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Graves, Kelly Elise

University of British Columbia

Temperate freshwater lakes that experience seasonal ice cover are thought to homogenize twice a year between summer and winter stratification (i.e. turnover). For lakes with weak winds, this breakdown of stratification is primarily due to penetrative convection driven by surface buoyancy flux (Bs). However, there may be insufficient time for the lake to homogenize before surface water temperature (Ts) cools to the temperature of maximum density, which stops convective deepening as Bs goes to zero, resulting in incomplete seasonal turnover. In this dissertation, we present five years of observations in Deeks Lake, a small, deep, temperate lake, which reveal interannual variation of final turnover depth. We use a theoretical model to gain insight into the deepening and cooling of the surface mixed layer (SML), which we use as a proxy for the extent of autumnal mixing. We show that the SML did not reach the lake bottom during autumnal turnover, and there was interannual variation in final turnover depth, which ranged from 33 to 71 m of the maximum lake depth of 100 m. The Wedderburn number and Monin-Obukhov depth during the periods of interest suggest that SML deepening is mainly driven by one-dimensional convection. The modelling results demonstrate the importance of the feedback between Ts and the SML deep- ening rate through the temperature dependence of the thermal expansion coefficient (α) in the definition of Bs. The results also show that the time required to erode the metalimnion directly affects whether the SML deepens to the bottom of the lake. This is because the longer it takes for the SML to erode the metalimnion, the more Ts decreases, which causes α, and thus Bs, to approach zero. With time, Bs decreases so that it is not strong enough to further erode the stratification or Bs = 0 and the convective deepening stops. The work presented in this dissertation indicates that a subset of lakes that would have previously been assumed to mix completely at turnover may not mix completely. This understanding will help model current lake hydrodynamics and improve predictions of how lakes may change due to climate change.

Text

2024-10-24

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/89505

Civil Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/