Water Quality at "The Lake"

(continued from previous)

Limnological characteristics of Lake of the Ozarks, Missouri II: Measurements following formation of a large reservoir upstream. 1988. J.R. Jones and M.S. Kaiser.

The main objective of this study was to determine if water quality in LOTO changed after the completion of Truman Reservoir. The hypothesis was that Truman Reservoir would act as a settling basin, decreasing the amount of inorganic suspended sediment entering LOTO. Lower turbidity would allow more light to penetrate into the water column and lead to more algal growth, especially up-lake.

Data were collected during the summers of 1980-1986, with samples coming from the six previously monitored sites as well as two additional main lake sites (18- and 28-mile markers). Data showed that shifts in water quality were minimal and the strong longitudinal gradient in the main lake was still present during 1980-1986. Because inorganic suspended sediment measurements were limited to only one summer prior to the completion of Truman Dam, changes in suspended sediment concentrations could not be quantified.

Monitoring of surface water near the dam on Truman Reservoir revealed lower phosphorus and inorganic suspended sediment values than measured at the up-lake LOTO site. As expected, Truman Reservoir was acting as a settling basin with outflows containing moderate levels of nutrients and suspended sediment. It was theorized that releases from Truman Reservoir scour the Osage River bed, increasing the phosphorus and inorganic suspended sediment concentrations in water moving down-lake.

Chlorophyll concentrations at the up-lake site increased from 14.2 to 21.1 µg/L (average values for the two studies), even though phosphorus levels decreased from 92 to 78 µg/L. One possible explanation for the increase in chlorophyll relative to phosphorus (ratio increased to 0.27) is that a shift in the algal community might have occurred. Prior to the impounding of Truman Reservoir, the algal community in the up-lake portion of LOTO would have been dominated by species that were best suited for conditions in the Osage River. After 1979, the composition of the algal community in the upper portion of LOTO reflected that found in Truman Reservoir: species better suited for lake conditions.

scale

Fewer sediments suspended in the water allowed light to penetrate
deeper into the water column, which led to increased algal growth.
Water clarity remained constant.

When water quality data were adjusted to account for differences in seasonal inflow, sites across the lake were determined to have higher chlorophyll concentrations (about 25% higher), even though phosphorus levels had dropped by about 26%. Secchi transparency values across the lake remained relatively unchanged after the completion of Truman Reservoir. Along with potential shifts in the algal community, lower inorganic suspended sediment concentrations in LOTO may account for increased algal production. In Missouri, both inorganic suspended sediment levels and algal biomass determine Secchi transparency. Because algal chlorophyll concentrations increased across the lake during 1980-1986 while Secchi measurements remained unchanged, it seems reasonable to surmise that inorganic suspended sediment levels decreased. In essence, the increased clarity associated with decreased inorganic suspended sediment was countered by increased algal biomass, which was made possible by the more favorable light environment that resulted from lower inorganic suspended sediment levels.

 

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