The dyes are superior tracers for experiments in which the final volume of the tracer patch is on the order of 1 cubic kilometer. In the stratified open ocean, this implies experimental durations of 5 days or so. 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.