Implications of Periodic Weak Thermal Stratification in the Epilimnion of Lake Opeongo

Episodic weak stratification is a persistent and important feature of the epilimnion of Lake Opeongo. Field studies were conducted in Lake Opeongo in 2009 and 2010 to assess the effect of the epilimnetic weak thermal stratification on turbulent mixing and ecological processes. Near surface thermoclines (as defined by dT/dz > 0.2°C m-1 between 1 m and 2 m) occur approximately 20% of the time and correspond to periods of high values of gradient Richardson number (Rig). Extended periods of near surface stratification (> 1 hr), account for more than 80% of the stratified period. We compare these findings with previous observations from the Experimental Lakes Area and discuss the biological implications of near surface thermoclines.;The persistent weak temperature stratification that characterizes the entire epilimnion of Lake Opeongo, acts to reduce the magnitude of turbulent mixing. During these stratified periods, the values of Ri g, increase, with a corresponding decrease in the rates of dissipation of turbulent kinetic energy, (epsilon), the turbulence activity parameter, I = epsilon/nuN2, and vertical eddy diffusivity (Kz). Mixing during cold fronts occur over time scales of minutes to hours, which work to erode diurnal thermoclines. The direct implications of weak stratification on aquatic organisms are also assessed.;The presence of weak stratification also allows for the existence of internal waves within the epilimnion. A key observation in this thesis is the relationship between enhanced small-scale spatial variability in zooplankton distribution and the presence of internal waves in the weakly stratified epilimnion. To quantify this physical-biological coupling, we compare variance of isotherm displacement and gradient Richardson number (Ri g) with small-scale spatial distributions of zooplankton. For smaller size ranges of zooplankton (284--450 microm), we find that spatial variability is statistically greatest for the highest values of variability of isotherm displacement. As vertical velocities estimated from wave characteristics are faster than swimming speeds of small zooplankton, these organisms become passively advected by the internal waves leading to increased spatial variability.