Biota of the Upper Mississippi River Ecosystem

Biota of the Upper Mississippi River Ecosystem
		by *James Wiener* National Biological Service *Teresa Naimo* *Carl Korschgen* National Biological Service *Robert Dahlgren* U.S. Fish and Wildlife Service *Jennifer Sauer* *Kenneth Lubinski* *Sara Rogers* National Biological Service *Sandra Brewer* University of Wisconsin-LaCrosse
The Mississippi River is one of the world's major river systems in size, habitat and biotic diversity, and biotic productivity. The navigable Upper Mississippi River, extending 1,370 km (850 mi) from St. Anthony Falls (Minnesota) to the confluence with the Ohio River, has been impounded by 27 locks and dams to enhance commercial navigation. The reach between two consecutive locks and dams is termed a "pool." The upstream portions of many pools are similar to the unimpounded river, whereas the downstream reaches are similar to reservoirs.
The Upper Mississippi contains a diverse array of wetland, open-water, and floodplain habitats, including extensive national wildlife and fish refuges. Human activities, though, have greatly altered this river ecosystem; much of the watershed is intensively cultivated, and many tributary streams deliver substantial loads of nutrients, pesticides, and sediment from farmland. Pollutants also enter the river from point sources.
We examine recent temporal trends in the abundance of several key groups of organisms in the Upper Mississippi River and show that certain flora and fauna have declined along substantial reaches of the river. Our analysis is spatially constrained by available data to the reach of river extending from Pool 2 (near Minneapolis-St. Paul, Minnesota) to Pool 19 (near Keokuk, Iowa).
Information on the abundance of selected riverine biota was obtained by compiling historical data and by censusing or sampling. Bottom-dwelling invertebrates, collectively termed benthic macroinvertebrates, were extensively studied. Some of these organisms are important in the diets of fish and wildlife and are useful as biological indicators of toxic pollution. Data on densities of fingernail clams (Musculium transversum) from 1973 to 1992 were obtained from regional scientists, published literature, and sampling (Wilson et al. 1994) in selected pools (pools are numbered consecutively from upstream to downstream).
In 1992 benthic macroinvertebrates were sampled in soft sediments of five reaches of the Upper Mississippi and one reach of the Illinois River to estimate densities of fingernail clams and the burrowing mayfly (Hexagenia). In 1975 and 1990, benthic macroinvertebrates were extensively sampled in five habitats (marsh, bay, open water, side channel, and dredged side channel) in Pool 8 of the Upper Mississippi (near La Crosse, Wisconsin) to examine changes in abundance, biomass, and community structure (Brewer 1992). Data on the unionid mussel fauna in the river were obtained from the literature and other sources (Shimek 1921; Grier and Mueller 1922; Ellis 1931a, b; Dawley 1947; Finke 1966; Coon et al. 1977; Fuller 1978, 1979; Mathiak 1979; Perry 1979; Thiel et al. 1979; Ecological Analysts, Inc. 1981; Thiel 1981; Duncan and Thiel 1983; Holland-Bartels 1990).
Wildcelery plants (Vallisneria americana) were sampled each August during 1980-84 and 1989-93 in quadrants along 12 0.8-km (0.5-mi) transects in Lake Onalaska, a backwater lake in Pool 7. Numbers of canvasback ducks (Aythya valisineria) in Pool 7 during fall migration were determined by aerial surveys. Trends in the abundance of mink (Mustela vison) were assessed by examining indices of mink harvest per unit of trapping effort (total harvest/total number of trappers) on the Upper Mississippi River National Wildlife and Fish Refuge and in states along the river corridor (Dahlgren 1990).
Status and Trends
Benthic Macroinvertebrates

Densities of fingernail clams declined significantly (P ¾ 0.05) in five of eight pools examined (declines in Pools 2, 5, 7, 9, and 19; Figs. 1 and 2) along 700 km (435 mi) of river from Hastings, Minnesota, to Keokuk, Iowa. Densities in Pool 19, which had the longest historical record on fingernail clams, averaged 30,000/m² (2,800/ft²) in 1985 and decreased to zero in 1990 (Fig. 2). In 1992 densities of fingernail clams were still low in sampled areas on the Upper Mississippi and Illinois rivers, averaging 5-94 individuals/m² (0.5-8.7/ft²). Only 8% of 721 samples taken in 1992 had densities exceeding 100 fingernail clams/m² (9.3/ft²). Corresponding mean densities of burrowing mayflies in these areas ranged from 10 to 99/m² (0.9-9.2/ft²)

Fig. 1. Mean density of the fingernail clam Musculium transversum in Pools 2, 5, 5A, 6, 7, 8, and 9 of the Upper Mississippi River during 1973-91..

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 mechanisms by which low flow influences fingernail clam abundance may involve unfavorable changes in the chemistry of sediment pore water.

Fig. 2. Mean densities (±1 standard error) of the fingernail clam Musculium transversum in Pool 19 of the Upper Mississippi River during 1973-91. Mean densities that were too low to appear in the chart are 1989 (17 clams/m²), 1990 (0), and 1991 (18 clams/m²).

In Pool 8, the structure of benthic macroinvertebrate 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 imperiled by the zebra mussel (Dreissena polymorpha), 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 midwestern 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 (Myriophyllum spicatum) 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 contiguous 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 (Aythya valisineria), for example, decreased greatly when the abundances of their preferred foods, wildcelery and benthic invertebrates (Korschgen 1989), declined in the late 1980's (Fig. 3). A gradual increase in foods in 1992 resulted in increased use by canvasbacks (C.E. Korschgen, unpublished data).

Fig. 3. Numbers of canvasback ducks (Aythya valisineria) in Pool 7 of the Upper Mississippi River during four consecutive fall migrations (1989-92) varied in relation to the abundance of plant and animal food in the pool.

Numbers of other migratory waterfowl have also decreased along the river corridor, reflecting deterioration of habitat on the breeding grounds and the river. The decrease in the abundance of fingernail clams has adversely affected waterfowl that feed heavily on the small mollusk, particularly lesser scaup (Aythya affinis). 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 (Mustela vison) 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 contrast, mink populations in the adjoining states of Iowa, Minnesota, and Wisconsin were relatively stable during this period and did not exhibit the pattern of decline and partial recovery seen in populations on the refuge. These patterns indicate that some factor unique to the river corridor, 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 decline of mink on the refuge coincided with the probable period of most severe PCB contamination in the river. Conversely, the partial recovery of mink populations that began in the late 1970'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) estimate that a diet containing 33% fish, having PCB concentrations similar to those in the early 1970'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 transferred from the sediment to higher trophic levels via the benthic food chain (Steingraeber et al. 1994).
Ecosystem Health
The declines in these riverine flora and fauna signal a deterioration in the health of this ecosystem. In recent decades, populations of fingernail clams, unionid mussels, certain 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 navigation, 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 environmental degradation, declining flora and fauna, and human impact on this ecosystem need objective analysis and resolution. It is suspected that mink populations declined in response to PCB contamination and that fingernail clams declined in response to sediment toxicity, perhaps linked to low-flow conditions during 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 factors, for example, are suspected of contributing to declines in the unionid mussel fauna, including habitat modification and degradation, con-taminants, overharvest, commercial and recreational navigation, and poor water quality (Williams et al. 1993). The need for scientifically based, integrated resource management of the Upper Mississippi is illustrated by the economic and ecological effects of the flood of 1993 on the river floodplain and its inhabitants. Federal and state agencies involved with resource management need integrated, proactive policies based on an understanding of the ecological structure and functioning of this complex ecosystem.
		For further information: James Wiener National Biological Service Upper Mississippi Science Center PO Box 818 La Crosse, WI 54602

References
Aulerich, R.J., and R.K. Ringer. 1977. Current status of PCB toxicity to mink, and effect on their reproduction. Archives of Environmental Contamination and Toxicology 6:279-292. Brewer, S.K. 1992. Community structure of benthic macroinvertebrates in Navigation Pool no. 8, Upper Mississippi River: comparisons between 1975 and 1990. M.S. thesis, University of Wisconsin-La Crosse. 84 pp. Coon, T.G., J.W. Eckblad, and P.M. Trygstad. 1977. Relative abundance and growth of mussels (Mollusca: Eulamellibranchia) in Pools 8, 9, and 10 of the Mississippi River. Freshwater Biology 7:279-285. Dahlgren, R.B. 1990. Fifty years of fur harvest on the Upper Mississippi River National Wildlife and Fish Refuge: consistencies, anomalies, and economics. Pages 142-160 in Proceedings of the 46th Annual Meeting of the Upper Mississippi River Conservation Committee, Rock Island, IL. Dawley, C. 1947. Distribution of aquatic mollusks in Minnesota. American Midland Naturalist 38:671-697. Duncan, R., and P.A. Thiel. 1983. A survey of the mussel densities in Pool 10 of the Upper Mississippi River. Wisconsin Department of Natural Resources Tech. Bull. 139. 14 pp. Ecological Analysts, Inc. 1981. Survey of freshwater mussels (Pelecypoda: Unionacea) at selected sites in Pools 11 through 24 of the Mississippi River. Final Rep. 9031. U.S. Army Corps of Engineers, Rock Island, IL. 188 pp. Ellis, M.M. 1931a. A survey of conditions affecting fisheries in the Upper Mississippi River. U.S. Bureau of Fisheries Circular 5:1-18. Ellis, M.M. 1931b. Some factors affecting the replacement of the commercial fresh-water mussels. U.S. Bureau of Fisheries Circular 7:1-10. Ensor, K.L., W.C. Pitt, and D.D. Helwig. 1993. Contaminants in Minnesota wildlife 1989-1991. Minnesota Pollution Control Agency, St. Paul. 75 pp. Finke, A.H. 1966. Report of a mussel survey in Pools 4 (Lake Pepin), 5, 6, 7, and 9 of the Upper Mississippi River during 1965. Wisconsin Department of Natural Resources, La Crosse. 5 pp. Fuller, S.L.H. 1978. Fresh-water mussels (Mollusca: Bivalvia: Unionidae) of the Upper Mississippi River: observations at selected sites within the 9-foot channel navigation project on behalf of the U.S. Army Corps of Engineers. Final Rep. 78-33. Academy of Natural Sciences, Philadelphia, PA. 400 pp. Fuller, S.L.H. 1979. Historical and current distribution of fresh-water mussels (Mollusca: Bivalvia: Unionidae) in the Upper Mississippi River. Pages 71-119 in J.L. Rasmussen, ed. Proceedings of the UMRCC Symposium on Upper Mississippi River Bivalve Mollusks. Upper Mississippi River Conservation Committee, Rock Island, IL.	Grier, N.M., and J.F. Mueller. 1922. Notes on the naiad fauna of the Upper Mississippi River. II. The naiades of the Upper Mississippi drainage. Nautilus 36:46-49, 96-103. Holland-Bartels, L.E. 1990. Physical factors and their influence on the mussel fauna of a main channel border habitat of the Upper Mississippi River. Journal of the North American Benthological Society 9:327-335. Hora, M.E. 1984. Polychlorinated biphenyls (PCBs) in common carp (Cyprinus carpio) of the Upper Mississippi River. Pages 231-239 in J.G. Wiener, R.V. Anderson, and D.R. McConville, eds. Contaminants in the Upper Mississippi River. Butterworths, Stoneham, MA. Korschgen, C.E. 1989. Riverine and deepwater habitats for diving ducks. Pages 157-180 in L.M. Smith, R.L. Peterson, and R.M. Kaminski, eds. Habitat management for migrating and wintering waterfowl in North America. Texas Tech University Press, Lubbock. Mathiak, H.A. 1979. A river survey of the unionid mussels of Wisconsin, 1973-1977. Sand Shell Press, Horicon, WI. 75 pp. Perry, E.W. 1979. A survey of Upper Mississippi River mussels. Pages 118-139 in J.L. Rasmussen, ed. A compendium of fishery information on the Upper Mississippi River. 2nd ed. Upper Mississippi River Conservation Committee, Rock Island, IL. Platonow, N.S., and L.H. Karstad. 1973. Dietary effect of polychlorinated biphenyls on mink. Canadian Journal of Comparative Medicine 37:391-400. Shimek, B. 1921. Mollusks of the McGregor, Iowa, Region I. Iowa Conservationist 5:1. Steingraeber, M.T., T.R. Schwartz, J.G. Wiener, and J.A. Lebo. 1994. Polychlorinated biphenyl congeners in emergent mayflies from the Upper Mississippi River. Environmental Science and Technology 28:707-714. Thiel, P.A. 1981. Survey of unionid mussels in the Upper Mississippi River (Pools 3-11). Tech. Bull. 124. Wisconsin Department of Natural Resources, Madison. 24 pp. Thiel, P.A., M. Talbot, and J. Holzer. 1979. Survey of mussels in the Upper Mississippi River, Pools 3 through 8. Pages 148-156 in J.L. Rasmussen, ed. Proceedings of the UMRCC Symposium on Upper Mississippi River Bivalve Mollusks. Upper Mississippi River Conservation Committee, Rock Island, IL. Williams, J.D., M.L. Warren, Jr., K.S. Cummings, J.L. Harris, and R.J. Neves. 1993. Conservation status of freshwater mussels of the United States and Canada. Fisheries 18:6-22. Wilson, D.M., T.J. Naimo, J.G. Wiener, R.V. Anderson, M.B. Sandheinrich, and R.E. Sparks. 1994. Declining populations of the fingernail clam Musculium transversum in the Upper Mississippi River. Hydrobiologia. In press. Wren, C.D. 1991. Cause-effect linkages between chemicals and populations of mink (Mustela vison) and otter (Lutra canadensis) in the Great Lakes Basin. Journal of Toxicology and Environmental Health 33:549-585.

References

Aulerich, R.J., and R.K. Ringer. 1977. Current status of PCB toxicity to mink, and effect on their reproduction. Archives of Environmental Contamination and Toxicology 6:279-292.

Brewer, S.K. 1992. Community structure of benthic macroinvertebrates in Navigation Pool no. 8, Upper Mississippi River: comparisons between 1975 and 1990. M.S. thesis, University of Wisconsin-La Crosse. 84 pp.

Coon, T.G., J.W. Eckblad, and P.M. Trygstad. 1977. Relative abundance and growth of mussels (Mollusca: Eulamellibranchia) in Pools 8, 9, and 10 of the Mississippi River. Freshwater Biology 7:279-285.

Dahlgren, R.B. 1990. Fifty years of fur harvest on the Upper Mississippi River National Wildlife and Fish Refuge: consistencies, anomalies, and economics. Pages 142-160 in Proceedings of the 46th Annual Meeting of the Upper Mississippi River Conservation Committee, Rock Island, IL.

Dawley, C. 1947. Distribution of aquatic mollusks in Minnesota. American Midland Naturalist 38:671-697.

Duncan, R., and P.A. Thiel. 1983. A survey of the mussel densities in Pool 10 of the Upper Mississippi River. Wisconsin Department of Natural Resources Tech. Bull. 139. 14 pp.

Ecological Analysts, Inc. 1981. Survey of freshwater mussels (Pelecypoda: Unionacea) at selected sites in Pools 11 through 24 of the Mississippi River. Final Rep. 9031. U.S. Army Corps of Engineers, Rock Island, IL. 188 pp.

Ellis, M.M. 1931a. A survey of conditions affecting fisheries in the Upper Mississippi River. U.S. Bureau of Fisheries Circular 5:1-18.

Ellis, M.M. 1931b. Some factors affecting the replacement of the commercial fresh-water mussels. U.S. Bureau of Fisheries Circular 7:1-10.

Ensor, K.L., W.C. Pitt, and D.D. Helwig. 1993. Contaminants in Minnesota wildlife 1989-1991. Minnesota Pollution Control Agency, St. Paul. 75 pp.

Finke, A.H. 1966. Report of a mussel survey in Pools 4 (Lake Pepin), 5, 6, 7, and 9 of the Upper Mississippi River during 1965. Wisconsin Department of Natural Resources, La Crosse. 5 pp.

Fuller, S.L.H. 1978. Fresh-water mussels (Mollusca: Bivalvia: Unionidae) of the Upper Mississippi River: observations at selected sites within the 9-foot channel navigation project on behalf of the U.S. Army Corps of Engineers. Final Rep. 78-33. Academy of Natural Sciences, Philadelphia, PA. 400 pp.

Fuller, S.L.H. 1979. Historical and current distribution of fresh-water mussels (Mollusca: Bivalvia: Unionidae) in the Upper Mississippi River. Pages 71-119 in J.L. Rasmussen, ed. Proceedings of the UMRCC Symposium on Upper Mississippi River Bivalve Mollusks. Upper Mississippi River Conservation Committee, Rock Island, IL.

Grier, N.M., and J.F. Mueller. 1922. Notes on the naiad fauna of the Upper Mississippi River. II. The naiades of the Upper Mississippi drainage. Nautilus 36:46-49, 96-103.

Holland-Bartels, L.E. 1990. Physical factors and their influence on the mussel fauna of a main channel border habitat of the Upper Mississippi River. Journal of the North American Benthological Society 9:327-335.

Hora, M.E. 1984. Polychlorinated biphenyls (PCBs) in common carp (Cyprinus carpio) of the Upper Mississippi River. Pages 231-239 in J.G. Wiener, R.V. Anderson, and D.R. McConville, eds. Contaminants in the Upper Mississippi River. Butterworths, Stoneham, MA.

Korschgen, C.E. 1989. Riverine and deepwater habitats for diving ducks. Pages 157-180 in L.M. Smith, R.L. Peterson, and R.M. Kaminski, eds. Habitat management for migrating and wintering waterfowl in North America. Texas Tech University Press, Lubbock.

Mathiak, H.A. 1979. A river survey of the unionid mussels of Wisconsin, 1973-1977. Sand Shell Press, Horicon, WI. 75 pp.

Perry, E.W. 1979. A survey of Upper Mississippi River mussels. Pages 118-139 in J.L. Rasmussen, ed. A compendium of fishery information on the Upper Mississippi River. 2nd ed. Upper Mississippi River Conservation Committee, Rock Island, IL.

Platonow, N.S., and L.H. Karstad. 1973. Dietary effect of polychlorinated biphenyls on mink. Canadian Journal of Comparative Medicine 37:391-400.

Shimek, B. 1921. Mollusks of the McGregor, Iowa, Region I. Iowa Conservationist 5:1.

Steingraeber, M.T., T.R. Schwartz, J.G. Wiener, and J.A. Lebo. 1994. Polychlorinated biphenyl congeners in emergent mayflies from the Upper Mississippi River. Environmental Science and Technology 28:707-714.

Thiel, P.A. 1981. Survey of unionid mussels in the Upper Mississippi River (Pools 3-11). Tech. Bull. 124. Wisconsin Department of Natural Resources, Madison. 24 pp.

Thiel, P.A., M. Talbot, and J. Holzer. 1979. Survey of mussels in the Upper Mississippi River, Pools 3 through 8. Pages 148-156 in J.L. Rasmussen, ed. Proceedings of the UMRCC Symposium on Upper Mississippi River Bivalve Mollusks. Upper Mississippi River Conservation Committee, Rock Island, IL.

Williams, J.D., M.L. Warren, Jr., K.S. Cummings, J.L. Harris, and R.J. Neves. 1993. Conservation status of freshwater mussels of the United States and Canada. Fisheries 18:6-22.

Wilson, D.M., T.J. Naimo, J.G. Wiener, R.V. Anderson, M.B. Sandheinrich, and R.E. Sparks. 1994. Declining populations of the fingernail clam Musculium transversum in the Upper Mississippi River. Hydrobiologia. In press.

Wren, C.D. 1991. Cause-effect linkages between chemicals and populations of mink (Mustela vison) and otter (Lutra canadensis) in the Great Lakes Basin. Journal of Toxicology and Environmental Health 33:549-585.

Biota of the Upper Mississippi River Ecosystem

Status and Trends