DECLINES IN AQUATIC VEGETATION IN NAVIGATION POOL NO-8, UPPER MISSISSIPPI RIVER BETWEEN 1975 AND 1991

The biomass and frequency of occurrence of aquatic macrophytes observe d during 1975 and 1991 in Navigation Pool No. 8 of the upper Mississip pi River were compared. Samples were collected from contiguous backwat er, isolated backwater and impounded habitat types. Overall declines w ere observed in both frequency and biomass between the two years. Chan ges in depth, water clarity and bed sediment texture were also observe d. About 50% of the samples collected in 1991 had no vegetation, compa red with 20% in 1975. The greatest reduction in frequency occurred amo ng submergent taxa (77% in 1975 to 31% in 1991), whereas only slight d eclines were observed for emergent taxa. However, the mean biomass of both lifeforms decreased substantially (48 to 1 g/m(2) for submergent and 61 to 27 g/m(2) for emergent lifeforms). The mean total biomass de clined from 114 g/m(2) in 1975 to 35 g/m(2) in 1991. Declines were mos t evident in the impounded habitat, where the frequency of occurrence of submergent taxa decreased from 83% in 1975 to 11% in 1991, and mean biomass decreased from 90 to 1 g/m(2). Similar but less extensive red uctions in frequency and biomass of submergent taxa were observed in t he contiguous backwater habitat. In contrast, both the frequency and b iomass of floating-leaved macrophytes increased slightly in contiguous backwater habitats. Changes in frequency and biomass of macrophytes i n isolated backwater habitats were neligible between 1975 and 1991. It is hypothesized that changes in the macrophyte community structure ma y be related to physical changes associated with 57 years of impoundme nt.