Declines in aquatic vegetation in Navigation Pool No. 8, UMR, between 1975 and 1991 Fischer, J. R. 1995. Declines in aquatic vegetation in Navigation Pool No. 8, Upper Mississippi River, between 1975 and 1991. M.S. thesis submitted to the faculty of the graduate school of the University of Wisconsin-La Crosse. Reprinted by U.S. Geological Survey, Environmental Management Technical Center, Onalaska, Wisconsin, March 1997. LTRMP 97-R003. 47 pp. (NTIS #PB96-197884) ABSTRACT I compared the taxonomic composition and biomass of aquatic macrophytes observed during 1975 and 1991 in Navigation Pool No. 8 of the upper Mississippi River. In 1991 aquatic vegetation and sediment samples were collected from contiguous backwater, isolated backwater, and impounded habitat types. Depth was also measured in each habitat, and all data were compared to a 1975 database established for the same sites. I observed declines in frequency of occurrence, richness, and biomass of vegetation between the two years. About 50% of the samples collected in 1991 had no vegetation compared to 20% in 1975. The greatest difference in frequency of occurrence was among submergent taxa (77% in 1975 and 31% in 1991), whereas the difference was slight for emergent and floating-leaved taxa. The mean biomass of total vegetation was significantly lesser in 1991 than in 1975 (p<0.001), with 97% and 56% reductions in submergent and emergent biomass, respectively. The most evident declines were observed in the impounded habitat--the frequency of occurrence of submergent taxa decreased from 83% in 1975 to 11% in 1991, and mean submergent biomass decreased by about 99%. Similar but less extensive declines in frequency and biomass of submergent taxa were observed in the contiguous backwater habitat. In contrast differences in the aquatic vegetation of the isolated backwater habitat were negligible. Mean depth decreased by 20 cm, 30 cm, and 70 cm in the impounded, contiguous backwater, and isolated backwater habitats, respectively. The silt-clay fraction of the sediment was about 75% greater in 1991 than 1975 in the isolated and contiguous backwater habitats, but was similar between years in the impounded habitat. Although numerous factors probably contributed to the decline, I propose that disturbance from wind action and nutrient limitation during low river flows were probably the most important factors affecting vegetation growth in the impounded habitat, whereas loss of depth, increased channelization, and decreased light penetration were likely the most critical factors in the contiguous backwaters. The isolated backwater habitat was less susceptible to change because of its isolation from direct river flow--most successional changes had probably already occurred by 1975. The differences I observed between 1975 and 1991 may be indicative of more widespread ecological and physical changes associated with impoundment; therefore, further study is recommended to monitor future changes and better understand the relationship between biological and physical dynamics of the upper Mississippi River. KEYWORDS Aquatic vegetation, aquatic macrophytes, Mississippi River, sediment, water depth, reservoirs