Upper Midwest Environmental Sciences Center
Products and Publications
Fiscal
Year 1999 Findings for the Long Term Resource Monitoring ProgramInterpretation of Ecological Conditions
The Upper Mississippi River Basin has been implicated as contributing disproportionately to nitrate delivery and subsequent environmental degradation in the Gulf of Mexico. In addition, high loading of nitrogen, phosphorus, and sediment to streams, lakes, and reservoirs within the Upper Mississippi River Basin may adversely affect local and regional resources. To manage sediment and nutrient movement within the basin, information is needed on the present and historical levels of these constituents and the mechanisms that control their movement. The LTRMP is uniquely qualified to contribute to such investigations.
During 1988–89, the abundance of submersed aquatic vegetation declined precipitously in extensive reaches of the Upper Mississippi River, coincident with a severe midwestern drought. This decline continued through 1994 (Wiener et al. 1998). The causes of this decline are unknown, but may include decreased light availability because of phytoplankton blooms, depletion of nutrients during low flows, herbicides, and grazing by fish (Wiener et al. 1998). The recovery of aquatic vegetation was hindered by high turbidity in backwaters, which reduced light availability (Kimber et al. 1995; Owens and Crumpton 1995), and by high water levels during the 1993 flood.
Yin et al. (2000a) concluded that in 1998 submersed aquatic vegetation was at or near the peak abundance recorded since 1991. The abundance of submersed aquatic vegetation was similar between 1998 and 1999 in Pools 4, 8, and 13. Collectively, these observations indicate that the Upper Mississippi River above Dam 13 has rebounded from the severe declines in aquatic vegetation that occurred during the drought of 1987–89 and the flood of 1993.
Estimated mean densities of fingernail clams in 1999 increased in Pools 4 and 8, relative to 1998, but decreased in Pools 13 and 26 and La Grange Pool (Figure 8). The reason for the pronounced increase in the mean densities of fingernail clams in Pool 8 is unknown, but could be related to improved conditions within the sediments. The abundance, standing crop, and taxonomic composition of benthic macroinvertebrates declined in some reaches during recent decades (reviewed by Wiener et al. 1998). In particular, populations of the fingernail clam declined significantly along a 700-km stretch of river from Minneapolis, Minnesota, to Keokuk, Iowa (Wilson et al. 1995). These declines and the subsequent slow recovery of fingernail clams may have been caused in part by the periodic buildup of toxic concentrations of ammonia in bottom sediments (Wilson et al. 1995; Wiener et al. 1998).
Increases in the abundance of zebra mussels may adversely affect certain native biota in the Upper Mississippi River. In particular, the river's unionid mussel fauna is seriously threatened by the zebra mussel, which attaches to the shells of native mussels. Since 1991, the frequency and intensity of unionid mussel infestation by zebra mussels have increased greatly in many areas of the Mississippi River. In other North American waters, unionid mussels have suffered severe mortality (up to 100%) within 1 to 2 years after heavy infestation by zebra mussels (Nalepa 1994; Schloesser and Nalepa 1994; Ricciardi et al. 1995; Whitney et al. 1995).
High densities of zebra mussels can also degrade water quality, indirectly harming other organisms. In particular, zebra mussels have caused major depletion of dissolved oxygen in affected reaches of the Seneca River in New York (Effler and Siegfried 1994) and the Illinois River (Sparks et al. 1994) where populations exceeded 30,000 mussels per square meter. The abundances of certain other exotic (nonnative) species has also increased in the river, posing potential threats to native fauna and flora (Wiener et al. 1998).
March 4, 2002Page Last Modified: April 17, 2018