Flushing the Lake Littoral Region: The Interaction of Differential Cooling and Mild Winds

The interaction of a uniform cooling rate at the lake surface with sloping bathymetry efficiently drives cross-shore water exchanges between the shallow littoral and deep interior regions. The faster cooling rate of the shallows results in the formation of density-driven currents, known as thermal siphons, that flow downslope until they intrude horizontally at the base of the surface mixed layer. Existing parameterizations of the resulting buoyancy-driven cross-shore transport assume calm wind conditions, which are rarely observed in lakes and thereby restrict their applicability. Here, we examine how moderate winds (≲5 m s −1) affect this convective cross-shore transport. We derive simple analytical solutions that we further test against realistic three-dimensional numerical hydrodynamic simulations of an enclosed stratified basin subject to uniform and steady surface cooling rate and cross-shore winds. We show cross-shore winds modify the convective circulation, stopping or even reversing it in the upwind littoral region and enhancing the cross-shore exchange in the downwind region. The analytical parameterization satisfactorily predicted the magnitude of the simulated offshore unit-width discharges in the upwind and downwind littoral regions. Our scaling expands the previous formulation to a regime where both wind and buoyancy forces drive cross-shore discharges of similar magnitude. This range is defined by the non-dimensional Monin-Obukhov length scale, χMO: 0.1 ≲ χMO ≲ 0.5. The information needed to evaluate the scaling formula can be readily obtained from a traditional set of in situ observations.Swiss National Science Foundation (SNSF) European Commission 175919ETH-Bereich Forschungsanstalte

Development of overturning circulation in sloping waterbodies due to surface cooling

Cooling the surface of freshwater bodies, whose temperatures are above the temperature of maximum density, can generate differential cooling between shallow and deep regions. When surface cooling occurs over a long enough period, the thermally induced cross-shore pressure gradient may drive an overturning circulation, a phenomenon called 'thermal siphon'. However, the conditions under which this process begins are not yet fully characterised. Here, we examine the development of thermal siphons driven by a uniform loss of heat at the air-water interface in sloping, stratified basins. For a two-dimensional framework, we derive theoretical time and velocity scales associated with the transition from Rayleigh-Benard type convection to a horizontal overturning circulation across the shallower sloping basin. This transition is characterised by a three-way horizontal momentum balance, in which the cross-shore pressure gradient balances the inertial terms before reaching a quasi-steady regime. We performed numerical and field experiments to test and show the robustness of the analytical scaling, describe the convective regimes and quantify the cross-shore transport induced by thermal siphons. Our results are relevant for understanding the nearshore fluid dynamics induced by nighttime or seasonal surface cooling in lakes and reservoirs

Lake surface cooling drives littoral-pelagic exchange of dissolved gases

The extent of littoral influence on lake gas dynamics remains debated in the aquatic science community due to the lack of direct quantification of lateral gas transport. The prevalent assumption of diffusive horizontal transport in gas budgets fails to explain anomalies observed in pelagic gas concentrations. Here, we demonstrate through high-frequency measurements in a eutrophic lake that daily convective horizontal circulation generates littoral-pelagic advective gas fluxes one order of magnitude larger than typical horizontal fluxes used in gas budgets. These lateral fluxes are sufficient to redistribute gases at the basin-scale and generate concentration anomalies reported in other lakes. Our observations also contrast the hypothesis of pure, nocturnal littoral-to-pelagic exchange by showing that convective circulation transports gases such as oxygen and methane toward both the pelagic and littoral zones during the daytime. This study challenges the traditional pelagic-centered models of aquatic systems by showing that convective circulation represents a fundamental lateral transport mechanism to be integrated into gas budgets. Cooling-induced horizontal circulation redistributes gases daily between littoral and pelagic lake waters under calm conditions

Characterization of the bottom turbid layer in a pit lake and its response to convective cooling

Base Mine Lake (BML) is a pit lake used to store the Fluid Fine Tailings (FFT) generated by the oil sands industry, in the Athabasca region (northern Alberta, Canada). It is well studied to try understanding its evolution over time and to determine if an effective separation of the FFT from the cap water will eventually be reached. The mixing mechanisms taking place in the lake and affecting the suspended solids are of particular interest. Turbidity increases at the bottom of the water column suggesting the presence of a turbid layer which might be subject to mixing. The solids concentration exceeds the maximum limit of the sensors and deeper values cannot be recorded which prevents to fully understand the vertical structure of the near FFT interface region. To estimate the suspended solids concentration (CV ) over the entire water column, a laboratory calibration of the turbidity sensors is performed and the linear relationship CV [mg=L] = 0.84 x Turb [NTU] is obtained. This corresponds to concentrations of approximately 200mg=L for the region located just above the peak of turbidity, at the bottom of BML. The point of zero turbidity corresponding to the upper limit of the sensors is estimated at approximately 40 g=L. This value must be considered as a minimum solids concentration for the region located below the peak of turbidity. A small optical device is built using a waterproof camera in order to validate the previous results by applying another method than turbidity. The values obtained are comparable and this visually-based estimation can be useful to better understand turbidity variations. Convective cooling, which is one of the processes possibly affecting the bottom turbid layer during fall turnover, is studied in the laboratory on a kaolinite suspension. A simplified experiment using the melting of an ice cube to generate surface cooling shows the rapid rise of the turbid layer as cold water penetrates across its interface. This results in a decrease of bottom turbidity and an increase of mid-depth turbidity. The convective process is then roughly modelled using plume theory and conservation equations. The results obtained are in the same order of magnitude than the values observed. A more realistic experiment with a more uniform and gradual cooling of the water surface suggests that convective cooling might affect the bottom turbid layer by eroding its interface and slowly entraining particles to the water above

Density currents driven by differential cooling in lakes: occurrence, dynamics and implications for littoral-pelagic exchange

Cross-shore flows exchange water laterally in lakes, with ecological implications for the ecosystem. One example is the convective circulation induced by differential cooling, also known as the thermal siphon. This lateral flow forms when the sloping sides of lakes experience surface cooling. Shallow areas cool faster, which generates lateral density gradients and drives a cold downslope gravity current and an onshore surface flow. The role of this two-layer-circulation in lateral exchange in lakes is poorly understood, due to the lack of long-term and high-resolution measurements of thermal siphons. This thesis aims at filling this gap by investigating the occurrence and dynamics of thermal siphons from extensive in situ observations in a wind-sheltered Swiss lake (Rotsee). We first quantified the seasonality of thermal siphons from one-year-long observations in Rotsee. Thermal siphons occurred on a daily basis from late summer to winter and flushed the littoral region in ~ 10 h. Their duration increased but their intensity decreased over the same period. We linked this seasonality to the change in forcing conditions by testing scaling relationships from theoretical and laboratory-based studies. We then focused on the short-term temporal variability of the transport by quantifying the dynamics of thermal siphons over a diurnal cycle and their interaction with convective plumes. Our results reveal that convective plumes penetrated into the gravity current at night and eroded its upper interface. This vertical mixing generated vertical interface fluctuations and reduced the lateral transport at night. The maximal transport was delayed to daylight conditions when radiative heating weakened penetrative convection. After having quantified the physical water transport, we assessed the role of thermal siphons in the lateral exchange of dissolved gases. We found that both branches of the circulation were capable of transporting gases laterally. The downslope current brought littoral gases to the base of the mixed layer in the stratified region whereas the surface flow transported gases towards the shore. We quantified this exchange for oxygen and methane. Finally, we generalized our observations to other lakes with different bathymetry and forcing conditions. The frequent occurrence of thermal siphons observed in six lakes confirmed that this lateral transport process is ubiquitous in lakes with shallow littoral regions. We showed that our results from Rotsee were applicable to other systems, where the same scaling relationships predicted the formation and intensity of thermal siphons. Based on these results, we proposed a procedure to predict the contribution of thermal siphons for lateral transport in any lake. This thesis provides a comprehensive understanding of the formation and dynamics of thermal siphons and paves the route for integrating this lateral transport process in lake ecosystem research.LH

Seasonality of density currents induced by differential cooling

This study was financed by the Swiss National Science Foundation ("Buoyancy driven nearshore transport in lakes" project; HYPOlimnetic THErmal SIphonS, HYPOTHESIS, grant no. 175919).When lakes experience surface cooling, the shallow littoral region cools faster than the deep pelagic waters. The lateral density gradient resulting from this differential cooling can trigger a cold downslope density current that intrudes at the base of the mixed layer during stratified conditions. This process is known as a thermal siphon (TS). TSs flush the littoral region and increase water exchange between nearshore and pelagic zones; thus, they may potentially impact the lake ecosystem. Past observations of TSs in lakes are limited to specific cooling events. Here, we focus on the seasonality of TS-induced lateral transport and investigate how seasonally varying forcing conditions control the occurrence and intensity of TSs. This research interprets 1-year-long TS observations from Rotsee (Switzerland), a small wind-sheltered temperate lake with an elongated shallow region. We demonstrate that TSs occur for more than 50 % of the days from late summer to winter and efficiently flush the littoral region within similar to 10 h. We further quantify the occurrence, intensity, and timing of TSs over seasonal timescales. The conditions for TS formation become optimal in autumn when the duration of the cooling phase is longer than the time necessary to initiate a TS. The decrease in surface cooling by 1 order of magnitude from summer to winter reduces the lateral transport by a factor of 2. We interpret this transport seasonality with scaling relationships relating the daily averaged cross-shore velocity, unit-width discharge, and flushing timescale to the surface buoyancy flux, mixed-layer depth, and lake bathymetry. The timing and duration of diurnal flushing by TSs relate to daily heating and cooling phases. The longer cooling phase in autumn increases the flushing duration and delays the time of maximal flushing relative to the summer diurnal cycle. Given their scalability, the results reported here can be used to assess the relevance of TSs in other lakes and reservoirs.Swiss National Science Foundation (SNSF) European Commission 17591

Seasonality of density currents induced by differential cooling

When lakes experience surface cooling, the shallow littoral region cools faster than the deep pelagic waters. The lateral density gradient resulting from this differential cooling can trigger a cold downslope density current that intrudes at the base of the mixed layer during stratified conditions. This process is known as a thermal siphon (TS). TSs flush the littoral region and increase water exchange between nearshore and pelagic zones; thus, they may potentially impact the lake ecosystem. Past observations of TSs in lakes are limited to specific cooling events. Here, we focus on the seasonality of TS-induced lateral transport and investigate how seasonally varying forcing conditions control the occurrence and intensity of TSs. This research interprets 1-year-long TS observations from Rotsee (Switzerland), a small wind-sheltered temperate lake with an elongated shallow region. We demonstrate that TSs occur for more than 50 % of the days from late summer to winter and efficiently flush the littoral region within ∼10 h. We further quantify the occurrence, intensity, and timing of TSs over seasonal timescales. The conditions for TS formation become optimal in autumn when the duration of the cooling phase is longer than the time necessary to initiate a TS. The decrease in surface cooling by 1 order of magnitude from summer to winter reduces the lateral transport by a factor of 2. We interpret this transport seasonality with scaling relationships relating the daily averaged cross-shore velocity, unit-width discharge, and flushing timescale to the surface buoyancy flux, mixed-layer depth, and lake bathymetry. The timing and duration of diurnal flushing by TSs relate to daily heating and cooling phases. The longer cooling phase in autumn increases the flushing duration and delays the time of maximal flushing relative to the summer diurnal cycle. Given their scalability, the results reported here can be used to assess the relevance of TSs in other lakes and reservoirs.APHY

Near-bed stratification controls bottom hypoxia in ice-covered alpine lakes

In ice-covered lakes, near-bottom oxygen concentration decreases for most of the wintertime, sometimes down to the point that bottom waters become hypoxic. Studies insofar have reached divergent conclusions on whether climate change limits or reinforces the extent and duration of hypoxia under ice, raising the need for a comprehensive understanding of the drivers of the dissolved oxygen (DO) dynamics under lake ice. Using high-temporal resolution time series of DO concentration and temperature across 14 mountain lakes, we showed that the duration of bottom hypoxia under ice varies from 0 to 236 d within lakes and among years. The variability of hypoxia duration was primarily explained by changes in the decay rate of DO above the lake bottom rather than by differences in DO concentration at the ice onset or in the ice-cover duration. We observed that the DO decay rate was primarily linked to physical controls (i.e., deep-water warming) rather than biogeochemical drivers (i.e., proxies for lake or catchment productivity). Using a simple numerical model, we provided a proof-of-concept that the near-bed stratification can be the mechanism tying the DO decay rate to the sediment heat release under the ice. We ultimately showed that the DO decay rate and hypoxia duration are driven by the summer light climate, with faster oxygen decline found under the ice of clearer cryostratified alpine lakes. We derived a framework theorizing how the hypoxia duration might change under the ice of alpine lakes in a warmer climate.APHY

Near-bed stratification controls bottom hypoxia in ice-covered alpine lakes

In ice-covered lakes, near-bottom oxygen concentration decreases for most of the wintertime, sometimes down to the point that bottom waters become hypoxic. Studies insofar have reached divergent conclusions on whether climate change limits or reinforces the extent and duration of hypoxia under ice, raising the need for a comprehensive understanding of the drivers of the dissolved oxygen (DO) dynamics under lake ice. Using high-temporal resolution time series of DO concentration and temperature across 14 mountain lakes, we showed that the duration of bottom hypoxia under ice varies from 0 to 236 d within lakes and among years. The variability of hypoxia duration was primarily explained by changes in the decay rate of DO above the lake bottom rather than by differences in DO concentration at the ice onset or in the ice-cover duration. We observed that the DO decay rate was primarily linked to physical controls (i.e., deep-water warming) rather than biogeochemical drivers (i.e., proxies for lake or catchment productivity). Using a simple numerical model, we provided a proof-of-concept that the near-bed stratification can be the mechanism tying the DO decay rate to the sediment heat release under the ice. We ultimately showed that the DO decay rate and hypoxia duration are driven by the summer light climate, with faster oxygen decline found under the ice of clearer cryostratified alpine lakes. We derived a framework theorizing how the hypoxia duration might change under the ice of alpine lakes in a warmer climate.ISSN:0024-3590ISSN:1939-559