaptation. — Among schemes of 



14 



