High‐Frequency Observations of Temperature and Dissolved Oxygen Reveal Under‐Ice Convection in a Large Lake

Abstract

Detailed observations of thermal structure over an entire winter in a large lake reveal the presence of large (10-20 m) overturns under the ice, driven by diurnal solar heating. Convection can occur in the early winter, but the most vigorous convection occurred near the end of winter. Both periods are when our lake ice model suggest thinner ice that would have been transparent. This under-ice convection led to a deepening of the mixed layer over time, consistent with previous short-term studies. During periods of vigorous convection under the ice at the end of winter, the dissolved oxygen had become supersaturated from the surface to 23 m below the surface, suggesting abundant algal growth. Analysis of our highfrequency observations over the entire winter of 2015 using the Thorpe-scale method quantified the scale of mixing. Furthermore, it revealed that changes in oxygen concentrations are closely related to the intensity of mixing.

Ice‐Free Radiative Convection Drives Spring Mixing in a Large Lake

“…Recent work has established radiative convection as a dominant mixing mechanism in ice-covered lakes Jonas, Terzhevik, et al, 2003;Yang et al, 2017), but its potential importance in nearly ice-free dimictic lakes has been largely speculative (e.g., Boyce et al, 1989). Although ice-free radiative convection has been observed throughout the spring heating period (March-June; see Figure 1) in the Laurentian Great Lakes (Lake Michigan: Church, 1947;Beletsky & Schwab, 2001; Lake Superior: Bennett, 1978;Austin, 2019; and Lake Ontario: Scavia & Bennett, 1980), there have been, to the best of our knowledge, no direct observations of the turbulence characteristics associated with this mixing mechanism, despite its potential importance for both biological and chemical properties, including phytoplankton and zooplankton biomass (Sommer et al, 2012;Vanderploeg et al, 2010), dissolved oxygen (Yang et al, 2017), and nutrients (Hampton et al, 2017). Although ice-free radiative convection has been observed throughout the spring heating period (March-June; see Figure 1) in the Laurentian Great Lakes (Lake Michigan: Church, 1947;Beletsky & Schwab, 2001; Lake Superior: Bennett, 1978;Austin, 2019; and Lake Ontario: Scavia & Bennett, 1980), there have been, to the best of our knowledge, no direct observations of the turbulence characteristics associated with this mixing mechanism, despite its potential importance for both biological and chemical properties, including phytoplankton and zooplankton biomass (Sommer et al, 2012;Vanderploeg et al, 2010), dissolved oxygen (Yang et al, 2017), and nutrients (Hampton et al, 2017).…”

“…Large radiative heat fluxes in the Laurentian Great Lakes, attributed to high water clarity (Barbiero et al, 2018) and low ice cover (Wang et al, 2011), make them particularly susceptible to this phenomenon. Although ice-free radiative convection has been observed throughout the spring heating period (March-June; see Figure 1) in the Laurentian Great Lakes (Lake Michigan: Church, 1947;Beletsky & Schwab, 2001; Lake Superior: Bennett, 1978;Austin, 2019; and Lake Ontario: Scavia & Bennett, 1980), there have been, to the best of our knowledge, no direct observations of the turbulence characteristics associated with this mixing mechanism, despite its potential importance for both biological and chemical properties, including phytoplankton and zooplankton biomass (Sommer et al, 2012;Vanderploeg et al, 2010), dissolved oxygen (Yang et al, 2017), and nutrients (Hampton et al, 2017).…”

Observations of radiatively driven convection in a deep lake

“…While most of these references address bulk response of the convective layer, focusing on vertical structure, convective process can have relatively short horizontal scales of variability (Forrest et al ), which cannot be resolved with traditional point moorings. Impacts of RDC on biogeochemical processes have also been considered, including primary productivity (Kelley ; Vehmaa and Salonen ) and dissolved oxygen distributions (Yang et al ). Recent work (Ulloa et al ) has provided insights into the energy budget, showing that about 65% of the energy input through the radiative buoyancy flux is converted into changes in potential energy.…”

Mixing processes in small arctic lakes during spring

“…Heat fluxes and mixing Penetrative convection is usually identified from time series temperature measurements when the water column becomes isothermal at progressively increasing depths beginning immediately below the surface boundary layer (Farmer 1975) and interior convection when isothermal regions develop deeper in the water column (Malm et al 1997). Using high-resolution sensors, Yang et al (2017) inferred active mixing if there were instabilities in the time series data and assumed that isothermy only implied previously mixed water. We similarly expected mixing to occur when the temperature at one depth exceeded that below such that density instabilities formed due to increased temperature below 4 C. If isothermy resulted between the two thermistors over the next several hours, we could assume mixing that had a length scale equal to the separation between thermistors.…”