Among the many benthic diatom communities 

 which have been named, I shall mention only a few. 

 The Pi a to ma vulgare - Melosira varians community, 

 characterized by a substantial development of 

 species of alkaline and eutrophic waters . Under 

 typical conditions, at least in the Belgian streams 

 studied by Symoens (1957), few non-diatomaceous 

 algae are included. With time, Ulothrix , Clado - 

 phora , Vaucheria and Oscillatoria make their ap- 

 pearance. The latter is associated especially with 

 organically enriched waters . When these algae be- 

 come dominant, the physiognomy of the vegetation 

 changes completely, and its formal classification 

 under the alliances listed by Symoens would shift 

 from group 2 (benthic diatoms) to group 3 (green and 

 red filamentous algae) (Symoens, 1957). 



Another diatom community is the Diameto r 

 Meridionetum of Margalef (1948) and others . It is 

 represented in various forms in different streams, 

 with many species included, but the diagnostic 

 ones are Diatoma hiemale and Meridion circulare 

 Ag. In the Ardennes, Symoens (1957) considers 

 this community to be associated with considerable 

 elevation (150-2500 m.) and Margalef states that 

 it Is confined to waters of temperature between 10° 

 and 15° C. 



Under the associations of green and red fila- 

 mentous algae Symoens has listed, among others a 

 Cladophora qlomerata association, known from 

 Belgium, France, Western Germany, Catalonia, 

 Switzerland, Balearic Isles, the U.S.A., and, with 

 associated Podostemaceae, from central Africa. 

 The diatoms associated in the Ardennes region are 

 those of the Diatoma vulgaris - Melosira associa- 

 tion . Cocconeis pediculus is a widespread, epi- 

 phyte on the Cladophora . 



Another filamentous association listed is 

 that of lime-rich waters dominated by Vaucheria 

 spp. , especially (in the Ardennes) V. debaryana 

 Woronin, and characterized by much the same dia- 

 toms that are associated with the Cladophora vege- 

 tation. 



An additional Vaucheria Association is listed. 

 However, this is Vaucheria of soft waters and is 

 accompanied by diatoms of the Diatoma hiemale- 

 Meridion circulare community . 



It is clear from these brief examples taken 

 from the literature that a beginning has been made 

 in the cataloguing and classification of stream 

 communities. Work in future years will, no doubt, 

 increase our knowledge of the validity and floris- 

 tic affinities of the associations named thus far 

 and give us more than a pioneer explorer's view of 

 the vegetation of streams of the world . 



DISTRIBUTION OF ALGAL POPULATIONS IN TIME 



Colonization and Succession. — From his 

 work on the Hull (England), Butcher has concluded 



that colonization there reaches a maximum in May. 

 Individual dominant species have periods of most 

 rapid reproduction in various months from March 

 through October, and the period of least coloniza- 

 tion is in winter. In summer, colonization appears 

 to be complete in about 20 days, but in cold months 

 when growth is slower, 30 to 40 days are required. 

 It might be supposed that vigorous current would 

 impede algal colonization on so smooth a surface 

 as a glass slide, but more algae are produced on 

 the slides in regions of rapid flow than elsewhere. 



In southern Michigan, colonization of rock 

 surfaces by winter-dominant diatoms is very rapid, 

 and macro scopically visible colonies can form in as 

 little as ten days . The period within which Gom- 

 phonema olivaceum colonized bare rock surfaces 

 extended from late November to early April, and 

 colonization appeared to be possible at any time 

 within this period. No evidence of succession 

 was found prior to the establishment of this com- 

 munity, and the same may be said for the commun- 

 ity of Diatoma vulgare characteristic of late fall — 

 both these forms were at one and the same time 

 colonists and seasonal dominants (Blum, 1956). 



In the work of Butcher there is likewise little 

 evidence of succession in the algal communities he 

 has investigated . In the Cocconeis — Chamaesi - 

 phon community, Chamaesiphon arrives in the en- 

 semble later than the other forms, but there is no 

 true succession. Certain of these communities are 

 regarded as representing a true climax, but their 

 development is apparently accomplished with no 

 steps separating invasion from climax conditions 

 (Butcher, 1946) . 



Periodicity of Algal Populations. — The plank- 

 ton algae of a great many streams have now been 

 investigated for sustained periods. Most of these 

 streams are in Europe, but some of those in the 

 United States have been extensively studied also, 

 and a few tropical streams are represented. Many 

 streams exhibited considerable consistancy, during 

 the period and to the extent of the observations 

 made, as regards the time of development of the 

 greatest densities of their plankton or of their ben- 

 thic vegetation, whereas other studies have pointed 

 out sharp differences in the year-to-year develop- 

 mental pattern. A mere year's work upon a single 

 stream may prove inadequate in that it reveals con- 

 ditions essentially unlike the preceding and fol- 

 lowing years . In nearly all streams investigated , 

 the principal phytoplankton pulse, if there is one, 

 is to be expected at some time during the warm 

 season, but individual plankters, e.g., Crucigenia 

 rectangularis , Pediastrum boryanum , Fraqilaria 

 capucina , Meridion circulare and Synedra ulna , 

 frequently exhibit population maxima in winter. 

 Many streams are characterized by two separate 

 periods of maximum plankton abundance. Some 

 species of algae exhibit great variability in pro- 

 duction, remaining uncommon in one year and 



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