species present: 



During the progress of this work, which was begun in 1894, 

 every effort was made to secure all pertinent literature bearing 

 on the genera of plants and animals represented in the plank- 

 ton, and so far as possible in the enumeration of the col- 

 lections the individuals were referred to "species" already ^ 

 described, or, in default of this, recorded as unidentified. 

 In some groups — notably the desmids, diatoms, and unicel- 

 lular algae — it was not possible under the conditions of 

 plankton enumeration to apply to all the individuals enumera- 

 ted the fine distinctions which specialists in these groups 

 have made. ... the difficulty lies not so much in finding 

 representatives of these closely related species, but, rather, 

 in drawing the lines between them and placing every individual 

 enumerated in the proper pigeon-hole. To avoid this difficulty, 

 the separation was not attempted in every case. 



In Table 1, the increase in the number of species of diatoms (Bacil- 

 lariophyta ) in particular from four species in 1898 to 40 species in 

 1974 has more to do with the publication of The Diatoms of the United 

 stat es (Patrick and Reimer, 1966) than with a real change in the 

 phytoplankton. 



The number of algae per liter, taken in Number 20 plankton nets, 

 apparently did decline dramatically from an average of 176,887 in 1898 

 to 3,230 in 1974 (Table 1 ) . The number of diatoms (Bacillariophyta) 

 remained about the same, while the green algae (Chlorophyta) declined 

 by a factor of 10 and the blue-green algae (Cyanophyta) declined by 

 a factor of 1,000. Note that the results obtained by Colbert et al. at mile 2.5 

 (1975: E-28) were excluded from Table 1 because the number of organisms 

 and number of species were much lower than at their other six sampling 

 stations. The reduction in current velocity associated with the 

 construction of the dam at Alton should have favored the green and 

 blue-green algae. However, the potentially beneficial effects of the 

 dam on phytoplankton have apparently been overridden by the increasing 

 turbidity of the river (Mills et al. , 1966: 7). Turbidity reduces 

 light penetration, thus depressing photosynthesis by algae, and the 

 suspended sediment may physically abrade algae. 



13 



