4. Discussion 



In the first year of the study we detected chronic sediment toxicity in porewaters collected 

 from the ALCOA site. Those porewaters significantly reduced the reproductive rate of 

 Ceriodaphnia dubia. Since chronic toxicity was detected in sediment from that location, a study 

 involving infaunal organisms (i.e., deertoe mussel. Truncilla truncata) was conducted during the 

 second year to determine if the chronic toxicity observed in C. dubia, a laboratory animal, could 

 also be detected with an infaunal organism. Unfortunately, because of poor health of the culture 

 and experimental error, no direct comparison of sediment toxicity was accomplished between 

 parallel tests involving C. dubia and T. truncata during the second year. 



Observation of the toxicity assays conducted both years of the study provided some 

 information concerning the quality of the sediments in Reach 15. In year one. organisms 

 exposed to ALCOA porewaters displayed the effects of chronic toxicity and in year two. chronic 

 toxicity responses were observed in organisms exposed to Campbell's Slough and Sylvan Slough 

 sediment porewaters. Since the sediments used in the studies were collected approximately one 

 year apart, we expected to see some difference in the results. Sediment toxicity episodes may be 

 brief and infrequent, allowing organisms to colonize between episodes (Sparks and Dillon 1993). 

 Also, toxic hot spots, as well as benthic organism distribution are known to be patchy (Sparks et 

 al. 1992: Sparks and Dillon 1993). 



The mussel filtering assay used in this study indicated mussel filtering rates were affected by 

 porewaters from Reach 15 of the Upper Mississippi River. Although we can assume a 53% 

 decrease in filtering rate indicates a chronic toxic effect, there is no information on the effect of a 

 decrease in filtering rate on the survival of a mussel species, such as a threshold filtering rate 



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