Diffusive and Kinetic Fluxes of Phosphorus from Sediments in Relation to Phosphorus Dynamics in Lake Pepin, Upper Mississippi River

摘要

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 <15 percent of both the external total P and SRP loads to Lake Pepin. It did, however, account for 30 to 56 percent of the net SRP export from the lake during the summer. Sediments in Lake Pepin also had a high equilibrium P concentration (EPC = 0.162 mg/L +/- 0.018 S.E.) and native adsorbed P pool (NAP = 0.178 mg/g +/- 0.058 S.E. ) at equilibrium.