Abstract
Recent experimental and modeling work predicted salt fingers, known in saline water bodies, would form under ice in freshwater lakes with specific conductance (SC) as low as 50 μ S cm −1 . To test this prediction, Toolik Lake, Alaska (summer SC 60–90 μ S cm −1 ) was instrumented with temperature‐conductivity arrays. Calculations of solutes excluded with ice formation and a mass balance of changes in concentration of solutes within the lake indicated 90% to 100% of increase in solutes for several months following ice‐on was from cryoconcentration. Two metrics based on the ratio of density gradients of temperature and solutes, R ρ and the Turner angle (Tu), obtained by conductivity, temperature, depth (CTD) and microstructure profiling, and Ɍ , ratio of solute and heat fluxes at the ice‐water interface, had values indicative of salt fingers below ice. R ρ and Tu were in the range for salt fingers and the diffusive mode of double diffusion in intrusive‐features in lower water column. Step‐like changes in temperature and SC provide further evidence for double diffusion. Rates of dissipation of turbulent kinetic energy below ice and in intrusions were between 10 −12 and 10 −10 m 2 s −3 . Increases in SC above the sediments following ice‐on at sites 4, 10 and 15 m deep in the 24 m deep lake imply that salt fluxes created localized increases in density conducive for intrusive flows. These results are the first for freshwater lakes illustrating formation of salt fingers and complex intrusive flows and indicate the need to revise models of under‐ice circulation.