Vertical eddy diffusion calculated by the flux gradient method: Significance of sediment-water heat exchange

Periodically measured temperature profiles were used to calculate vertical eddy diffusivities by the heat flux gradient method in the stratified portion of a dimictic New England lake. Loss of heat from the water column to sediments, a term commonly neglected, was explicitly calculated with a simple numerical model. This term dominated the heat budget of the stratified lake and would have caused large errors in calculated eddy difisivities had it been ignored. Heat exchange with sediments should be considered when thermal flux gradient analysis is used to calculate eddy diffusion coefficients for lakes with combined metalimnetic plus hypolimnetic thicknesses up to- 15 m.-In the stratified portion of lakes, the distribution of chemicals is controlled by source and sink functions and by transport via turbulent mixing. For simplicity and mathematical tractability, the combined effect of these poorly understood processes is commonly combined into an eddy diffusivity, which can then be used in a diffusion equation. Lake hypolimnia are poorly mixed, and vertical concentration gradients of dissolved materials are com-monly observed in such regions. To understand the geo-chemical cycling of chemical components as reflected by their fluxes and patterns of distribution within the hy-polimnion, it is first necessary to know the magnitude of eddy diffusion as a function of depth and time. Calculation of eddy diffusion can be carried out by monitoring the change in distribution of a stable, con