classification which have been devised for the 

 forms of plant body possessed by rheobiontic algae, 

 that of Cedergren, sketched in brief form In 1938, 

 is worth attenUon. Cedergren points out that ben- 

 thic algae, which inhabit a current, exhibit adap- 

 taUons of four principal types: (1) Richly branched 

 filaments. These have strong anchorage and water 

 may pass freely between adjacent filaments. The 

 presence of a slippery gelatinous external coat is 

 noteworthy in many of these such as Batrachosper - 

 mum, Draparnaldia , and Stigeoclonium . (2) Long 

 flexible cylinders. These align themselves per- 

 fectly with the current and thus offer little resis- 

 tance to the water. (Encyonema spp. , Tetraspora 

 cylindrica (Wahlenb.) Ag . or Splrogyra fluviatilis 

 Hilse) The presence of a firm attachment and a 

 slippery exterior is remarkable in this group. 

 (3) Spheric, wart-like or cushion-like colonies. 

 These show numerous contained filaments as in 

 Chaetophora or Nostoc which, because of the 

 smooth external surface of the colony, offer little 

 resistance to water movement. (4) Reduced, sim- 

 plified, plate-like forms. These grow in a thin, 

 appressed sheet. Although diverse in internal 

 structure, externally such sheets are similar in 

 usually following the contours of the rock or other 

 substrate and in exposing relatively little surface 

 as a colony to the water in relation to the enormous 

 surface of the individual filaments or other units . 

 Hildenbrandia and Phormidium are examples . 



These four types can actually be further re- 

 duced. Groups 1 and 2 are those with flexible 

 thalli which permit water to run through them and 

 thus expose a large surface to the water. Groups 

 3 and 4 have inflexible bodies and a greatly re- 

 duced surface . A number of low-growing green and 

 blue-green algae do not seem to fit into this system 

 very well. Nevertheless, a great majority of 

 stream algae would fall under Cedergren 's groups 

 1 and 4 . 



Named Benthic Communities of Algae. — A 

 number of authors have catalogued stream commun- 

 ities which have come under their observation, but 

 relatively few have made a synthesis of observa- 

 tions from other regions with their own. Among re- 

 cent contributors to algal sociology, Symoens, who 

 has done important work on streams in Belgium, has 

 presented an outline of algal communities based on 

 (1) floristics, (2) aspect (physionomie ) , (3) syn- 

 genetic relationships, and (4) synecology. The 

 system includes eighteen alliances which he lists 

 "timidly and provisionally. " These include three 

 alliances of stream algae, as follows: (1) epil- 

 ithic algae and crustose lichens, (2) benthic dia- 

 toms and (3) filamentous green and red algae 

 (Symoens 1951) . Mention was made of certain 

 associations represented by each heading, but 

 without extensive discussion or description. These 

 associations were amplified in a subsequent study 

 of streams in the Ardennes region (Symoens, 1957) . 



In England, Butcher (1946) has recorded what 

 he considers to be two distinctive communities. 

 The first of these, Achnanthes microcephala — 

 Chaetopeltis , with associated Diatoma hiemale and 

 and Eunotia, is the characteristic oligotrophic com- 

 munity in the streams studied. With increasing 

 eutrophy in downstream areas, it may be replaced 

 there by the Cocconeis - Ulvella - Chamae siphon 

 community. This is a eutrophic association which 

 shows no seasonal periodicity and which is com- 

 posed principally of Cocconeis placentula Ehr . , 

 Achnanthes minutissima Kiitz . , Ulvella frequens , 

 Chamaesiphon incrustans , Grun . and Chamaesi - 

 phon irregularis . 



Butcher's group observed the dominance of 

 the first of these communities in the upper reaches 

 of the Tees. This was replaced by the second 

 wherever sewage drains flowed into the river in 

 quantity. The first, or Achnanthes - Chaetopeltis , 

 reappeared wherever the dilution was sufficient and 

 decomposition rapid. Margalef (1948) has referred 

 under a different name (Asoclation Hydrococcetum 

 rivularis) to what appears to be a directly com- 

 parable community living in streams of the 

 Pyrenees . 



Another community characteristic of the upper 

 reaches of hard water streams is a crustose com- 

 munity dominated by Phormidium , Schizothrix , and 

 Audouinella , and which is known also as the Chan- 

 transieto-Phormidietum incrustans (Symoens, 1957). 

 It has been identified in Austria, Belgium, England, 

 and the United States of America. It is probably 

 widespread in hard water streams, perhaps through- 

 out the world. The principal blue-green components 

 of this crust, Schizothrix fa sciculata , S . pulvinata 

 and S. lacustris grow on submerged rocks, forming 

 dense tufts of radiating filaments which may be 

 only .5 mm. in height. As the Schizothrix grows, 

 two other species frequently invade the tufts: Phor - 

 midium incrustatum , whose filaments mingle with 

 the tufts and grow, with either horizontal or verti- 

 cal orientation, twisted in and out between the 

 Schizothrix filaments; and Audouinella sp. , which 

 grows radially like the Schizothrix out to the limits 

 of the tufts, where it branches while its upward 

 growth keeps pace with, or slightly surpasses, that 

 of the Schizothrix and Phormidium species . These 

 four or five organisms constitute a single layer, and 

 apparently all of them may secrete calcium carbon- 

 ate abundantly, thus converting the entire tuft, or 

 stratum of continuous tufts, into a crust. This 

 crust becomes especially stony and resistant in the 

 presence of a dense admixture of Phormidium in- 

 crustatum . After growth for a year or more, the 

 tufted layer may eventually attain a thickness of 

 4-5 mm. Frequently, at seasons when other ben- 

 thic algae are abundant within the stream, it 

 becomes an inferior layer, shaded and more or less 

 enveloped by temporary dominants such as Clado- 

 phora qlomerata (Blum, 1956). 



15 



