Boundary versus internal diapycnal mixing in stratified natural waters

Using the fluorescent dye uranin, tracer release experiments to study the contribution of bottom boundary mixing to diapycnal transport in stratified natural waters were performed in Lake Alpnach (central Switzerland) during 1992-1995. A first experiment involved injecting the tracer from a point source into the center of the hypolimnion (that part of the lake below the surface mixed layer). An in situ fluorometer was then employed to detect the horizontal and vertical spreading of the tracer cloud, allowing rates of diapycnal diffusivity to be determined. As long as the tracer was confined to the interior water region, the diapycnal diffusivity was relatively small. However, after the tracer cloud had reached the lake boundary, the diapycnal diffusivity increased by approximately one order of magnitude. In a second experiment, the tracer was released near the sediment-water interface. In this case the dynamics of vertical tracer spreading were opposite. During the first few hours after tracer release, diapycnal diffusivities were large, subsequently decreasing as the tracer cloud drifted away from the lake boundary. Basin-wide diapycnal diffusivities calculated from heat flux measurements based on temperature profiles obtained from thermistor chains or conductivity-temperature-depth casts agreed well with the values obtained from the vertical tracer diffusion after horizontal homogenization. The results of the tracer experiments corroborate the hypothesis that diapycnal fluxes are determined predominantly by mixing in the bottom boundary region.