Abstract
Turbulent mixing controls the vertical transfer of heat, gases and nutrients in stratified water bodies, shaping their response to environmental forcing. Nevertheless, due to technical limitations, the redistribution of wind-derived energy fuelling turbulence within stratified lakes has only been mapped over short (sub-annual) timescales. Here we present a year-round observational record of energy fluxes in the large Lake Geneva. Contrary to the standing view, we show that the benthic layers are the main locus for turbulent mixing only during winter. Instead, most turbulent mixing occurs in the water-column interior during the stratified summer season, when the co-occurrence of thermal stability and lighter winds weakens near-sediment currents. Since stratified conditions are becoming more prevalent –possibly reducing turbulent fluxes in deep benthic environments–, these results contribute to the ongoing efforts to anticipate the effects of climate change on freshwater quality and ecosystem services in large lakes.
Highlights
Turbulent mixing controls the vertical transfer of heat, gases and nutrients in stratified water bodies, shaping their response to environmental forcing
The standing knowledge builds upon a few integrative field studies aiming at resolving all relevant components of the turbulent kinetic energy (TKE) budget[28,29,30,31], interspersed with a larger number of process-oriented campaigns targeting one or a few specific processes[25,32,33,34,35]
The correlation between wind and near-surface water velocities indicates that annually γW = 0.36−0.43% of the downward wind energy flux (P10 ≈ 172 mW m−2) feeds the internal motions of Lake Geneva (RW = 0.61 − 0.72 mW m−2) (Fig. 2a), which store ~196 J m−2 of mechanical energy, giving a mean energy residence time of τ = ME/rate of wind work (RW) ≈ 3.5 days
Summary
Turbulent mixing controls the vertical transfer of heat, gases and nutrients in stratified water bodies, shaping their response to environmental forcing. Integrative studies have historically represented major breakthroughs and facilitated the parameterisation of internal lake mixing in one-dimensional computational models[36,37,38], which have been instrumental for predicting the long-term response of lakes to climatic changes[39,40,41]. Despite their relevance, integrative budgets have so far had limited temporal coverage. The continuous observations were interspersed with 447 regular (approximately weekly) microstructure turbulence profiles This unique dataset is used here to unravel the mixing pathways across seasons depending on varying stratification and wind conditions. Our findings substantiate that energy pathways are modulated by seasonal stratification, with the benthic boundary layers being highly turbulent in winter but isolated from wind-driven mixing during the stratified season
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