1. Introduction 



Reach 15 of the Upper Mississippi River contains one of seven mussel refuges 

 established by the Illinois Department of Natural Resources, formerly the Illinois Department of 

 Conservation, and is home to the federally-endangered Higgin's-eye pearly mussel, Lampsilis 

 higginsi (Blodgett and Sparks 1987a, b, and c). The refuges were established in 1988 to protect 

 endangered or threatened mussels, to provide a source of native mussel species to repopulate 

 other areas, and to serve as unharvested reference areas for comparison with harvested areas. 



A massive die-off of native mussels occurred in the Upper Mississippi River from 1981 

 through 1986 (Blodgett and Sparks 1987a, b, and c). Large numbers of two econo- 

 mically important mussels species, the threeridge (Amblema plicata) and the washboard 

 (Megalonaias gigantea). died in Reaches 14 and 15 of the Mississippi River (Fritz 1983; 

 Blodgett and Sparks 1987c). Subsequent research to identify the causes of this die-off was 

 unsuccessful (Sparks et al, 1990) and the reason for the die-off remains a mystery. 



This document reports the results of sediment toxicity studies performed using 

 Ceriodaphnia dubia to determine whether there were any toxic sediments that may threaten the 

 refuges, including any toxicity that might linger from the 1981 to 1986 die-offs. Toxicity 

 assays were conducted on sediment porewaters collected from Reach 15 of the Mississippi 

 River during 1994 and 1995. Ceriodaphnia dubia acute toxicity and survival/reproduction 7- 

 day assays were performed on porewaters of collected sediments in the first year. Although 

 short-term toxicity studies using a standard reference organism. C. dubia. were the only tests 

 planned, findings from the first year of the study (1994) provided enough evidence to warrant 

 further studies. Work in the second year consisted of C. dubia survival/reproduction 7-day 



