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

Abstract

Abstract 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 m 2 s − 1 ), and relatively low values in the metalimnion ( K z ~ 10 − 7 –10 − 6 m 2 s − 1 ) and the hypolimnion ( K z ~ 10 − 6 –10 − 5 m 2 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.