Vertical eddy diffusivity determined with ²²²Rn in the benthic boundary layer of ice‐covered lakes1

Abstract

Concentration gradients of 222 Rn and 226 Ra were analyzed in the benthic boundary of seven ice‐covered lakes to investigate vertical eddy diffusivity, K z . Sediment‐free samples, retrieved with a tubing sampler from as close to the bottom as 14 cm, showed that sharp 222 Rn gradients persist over the sediments. A three‐dimensional analytical mixing model indicated horizontal mixing would not supply significant amounts of 222 Rn to midlake profiles in some flat‐bottomed lakes under ice cover. One‐dimensional analysis of the profiles permitted observation of change in K z with height above the sediments. Computed K z values usually decreased in a given lake toward the sediments. The range in K z from all lakes studied was 0.30 cm 2 s −1 at 3.61 m above the bottom to 3.2 × 10 −4 cm 2 s −1 at 0.23 m, both in Lake Mendota. The correlation between K z and the buoyancy density, N 2 , was computed: K = 1.2 × 10 −5 ( N 2 ) −0.63 . This relationship indicates turbulence arising from shear flow influences the vertical mixing in the winter boundary layer. Retarded mixing in the profundal boundary layer restricts vertical solute transport in some ice‐covered lakes.