Benthos — Mississippi River 



Historical Perspective. Very little information is available con- 

 cerning the status of benthic populations in the study reach of the 

 Mississippi River prior to the construction of the navigation dams. 

 Many of the early benthos studies that were conducted on other parts 

 of the Mississippi prior to dam construction were concerned mainly 

 with effects from the increased amounts of pollution entering the 

 river. 



At least one early study (Wiebe, 1927) has documented a reduction 

 in benthic species diversity and an increase in pollution-tolerant forms. 

 Presumably, similar consequences would have resulted from increased 

 pollution in the study reach of the river. Another early study demon- 

 strated the degradation of bottom fauna in Lake Keokuk (Ellis, 1931a) 

 following closure of the dam. 



Pre-Dam and Post-Dam Surveys. The earliest reported survey which 

 includes benthos and was conducted near the study reach of the river is 

 that of Garman (1890). He surveyed several sloughs, backwater areas, 

 and bottomland lakes near Quincy, Illinois (river mile 327). He collec- 

 ted and identified 10 species of snails, 2 species of fingernail clams, 

 6 species of unionid clams, 6 species of chironomid larvae, and numerous 

 other insect larvae. Many of the species Garman identified, including 

 the snails Valvata tricarinata and Lioplax subcarinatus and the mayfly 

 Hexagenia bilineata , were classified as clean-water species by Richard- 

 son in his study of the Illinois River (1928: 408, 409). 



No other pre-dam benthic surveys were conducted in the region of the 



study area. However, in the section of the Mississippi from Minneapolis 



(river mile 850) to Winona, Minnesota (river mile 725) Wiebe documented 



a reduction in clean-water forms due to the influence of pollution from 



the Twin Cities (1927). Clean-water forms such as planaria and mayfly 



nymphs were first taken in this study below the Twin Cities at Red 



Wing, Minnesota (river mile 790). At this station fingernail clams, 



2 

 leeches, and tubificid worms numbered more than 1,700/m (Wiebe, 1927: 



146). In conjunction with these findings, Wiebe found more species of 



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