Book Description

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