Turbulent mixing in lakes plays a crucial role in nutrient transport (e.g. phosphorus) and thus influences the water quality within the lake. Field experiments were conducted in Lake Mendota in the summer 2008 to determine the interior and boundary mixing. The interior mixing is generally several orders of magnitude smaller than the boundary mixing because the thermal stratification inhibits vertical mixing through the metalimnion. However, interior mixing plays a critical role in controlling the vertical distribution of biological and chemical constituents through the water column. The interior mixing coefficient was estimated using a dye experiment. The Rhodamine WT was released at a specific depth within the metalimnion, and then its vertical dispersion was measured over time. The estimated interior mixing coefficient is about 5.45 ×10-3 cm2/s. To quantify the boundary mixing, five thermistor chains were moored to record the time series of water temperature profiles along a sloping bottom. In addition, an in-situ underwater particle image velocimetry (UWPIV) technique was applied to obtain spatial distribution of instantaneous velocities right above the water-sediment interface. The measured velocity field was used to estimate the near-bed turbulent kinetic energy and dissipation rate. The results show that the boundary mixing was intense (turbulent dissipation rate was about 2×106 m2s-3) when internal wave upwelling occurred.

Keywords: Turbulent mixing; Dye experiment; Underwater particle image velocimetry (UWPIV).