becoming a dominant in the next, exhibiting a 

 pulse at widely variable times throughout the year, 

 or a single pulse in one year and a bimodal one in 

 another. Certain diatoms tend to exhibit a pulse 

 in spring and/or in autumn rather than in midsum- 

 mer . 



Occasionally a river may develop a "bloom", 

 although this phenomenon is more frequently seen 

 in ponds. Organisms which have been found re- 

 sponsible for such blooms include Thalassiosira 

 fluviatilis (Weser, Germany), Synedra delicatis - 

 sima (Potomac, U.S.A.), Microcystis flos - aquae 

 (Bug, U.S.S.R.), Anabaena spiroides (Mayenne, 

 France), Aphanizomenon flosaquae (Don, U.S.S.R.) 

 and Pandorina morum (Cumberland, U.S.A., and 

 Kentucky, U.S.A.) (Blum, 1956). Lackey lists no 

 fewer than 25 separate blooms for the Clinch River 

 and adjacent waters in 1956 with a great variety 

 of organisms represented (Lackey, 1958). 



The best correlations which have been ob- 

 tained by plankton work and chemical analyses 

 over a period of years point to a time relationship 

 between abundant nutrients and abundant phyto- 

 plankton, the plankton pulse usually following the 

 period of highest concentration of the nutrients, as 

 nitrates and nitrites, in such a way that the de- 

 crease in nutrients precedes by a few days to a 

 few weeks the maximum development of phytoplank- 

 ton. 



A diurnal plankton pulse has been observed 

 in a polluted stream, apparently dependent upon 

 and produced by midday sunlight which causes 

 benthic forms to rise into the current and be car- 

 ried downstream . Evidence that planktonic forms 

 reproduce as they are carried downstream has been 

 presented by various workers, but there remains the 

 suspicion that much of the actual cell division oc- 

 curs on the bottom and that the apparent increase 

 in phytoplankton downstream is largely the result 

 of more extensive nutrient beds there, and of 

 denser populations of benthic individuals, many of 

 which rise every day into the plankton. The vege- 

 tative dissemination of Spirogyra and Oscillatoria 

 communities was observed by the author on warm 

 summer days in 195 2 and '5 3. These communities 

 were especially characteristic of quiet shoals or 

 bays of the stream. Here the algae remained on 

 the bottom in contact with nutrient-rich silt de- 

 posits, as masses of filaments easily visible from 

 a distance. The surface water of such shoals and 

 bays is usually in slow circular movement set up 

 by the main current of the stream, which by-passes 

 the shoal or the bay in a tangent to the circular 

 current which it produces there . At times of rapid 

 photosynthesis, individual masses of the algal 

 filaments are detached and buoyed upwards by 

 trapped oxygen bubbles . Once the algal mass has 

 quit the floor of such a shoal, it is carried slowly 

 along in the eddying surface water. After moving 

 for some time in this circular manner, it may even- 



tually be picked up by the tangential current of the 

 main stream which removes it definitively from the 

 shoal. As the algal mass travels downstream. It 

 disseminates live filaments along the way. The 

 progress of these filaments Is arrested on obstruc- 

 tions or on new shoal areas or other sediments 

 downstream, which in this way are themselves 

 colonized. The elevation of algal masses by en- 

 trapped bubbles can be observed from about noon 

 until about 2-3 p.m. on sunny days in summer, and 

 the movement downstream of these floating masses 

 can be observed throughout an entire afternoon 

 (Blum, 1956). 



Under benthic algae are included both sea- 

 sonal and perennial species. While a single alga 

 may be dominant over relatively long reaches of the 

 stream's course, it is more common to find a num- 

 ber of dominants with different parts of the stream 

 having different dominant communities. In some 

 streams the algal vegetation remains much the 

 same throughout the year, whereas in others there 

 are marked seasonal aspects. As with plankton 

 algae, the seasonal variation may be summarized 

 broadly as maximum development in warm months, 

 followed by minimum development in cold months. 

 But, many algae behave quite differently. To what 

 degree these seasonal changes are due to some 

 environmental factor, has not been established. 



DISTRIBUTION OF ALGAL POPULATIONS IN SPACE 



Depth zonation .--The relation of depth to 

 the algae of rivers has received relatively little 

 attention . On the bank of the polluted Mulde 

 River, Schroder (1939) has illustrated a series of 

 algal zones with Spirogyra spp . just below the 

 water surface, Stigeoclonium tenue and then Oscil- 

 latoria spp. further down, and Sphaerotilus spp. 

 and Nitzschia spp. in the deepest water. In cer- 

 tain lower portions of the Meuse, Symoens has de- 

 scribed a distinctive zonation consisting of 

 Rhizoclonium sp . just above the usual water level, 

 Banqia atropurpurea at the level washed by the 

 water's edge, in a band 10 to 20 cm. wide, and 

 finally Cladophora glomerata , below the water 

 level and mixed with certain mosses (Symoens, 

 1957). It is probable that nearly all streams which 

 maintain a given level for several weeks exhibit 

 some zonation of the attached algae. Figure 1 

 illustrates a type of zonation, involving two or 

 three species , which has been noted in southern 

 Michigan streams. Scheele (1954) has recorded 

 diatom zonation in drains (SchleusenwSnde) in 

 Germany . The most outstanding difference with 

 increasing depth was a decrease in numbers of 

 Nitzschia palea . N . frustulum was found most 

 commonly in a transition zone intermediate in 

 depth and in available light. 



17 



