13S 



Aquatic Ei-nsyslcms — Our Lniiiii Rfsoiiives 



40 



73 76 79 82 85 88 91 

 Year 



Fig. 2. Mean densilies (± I stan- 

 dard error) of the llngemail clani 

 Miisciiliiim iniiisvir.'.iiifi in Pool U» 

 of the Upper Mississippi River 

 during ly??-"^!. Mean densities 

 that were too low to appear in the 

 chart are 1981^ (17 elams/m-), 

 144(1 (0). and IWl ( IS clams/m")^ 



7 14 21 28 4 11 18 25 2 

 Oct Nov 



Fig. 3. Numbers of canvasback 

 ducks (Avlhya valtsineria) in Pool 

 7 of the Upper Mississippi River 

 during four consecutive fall migra- 

 tions (1984-92) varied m relation 

 to the abundance of plant and ani- 

 mal food in the pool. 



(Jecline o\' mink on the refuge coincideti with 

 the probable period of most severe PCB conta- 

 mination in the river. Conversely, the pailial 

 recovery of mink populations that began in the 

 late I970"s coincided with a period of declining 

 PCB levels in riverine fishes (Hora 1984). R.B. 

 Dahlgren and K.L. Ensor (U.S. Fish and 

 Wildlife Service, personal communication) esti- 

 mate that a diet containing 33% fish, having 

 PCB concentrations similar to those in the early 

 I970"s. would contain enough PCB to prevent 

 reproduction in mink, based on experimental 

 toxicity studies (Platonow and Karstad 1973). 

 In 1989-91. PCB concentrations in mink from 

 the Upper Mississippi River in Minnesota 

 exceeded those in mink from all other areas of 

 the state except Lake Superior (Ensor et al. 

 1993). Recent studies show that PCBs continue 

 to enter or cycle within the riverine ecosystem 

 and that they are transfen'ed from the sediment 

 to higher trophic levels via the benthic food 

 chain (Steingiaeber et al. 1994). 



Ecosystem Health 



The declines in these riverine tlora and fauna 

 signal a deterioration in the health of this 

 ecosystem. In recent decades, populations of 

 fingernail clams, unionid mussels, ceilain other 

 invertebrates, submersed vegetation, migratory 

 waterfowl, and mink have decreased along 

 extensive reaches of the river. The Upper 

 Mississippi is often heralded as a multiple-use 

 resource, and human use of the river for naviga- 

 tion, hydropower, discharge of wastes, and 

 other purposes may increase while inputs of 

 sediment, nutrients, and chemicals from the 

 watershed continue. Yet the cumulative impacts 

 of humans may already exceed the assimilative 

 capacity of this ecosystem. 



Many complex questions concerning envi- 

 ronmental degradation, declining tlora and 

 fauna, and human impact on this ecosystem 

 need objective analysis and resolution. It is sus- 

 pected that mink populations declined in 

 response to PCB contamination and that finger- 

 nail clams declined in response to sediment tox- 

 icity, perhaps linked to low-flow conditions dur- 

 ing droughts (Wilson et al. 1994). The factors 

 causing most of the observed biotic declines are 

 largely unknown, however, hampering the 

 application of corrective measures. Several fac- 

 tors, for example, aie suspected of contributing 

 to declines in the unionid mussel fauna, includ- 

 ing habitat modification and degradation, con- 

 taminants, overharvest. commercial and recre- 

 ational navigation, and poor water quality 

 (Williams et al. 1993). The need for scientifi- 

 cally based, integrated resource management of 

 the Upper Mississippi is illustrated by the eco- 



nomic and ecological effects of the flood of 

 1993 on the river floodplain and its inhabitants. 

 Federal and stale agencies involved with 

 resource management need integrated, proac- 

 tive policies based on an understanding of the 

 ecological structuie and functioning of this 

 complex ecosystem. 



References 



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 PCB toxicity to mink, and effect on their reproduction. 

 Archives of Environmental Contamination and 

 Toxicology 6:274-292. 

 Brewer. S.K. 1992. Community structure of benthic 

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 Mississippi River: comparisons between 1975 and 1990. 

 M.S. thesis. University of Wisconsin-La Crosse. 84 pp. 

 Coon. T.G., J.W. Eckblad. and RM. Trygstad. 1977. 

 Relative abundance and growth of mussels (Mollusca: 

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