Article \'. — Cliaiigcs in the Small Bottom Fauna of Peoria Lake, 

 ip^o to iij32. By Rcibekt E. Richardson. 



Introduction 



Early in July ]9v!2 the cooperative program of chemical and bio- 

 logical investigations of the middle Illinois River and Peoria Lake 

 begun in the summer of 1920 by the State Natural History Survey and 

 the State Water Survey was again taken up after an interval of two 

 years. The summer's work included, as one item among several others, 

 between the date of opening and the middle of September, quantitative 

 bottom dredgings at seventy-one stations in the channel and wide 

 waters of Peoria Lake and contiguous river between Chillicothe and 

 the Peoria and Pekin L'nion Railway Bridge opposite the lower part 

 of the city of Peoria. The dredging stations were distributed over 

 approximately the area covered by the thirty-five stations worked in 

 Peoria Lake in the summer of 1920, and covered a linear distance of 

 19.9 miles. Collections were made and handled as in 1920. Assist- 

 ance was furnished by Dr. C. P. Alexander in determining the larval 

 Chironomidae (midges), by Mr. F. C. Baker and by Dr. \^ Stei-ki on 

 snails of the family Sphaeriidac, and by Prof. Frank Smith and Dr. 

 J. P. Moore on the worms and leeches. The nitrogen- data on which 

 the 1921 curve of per cent free ammonia is based have been furnished 

 in advance of publication by the U. S. Public Health Service. 



Destruction of the Old Botto.m F.\un.\ 



In a paper* published in December, 1921, on the changes in the 

 bottom and shore fauna of the middle Illinois River, it was shown that 

 between the period 1913-1915 and the summer of 1920, there had been 

 an almost complete extermination of the older normal bottom popula- 

 tion over a stretch of around 90 miles of river, including Peoria Lake, 

 extending from Chillicothe to Beardsiown. The same jieriod also saw 



• Bui. ni. Nat. Hist. Surv., Vol. XIV. Art. 4. pp. 33-75. 



