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This study examined rates of internal phosphorus (P) loading from sediments in Lake Pepin, a natural impoundment located on the upper Mississippi River. Laboratory determinations of rates of P release from the sediment, measured using intact sediment core systems, were very high, with mean values ranging from 3.8 to 15 mg /sq m/day under oxic and anoxic conditions, respectively. Independently determined Fickean diffusional fluxes (mean of 4.9 mg /sq m/day), measured in situ under oxic conditions using sediment peepers, were not statistically different from those rates determined in the laboratory under similar temperature, pH, and oxygen conditions. The dominant inorganic P fractions in the sediments were NaOH-extractable P followed closely by HCl-extractable P. Under both oxic and anoxic conditions, significant correlations existed between rates of P release and the NHl4Cl- and NaOH extractable P fractions, suggesting that loosely bound and iron- and aluminum-bound sediment P contributed to P release from the sediments in this lake. Correlations between total sediment P and extractable sediment P fractions indicated that loosely bound and iron- and aluminum-bound P increased, while calcium-bound P decreased, with increasing total P content of the sediment. Results suggest that internal P loading from the sediments under oxic as well as anoxic conditions can play a very important role in the P economy of this system. jg p19.
Over a 3-year period (1994-96), suspended seston and phosphorus (P) dynamics were examined in the Upper Mississippi River (UMR) and naturally impounded Lake Pepin, with special attention to external and internal P loading processes that may be contributing to deteriorating water quality conditions. The Minnesota River accounted for most of the annual and summer suspended seston (^81 %) and total phosphorus (^44 %) load, while the Metropolitan Wastewater Treatment Plant (Metro Plant) accounted for ^18 % of annual soluble reactive phosphorus (SRP) loading to the UMR. External loading during the summer of suspended seston, total P, and SRP to Lake Pepin averaged ^30,000/m2/day, and ^60 mg/m2/day, respectively. The lake retained a substantial portion of the summer suspended seston load and was also a sink for total P. While total P concentrations generally declined in Lake Pepin from headwaters to outflow, SRP exhibited a trend of increasing concentration from headwaters to outflow with net SRP export during the summer. These contrasting patterns in total P and SRP indicated the occurrence of internal P loading and/or transformations in the water column of P from particulate to soluble phases. Internal diffusive P flux from profundal sediments, estimated from laboratory incubation systems under different temperature and redox conditions, averaged ^7.5 mg/m2/day during the summer under predominately oxic conditions over all years. Although this oxic rate was high relative to other eutrophic lakes, it accounted for
Over a 3-year period (1994-96), suspended seston and phosphorus (P) dynamics were examined in the Upper Mississippi River (UMR) and naturally impounded Lake Pepin, with special attention to external and internal P loading processes that may be contributing to deteriorating water quality conditions. The Minnesota River accounted for most of the annual and summer suspended seston (81 %) and total phosphorus (44 %) load, while the Metropolitan Wastewater Treatment Plant (Metro Plant) accounted for 18 % of annual soluble reactive phosphorus (SRP) loading to the UMR. External loading during the summer of suspended seston, total P, and SRP to Lake Pepin averaged 30,000/m2/day, and 60 mg/m2/day, respectively. The lake retained a substantial portion of the summer suspended seston load and was also a sink for total P. While total P concentrations generally declined in Lake Pepin from headwaters to outflow, SRP exhibited a trend of increasing concentration from headwaters to outflow with net SRP export during the summer. These contrasting patterns in total P and SRP indicated the occurrence of internal P loading and/or transformations in the water column of P from particulate to soluble phases. Internal diffusive P flux from profundal sediments, estimated from laboratory incubation systems under different temperature and redox conditions, averaged 7.5 mg/m2/day during the summer under predominately oxic conditions over all years. Although this oxic rate was high relative to other eutrophic lakes, it accounted for