length of time required for the water to pass down- 

 stream from headwaters sources, or, as he regarded 

 it, as the "age" of the water. Thus there is an 

 initial increase in plankton with time and distance 

 going downstream. Certain streams (e.g., the 

 Illinois River and the Rock River, both in U.S.A.) 

 exhibit a headwaters area low in plankton and a 

 middle region rich in plankton, followed by a con- 

 sistent decline in plankton in the lower course. 

 This decline is, however, not universal, and the 

 phenomenon can not yet be correlated with a given 

 length of course or degree of eutrophy. Conditions 

 in a mature stream may be expected to approach 

 those of late or middle-age ponds in the neighbor- 

 hood of the stream whose waters have a similar 

 chemical composition (Blum, 1956). 



An ambitious attempt to compare the flora of 

 different streams has been made by Budde (1930) 

 who reviewed several published works on European 

 and other rivers. Of the British streams studied by 

 Butcher and his co-workers, the Itchen, Test, 

 Bristol Avon, Hampshire Avon, and Lark seem to be 

 somewhat similar physico-chemically and to have 



many parallels in their algal vegetation (Butcher, 

 193 2) . The work of Scheele on diatoms of the 

 Fulda (Germany) has shown that the tolerant, ubiq- 

 uitous species increase from source to mouth and 

 that the species characteristic of springs show a 

 corresponding decrease (Scheele, 1952). 



The changes in benthic algae over the course 

 of a stream from source to mouth have been inves- 

 tigated by many workers. One of the more exhaus- 

 tive treatises of this nature is the work of Lauter- 

 born (1910, 1916, 1918) on the Rhine vegetaUon. 

 The great variability in size, profile, geology, de- 

 gree of pollution, and other atrributes of the 

 streams studied does not admit of many compari- 

 sons of their respective floras, and little informa- 

 tion of general application can be drawn from these 

 studies until an extensive analysis of the data con- 

 tained in them has been assembled. 



As an example of some of the changes which 

 may be expected in the algal flora on the profile of 

 a stream, Table 1, condensed from Butcher (1947) 

 shows some of the changes as determined quanti- 

 tatively in his work. The table shows the response 



TABLE 1 



(Condensed from Butcher, 1947) 



ALGAL COLONIZATION ON BLANK GLASS SLIDES SUBMERSED IN A POLLUTED STREAM 



Totals under the different algal species identified show average growth in numbers per square millimeter of 

 sUde surface on slides submersed in the River Trent (England) above and below outfalls from tar disUUing 

 industries. Collections were taken from slides at intervals of 20 days from April to October, 1938. The 

 river water showed a pH of about 7.5. The station above the source of pollution, and that at 35 miles 

 below this source of pollution may be considered oligosaprobic . Stations from 2-8 mi. below may be con- 

 sidered polysaprobic, 10-17 mi. below, mesosaprobic . 



19 



