Declines in aquatic vegetation in navigation pool no. 8, upper Mississippi River between 1975 and 1991

Abstract

The biomass and frequency of occurrence of aquatic macrophytes observed during 1975 and 1991 in Navigation Pool No. 8 of the upper Mississippi River were compared. Samples were collected from contiguous backwater, isolated backwater and impounded habitat types. Overall declines were observed in both frequency and biomass between the two years. Changes in depth, water clarity and bed sediment texture were also observed. About 50% of the samples collected in 1991 had no vegetation, compared with 20% in 1975. The greatest reduction in frequency occurred among subrnergent taxa (77% in 1975 to 31% in 1991), whereas only slight declines were observed for emergent taxa. However, the mean biomass of both lifeforms decreased substantially (48 t o 1 g/m2 for submergent and 61 to 27 g/m2 for emergent lifeforms).The mean total biomass declined from 1 14g/m2 in 1975 to 35 g/m2 in 1991. Declines were most 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/m2. Similar but less extensive reductions in frequency and biomass of subrnergent taxa were observed in the contiguous backwater habitat. In contrast, both the frequency and biomass of floating-leaved macrophytes increased slightly in contiguous backwater habitats. Changes in frequency and biomass of macrophytes in isolated backwater habitats were neligible between 1975 and 1991. It is hypothesized that changes in the rnacrophyte community structure may be related to physical changes associated with 57 years of impoundment.

Synthesis of Upper Mississippi River System submersed and emergent aquatic vegetation: past, present, and future

“…During the drought, a loss of SAV was documented in Pools 5, 7, 8, 9, 11, and 19 (Rogers, 1994). Fischer & Claflin (1995) documented declines in species richness and biomass from pre-(1975) to post-drought (1991) in Pool 8. Reasons for the rapid loss of SAV during the 3-year drought have yet to be determined, but may include one or both of the following: (1) increased nutrient concentrations in the water column due to low flows and high residence time may have triggered high densities of phytoplankton, epiphyton and filamentous algae (Vis et al, 2007), and (2) decreased nutrient concentrations in the substrate due to low runoff from agricultural landscapes (Rogers, 1994;Rogers et al, 1995a).…”

Plants as river system engineers

“…Aquatic species require inundated habitats and, even in still water environments, are sensitive to water depth and clarity (e.g. Fischer and Claflin, ; Sharp et al ., in press). In the flowing waters of Danish streams, Riis et al .…”

Seasonal effects of the zebra mussel (Dreissena polymorpha) on sediment denitrification rates in Pool 8 of the Upper Mississippi River

“…Side channels and contiguous backwaters have minimal water exchange with the main channel for much of the year, with the exception of flood periods (Sparks et al 1998). Backwaters are characterized by organic sediments and widespread macrophyte growth (Fischer and Claflin 1995). The impounded zone is a quasilentic section of open water created upriver of a low head dam ( Fig.…”