Internal phosphorus loading in Lake Pepin (Minnesota-Wisconsin)

Assessments of phosphorus (P) loadings are essential for determining the trophic state of lakes and reservoirs and potential management alternatives. Historically, these assessments have focused primarily on external P loading from the watershed (Vollenweider 1976) and internal P loading from anoxic hypolimnia (Nürnberg 1984). Increasing concerns over deteriorating water quality and accelerated eutrophication of Lake Pepin, a natural impoundment located on the upper Mississippi River, have led to a detailed examination of external P loadings and point source P contributions from the watershed (Minnesota Pollution Control Agency 1993a and b, Lung and Larson 1994). One P control measure presently under consideration is the reduction of point source P loads from the Metropolitan Wastewater Treatment Plant (Lung and Larson 1994), located 50 miles upstream of Lake Pepin in St. Paul, MN. The treatment plant accounts for about 20 percent of the mean total external P load to the lake (Minnesota Pollution Control Agency 1993a). Internal P loading from sediments can often delay lake recovery substantially following reductions in external P loading (Welch 1977, Larson et al. 1979, Ryding 1981). This is an important concern in evaluating P reduction measures for Lake Pepin. While internal loading via P release from anoxic sediments has been well documented (Nürnberg 1984, 1987), little is known about the role of P release from sediments under oxic conditions. While most lake restoration efforts have ignored the potential for P release from sediments under oxic conditions, there is strong evidence that even oxic sediments can contribute substantially to the P economy of lakes (Drake and Heaney 1987, James and Barko 1991). P release from sediments under oxic conditions may be a very important component of the P budget of Lake Pepin, which stratifies thermally only intermittently during the summer. Rates of P release from the sediments of Lake Pepin are estimated based on results of laboratory studies conducted under controlled redox and temperature conditions. These rates are compared with diffusional fluxes of P calculated from in situ profiles of P in the porewater. Finally, different sediment P pools and correlations with P release from the sediment are examined. The objective is to evaluate the importance of internal P loading from oxic as well as anoxic sediment as components of the P budget of Lake Pepin.