362 



Annals New York Academy of Sciences 



river which have the same types of ecological habitats are studied at the same 

 time. The kinds of species in common are more variable than the numbers. 

 For example, Stations 1 and 6 on the Savannah River which are about 30 miles 

 apart were studied in June of 1960 and 187 species were identified at Station 1 

 and 54 per cent of these were found at Station 6. At Station 6146 species were 

 identitied and 75 per cent of these were found at Station 1. In October of 1960 

 when these two areas were studied the number of species at Station 1 was 184 

 and the number at Station 6 was 185. However, 75 per cent of the species at 

 Station 1 were at Station 6 and 75 per cent of the species at Station 6 were at 

 Station 1. 



This same principle as to similarity of numbers of species but differences in 

 kinds of species also holds for the hard water rivers we have studied. Often 

 the numbers of species are slightly less in natural hard water rivers than in 



40 r 



INDIVIDUALS = 1-2 2-4 4-8 8-|6 16-32 32-64 64" 128" 256" 512" 1024- 2048-4096" 8192-16384-32768- 



128 256 512 1024 2048 4096 8192 16384 32768 65536 



INTERVALS =0 1 2 3 4 5 6 7 8 9 1 1 1 12 13 14 15 16 



Figure 1. Ridley Creek, Pennsylvania 



soft water rivers. For example, in the Potomac River, a hard water river, from 

 April of 1959 to October of 1960 in one area studied the observed species varied 

 from 130 to 148 (average 144) as contrasted with a variation from 118 to 185 

 (average 161) in the Savannah River which is a soft water river, over a similar 

 period of time. 



In brackish waters such as the estuary of the York River the numbers of 

 species composing a diatom community sometimes are a little less than in a 

 soft water river. From November of 1956 to May of 1959 the number of 

 observed species varied from 108 to 147 (average, 130). However, in all of 

 these three types of rivers — soft, hard, and brackish water — the communities 

 are made up of many species most of which have relatively small populations 

 if the rivers are natural and not polluted. 



A different picture is found when the structure of diatom communities in 

 dystrophic streams is examined. In these there is a restricted diatom flora 

 which can live in these naturally acid streams high in humates. They are 

 species largely confined to the genera Eunotia and Frustidia and certain species 



