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.
