Kofoid took plankton samples at one-week intervals throughout the 

 year in 1898, and the hydrograph for that year was "normal" (Kofoid, 

 1904: 18). We used only the samples Kofoid had taken in September in 

 the main channel, under stable low-flow conditions. The September 

 samples of Colbert et al. were likewise taken under stable, low-flow 

 conditions. Although Kofoid' s samples were taken upstream from 

 the study area, he conducted four studies (Kofoid, 1903: 273-283) of 

 the longitudinal distribution of plankton in the river (one study 

 included the lower 80 miles of the Illinois) and concluded that the 

 plankton in the main channel was remarkably uniform. Hence, Kofoid 's 

 samples from the Havana area are probably generally representative of 

 the main channel in both the middle and lower reaches of the Illinois 

 River. 



Kofoid used a pump to take an integrated vertical sample of 

 water from the main channel. He pumped 250 liters of water through 

 a plankton net made of Number 20 silk bolting-cloth. Colbert 

 et al. (1975: 25) also took an -integrated sample, consisting of 30 

 liters of water collected from the surface, mid-depth, and bottom 

 of the water column, with a three-liter Van Dorn bottle. The water 

 was poured through a Number 20 plankton net. Kofoid also used paper 

 filters to collect the small plankton that passed through the Number 

 20 net. The number of organisms per liter trapped on the filter 

 paper was sometimes 3 to 4 orders of magnitude greater than the number 

 trapped in the plankton net (Kofoid, 1904: 314-340). Fortunately, 

 Kofoid separated the counts obtained with the filter paper from the 

 counts obtained with the net (Kofoid, 1904: Table I, 314-340), so the 

 results reported in our Table 1 and Table 2 for the years 1898 and 

 1974 were both obtained using Number 20 plankton nets. 



Although Table 1 shows that the number of species of phytoplankton 

 apparently increased from 28 in 1898 to 68 in 1974, the increase is 

 attributable to advances in the taxonomy of algae, rather than to a 

 real change in the population. Kofoid (1904: 11-12) was well aware 

 of the taxonomic uncertainties and limitations existing in the 1890' s, 

 and indicated that he probably underestimated the actual number of 



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