Thomas O. Claflin | USGS Science

USGS2006

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 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 submergent 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 to 1 g/m 2 for submergent and 61 to 27 g/m 2 for emergent lifeforms). The mean total biomass declined from 114g/m 2 in 1975 to 35 g/m 2 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/m 2 . Similar but less extensive reductions in frequency and biomass of submergent 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 macrophyte community structure may be related to physical changes associated with 57 years of impoundment.

Mississippi River↗

USGS2006

Volume loss and mass balance for selected physicochemical constituents in Lake Pepin, upper Mississippi River, USA

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.

Minnesota, Wisconsin↗