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 s tratified natural waters were performed in Lake Alpnach (central Switz erland) during 1992-1995. A first experiment involved injecting the tr acer from a point source into the center of the hypolimnion (that part of the lake below the surface mixed layer). An in situ fluorometer wa s then employed to detect the horizontal and vertical spreading of the tracer cloud, allowing rates of diapycnal diffusivity to be determine d. As long as the tracer was confined to the interior water region, th e diapycnal diffusivity was relatively small. However, after the trace r cloud had reached the lake boundary, the diapycnal diffusivity incre ased by approximately one order of magnitude. In a second experiment, the tracer was released near the sediment-water interface. In this cas e the dynamics of vertical tracer spreading were opposite. During the first few hours after tracer release, diapycnal diffusivities were lar ge, 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 thermis tor chains or conductivity-temperature-depth casts agreed well with th e 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 mixin g in the bottom boundary region.