Our Livini; Resources — Aqiialic Ecosyslems 



237 



8% of 721 samplL's taken in 1992 had densities 

 exceeding 100 llngernail clanis/ni- (9.3/t't-). 

 Corresponding mean densities of burrowing 

 mayflies in these areas ranged from 10 to 99/m- 

 (0.9-9.2/ft-). 



Wilson et al. (1994) hypothesized that the 

 declines in fingernail clams in Pools 2 to 9 were 

 linked to point-source pollution, and that the 

 declines in Pool 19 were linked to low-flow 

 conditions during drought. The causal mecha- 

 nisms by which low flow influences fingernail 

 clam abundance may involve unfavorable 

 changes in the chemistry of sediment pore 

 water. 



In Pool 8. the structure of benthic macroin- 

 vertebrate communities changed between 1975 

 and 1990 in all five habitats studied. Standing 

 crop of the benthos decreased significantly in 

 both open-water and bay habitats, and diversity 

 and abundance decreased in open-water habitat 

 (Brewer 1992). These declines suggest that the 

 standing crop of invertebrates has decreased 

 substantially in Pool 8 because open-water 

 habitat was 45% of the total area of the pool. 



The biodiversity of the unionid mussel fauna 

 in the Upper Mississippi River drainage has 

 declined from about 50 to 60 species in the 

 early 1920's to about 30 species in the 

 mid-1980"s. Many of these species are com- 

 mercially important; others are threatened or 

 endangered. Unionid mussels are further imper- 

 iled by the zebra mussel (Dreissena pohmor- 

 plui). which recently invaded the Illinois and 

 Upper Mississippi rivers. 



Rooted Aquatic Plants 



The abundance of submersed aquatic 

 plants — including wildcelery. which produces a 

 vegetative tuber important as food for certain 

 migratory waterfowl — declined along extensive 

 reaches of the Upper Mississippi River in the 

 late 1980"s. This decline has been attributed to 

 changing environmental conditions caused by 

 the severe midwestem drought of 1988-89. In 

 Pool 7. the abundance of wildcelery was fairly 

 stable during 1980-84, but declined greatly after 

 the dry summer of 1988. In Pools 5 through 9, 

 more than 4,000 ha ( 10,000 acres) of wildcelery 

 beds were lost (C.E. Korschgen. Upper 

 Mississippi Science Center, unpublished data). 

 Overall, the abundance of wildcelery and many 

 other submersed plants declined along 600 km 

 (375 mi) of river from Pool 5 to Pool 19. 

 Coincidentally, the abundance of the exotic 

 plant Eurasian watermilfoil (MyriophyUum spi- 

 catiim) has seemingly increased, particularly in 

 locations formerly occupied by wildcelery or 

 other native submersed plants. 



Migratory Birds 



Millions of migratory birds use the 

 Mississippi River corridor during fall and 

 spring migration. The river is critical in the life 

 cycle of many migratory birds because of its 

 north-to-south orientation and its nearly con- 

 tiguous habitat. Diving ducks, swans, pelicans, 

 and cormorants use the river's open waters. 

 Dabbling ducks, geese, herons, egrets, terns, 

 bitterns, rails, and many resident and 

 Neotropical songbirds use the shallow riverine 

 wetlands. Bottomland forests support migrating 

 and nesting songbirds, and nesting raptors, 

 herons, egrets, and waterfowl. 



The primary factor affecting the use of the 

 river ecosystem by birds is the production of 

 food by various plants and animals. The number 

 of birds in riverine habitats decreases rapidly if 

 preferred food resources are unavailable. The 

 use of Lake Onalaska (Pool 7) by canvasback 

 ducks (Ayrhya valisineria). for example, 

 decreased greatly when the abundances of their 

 preferred foods, wildcelery and benthic inverte- 

 brates (Korschgen 1989), declined in the late 

 I980's (Fig. 3). A gradual increase in foods in 

 1992 resulted in increased use by canvasbacks 

 (C.E. Korschgen, unpublished data). 



Numbers of other migratory waterfowl have 

 also decreased along the river corridor, reflect- 

 ing deterioration of habitat on the breeding 

 grounds and the river. The decrease in the abun- 

 dance of fingernail clams has adversely affected 

 watertowl that feed heavily on the small mol- 

 lusk, particularly lesser scaup {Aythya affiiiis). 

 The peak number of lesser scaup on Pool 19 

 during fall migrations, for example, has 

 decreased from 300.000-500,000 in the 1970"s 

 to fewer than 25,000 in 1993. 



Mink 



The abundance of mink (Miistela visoii) on 

 the Upper Mississippi River Refuge declined 

 precipitously during 1959-65, remained low 

 until about 1970, and then began to slowly 

 increase to numbers that are now less than half 

 those of the 1950"s (Dahlgren 1990). In con- 

 trast, mink populations in the adjoining states of 

 Iowa, Minnesota, and Wisconsin were relative- 

 ly stable during this period and did not exhibit 

 the pattern of decline and pailial recovery seen 

 in populations on the refuge. These patterns 

 indicate that some factor unique to the river cor- 

 ridor, not present in the mostly agricultural 

 watersheds of the adjoining states, caused the 

 decline of mink populations on the refuge. 



The survival and reproduction of mink are 

 adversely affected by dietary exposure to small 

 doses of polychlorinated biphenyls, (PCBs; 

 Aulerich and Ringer 1977; Wren 1991). The 



100 



50 



50 



25 



73 78 83 



200- 



100- 



0- 



50- 



25- 



E 



I 0^ 



t 73 



78 83 



'■■ 200- 



100 



On 

 7 



300- 

 200- 



100- 

 0- 



73 78 



73 



78 



83 



600- 

 500- 

 400- 

 300- 

 200- 

 100- 



73 



Pool 5A 



73 78 83 88 91 



Pool 6 



Pool 7 



83 88 91 



Pools 



I I TT 

 88 91 



Pools 



78 83 88 91 



Year 



Fig. 1. Mean density of the fin- 

 gernail clam Muscutium trcmsvcr- 

 siim in Pools 2, ."i, 5A. 6. 7. 8. and 

 9 of the Upper Mississippi River 

 during 197.V91. 



