Observations and environmental implications of variability in the vertical turbulent mixing in Lake Simcoe

A field study was conducted in Lake Simcoe to examine variations in turbulent mixing rates for a 40-day period in late summer of 2011. High sampling frequency records of water currents and temperature profiles were acquired using an acoustic Doppler current profiler (ADCP) and a chain of fast-response temperature loggers respectively. The acquired data were processed to quantify the relative strengths of the stratification in relation to the velocity shear, expressed by the dimensionless gradient Richardson number (Ri_g). We then used the calculated Ri_g as a proxy to estimate vertical turbulent eddy diffusivity (K_z) in the stratified water column of the lake. K_z exhibited large spatial and temporal variability with frequent high values in the epilimnion (K_z ~ 10^{− 5}–10^{− 4} m² s^{− 1}), and relatively low values in the metalimnion (K_z ~ 10^{− 7}–10^{− 6} m² s^{− 1}) and the hypolimnion (K_z ~ 10^{− 6}–10^{− 5} m² s^{− 1}). We correlated the K_z variability with the environmental forcing through the Lake number (L_N), which expresses the relative strengths of the stratification to the wind forcing. Analysis of the environmental implications of K_z variability suggested that vertical turbulent transport of dissolved oxygen was 10% of the total oxygen depleted in the hypolimnion for the observation period. We concluded that at least some of the observed year-to-year variability of hypolimnetic oxygen depletion could be due to the variation in turbulent transport of dissolved oxygen across the thermocline.