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.
Lake Pepin is a large, natural riverine lake in the upper Mississippi River downstream of the Twin Cities metropolitan area and the confluence with the Minnesota River, which are sources of suspended sediments and pollutants (nutrients and potentially toxic materials). The lake has a history of water quality problems and has been an efficient trap for suspended sediment and sediment-associated contaminants. Based on bathymetric survey data, the loss of volume in Lake Pepin between 1897 and 1986 was estimated. The mass balance of the lake for total suspended solids, chlorophyll a, total and dissolved reactive phosphorus and total nitrogen for 9 June 1987 to 4 June 1988 was also estimated. Water was sampled at the inflow, mid-reach, and outflow of the lake. Lake Pepin is very eutrophic, based on concentrations of phosphorus, nitrogen and chlorophyll a. The lake volume decreased by approximately 21% between 1897 and 1986.Longitudinally, the greatest fraction of the whole-lake volume loss occurred in the upper lake (45%). Based on mass balance calculations, the lake trapped about half of the suspended solids entering the lake, but it had a small net export of chlorophyll a. The lake was a sink for phosphorus and nitrogen; however, it had a net export of total phosphorus at times during low flows in the summer of 1987. Internal loading of dissolved reactive phosphorus was prevalent during the summer of 1987. The only substantial export of total nitrogen occurred in June 1987 during a bloom of cyanobacteria. The lake should continue to be an efficient trap for suspended sediment and associated contaminants, but its trapping efficiency will continue to decline slowly as lake volume decreases. Lake Pepin will probably continue to experience water quality problems, such as nuisance algal growths and low dissolved oxygen, especially during summer low flows.
This study was conducted to determine some effects of commercial and recreational traffic on the resuspension of sediment in Navigation Pool No. 9 of the Upper Mississippi River. Fifty commercial vessel passages were examined at five different main channel locations and at side channels that were adjacent to each of the main channel locations. Sixteen recreational vessel passages were examined at one main channel location and its adjacent side channel and at a channel located in the backwaters. The backwater channel was not directly influenced by navigation in the main channel. Changes in total non-filterable residue (TNFR) and average particle size of suspended silts were used to assess some effects of navigation. Seventy-eight percent of the commercial vessel passages resulted in significant increases of TNFR and/ or average particle size in the water column. In the main channel, TNFR increased from 3.4% to 15% above ambient levels; in the side channels, increases ranged from 2.5% to 21.7%. The average diameter of the resuspended silts increased by 0.21 to 2.34 pm. In the main channel, 50% of the recreational vessel passages caused increased TNFR and all passages increased average particle size. In the backwater channel, all of the recreational passages caused increases in TNFR and average particle size. Total resuspended sediment transported downstream ranged from an estimated 0.82 to 1015.7 mTons/passage in the main channel for commercial vessels, 0.39 to 0.64 mTons/passage in the main channel for recreational vessels, 0.22 to 28.12 mTons/passage in side channels due to commercial vessels in the main channel, and 0.54 to 2.08 mTons/passage in the backwater channel for recreational vessels. Bed-sediment composition, location of the vessels in the channel, channel geometry, the number of successive passages, and vessel speed were identified as factors that affected the magnitude of the resuspension.
The diversity, evenness, similarity, standing crop and abundance of benthic macroinvertebrates in Navigation Pool 8 of the upper Mississippi River were compared in 1975 and 1990. Macroinvertebrates were collected in midsummer from four habitats: marsh, bay, open water and side channel. These habitat types accounted for 64% of the total habitat area in the impoundment. The community structure changed in all four habitats; the percentage similarities between 1975 and 1990 were lowest in open water (19%), intermediate in bays (44%) and side channels (50%) and highest in marshes (62%). Macroinvertebrate diversity and evenness were not significantly different between 1975 and 1990. The standing crop decreased in all habitat types. The abundance decreased in all habitats except bays between 1975 and 1990. The greatest changes in standing crop and community composition occurred in bays and open water. Because the bays and open water habitat comprise 55% of the total area, it is possible that these declines may signal degradation of the aquatic habitat in the entire reservoir. Changes in macroinvertebrate community structure and standing crop may have been due to unstable sediment conditions caused by erosion of islands, resuspension of sediments and loss of aquatic macrophytes and depth.
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