Another comparison of Kofoid's 1898 data with a more recent survey 

 shows a decline in blue-green algae, but not in green algae or in total 

 number of algae. WAPORA (1974: 11-12) took 14 10-liter samples of water 

 during low-flow conditions on September 13 and 21, 1973 at Meredosia 

 (river mile 71). The water was passed through a ten-micron mesh nano- 

 plankton net, which would trap small plankton. The results may approxi- 

 mate those Kofoid obtained by using filter paper. The WAPORA biologists 

 took all of their samples from the surface. If the water column were 

 uniformly mixed, the samples would be comparable to those taken by 

 Kofoid. However, if more plankton occurred on the surface than in 

 deeper water, the WAPORA method would overestimate the abundance of 

 plankton in the water column. 



Number Per Liter 

 1898 a _ 1973 b 



75,430 

 252,214 

 2,140 

 30,930 

 360,714 



kofoid, 1904: 314-340. Includes filter paper collections. 

 b WAP0RA, 1974: D53-D54. Average of 14 samples taken Sept. 13 and 21, 1973. 



The above table shows that when small algae are included in the 

 plankton collections, the total number of algae has not shown a 

 decline between 1898 and 1973. However, there apparently has been a 

 marked decline in blue-green algae (Table 1 shows the same trend for 

 large blue-green algae) , which has been compensated for by increases 

 in other groups, most notably in diatoms. Many phycologists believe 

 diatoms are more tolerant of reduced light penetration and increased 

 abrasion associated with suspended sediment than other types of algae 

 (Colbert et al. , 1975: 59). Some species of blue-green algae are 

 considered nuisances when they grow to bloom proportions in nutrient- 

 rich waters. The reasons for the marked decline in blue-green algae 

 in the lower Illinois River are unknown. 



15 



