Two dyes have been used so far by our laboratory, Rhodamine-WT and fluorescein. WT and fluorescein are both nontoxic, and are accepted for use in tracer work in drinking water supplies. Rhodamine is stable in sunlight, but it is expensive and it suffers interference from background fluorescence in natural waters. Fluorescein is inexpensive, suffers much less from background, but decays rapidly in sunlight. Neither dye suffers significant adsorption onto particulates during a 5-day experiment, although fluorescein is reputed to be superior in this respect.
Fluorescein is clearly the tracer to choose for an experiment in conditions of low light.
The goal of most of the dye experiments performed thus far has been similar to that of the sulfur hexafluoride experiments, namely to measure the diapycnal diffusivity and the lateral dispersion in the stratified ocean, and to place these in a dynamical context. The first such experiment was performed in July 1993 in Massachusetts Bay at the outfall site of sewage treatment plant that is just being completed for the metropolitan Boston area. This was followed in 1995 by an experiment near the south shore of Massachusetts, in the far field of the outfall site. These experiments were jointly funded by the Massachusetts Water Resources Authority and the WHOI Sea Grant Program of the National Oceanographic and Atmospheric Administration. In these experiments, dye was released at various levels over the mid continental shelf south of Martha's Vineyard to measure diapycnal diffusivity.