15. Between 1915 and 1964, the average density of midges in the 

 study reach of the Illinois River increased from 1.3 to 353 per square 

 meter, and the number of oligochaete worms increased from 2.6 to 2,579 

 per square meter, indicating the presence of soft mud bottoms, an in- 

 crease in the organic load in the river, and slight decreases in the 

 average dissolved oxygen levels. Snails and fingernail clams also 

 increased between 1915 and 1964, as did leeches, some of which prey 



on clams and snails. Fingernail clams generally thrive in areas where 

 there is a moderate amount of organic pollution, and where soft mud 

 bottoms are available. Between 1964 and the 1970' s, the fingernail clam 

 populations declined slightly (the declines are very slight, and perhaps 

 insignificant), Asiatic clams invaded the river, and the snails 

 either disappeared entirely or were reduced to such low numbers that 

 they did not show up in the recent collections. Asiatic clams probably 

 first entered the Illinois River in 1970-1971. Some toxic agent 

 present in the lower Illinois River may be eliminating the snails. 

 Pesticides may be implicated, since snails exposed to Illinois River 

 water rapidly accumulate dieldrin (within 24 hours) . 



16. Benthic studies done prior to the construction of the navi- 

 gation dams on the Mississippi were concerned with areas outside the 

 study reach. 



17. Since we were not able to find any surveys of the plankton in 

 the study reach of the Mississippi prior to the mid-1930's, we were, 

 not able to compare plankton populations in the Mississippi before and 

 after construction of the navigation dams. 



18. Comparison of plankton data on the Illinois River gathered by 

 Kofoid (1903 and 1904) in 1898 with recent data gathered at the same 

 season, under comparable low-flow conditions, shows that: (a) the 

 average number of small diatoms (Bacillariophyta ) doubled, (b) the number 

 of large diatoms remained the same, (c) small green algae ( Chlorophyta ) 

 increased, (d) large green algae declined, (e) blue-green algae 



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