Lake Mixing and Its Relationship to Epilimnetic Phosphorus in Shagawa Lake, Minnesota

Publisher Website
Google Scholar
Abstract
Using thermometry, the mixing regime was intensively studied in each of the two principal basins of Shagawa Lake. After the thermocline had migrated past the interbasin sill depth (6 m), it migrated more rapidly in the larger, windswept western basin than in the smaller, sheltered eastern basin. The metalimnetic eddy conductivities (Kz) were also larger in the west basin, ranging from 0.025 cm2∙s−1 during intervals of low wind energy up to 0.25 cm2∙s−1 during intervals bracketing powerful cold fronts. The bottom temperature increased rapidly in the west basin in response to mixing, reducing total water column stability and increasing susceptibility to a sudden "turnover." Because of these warm bottom temperatures and drainage basin influence, density currents sometimes carry cold water into the benthic zone of the west basin following heavy, cold rains in midsummer. Epilimnetic phosphorus concentrations in Shagawa Lake depend on linkages among weather, basin morphometry, and water column redox. Time series analysis of 8 yr of record reveals the special importance of two factors. First, the anoxic zone below the mixed layer must include an important fraction of the metalimnetic shelf sediments in the western basin (6–10 m zone). Second, the mixed layer must migrate downward and directly entrain water overlying the anoxic sediments. Time series analysis excludes the possibility of sediment release of phosphorus within the epilimnetic zone as the principal control on phosphorus in the pelagic mixed layer. For lakes with similar morphometry and climatic setting, and a previous history of sewage inputs, lake mixing complicates the trophic adjustment of the lake to a reduction in external phosphorus loading.