Tim Duda

Senior Scientist
Applied Ocean Physics & Engineering
Woods Hole Oceanographic Institution


These are photos of the aftermath of one of Jim Ledwell's ONR Coastal
Mixing and Optics dye injection mixing studies from the summers of 1995, 1996, and 1997.

About the dyes and release techniques (Reprinted from Journal of Geophysical Research, Copyright
American Geophysical Union, 2004):

2.1. Dyes

[6]   Two dyes were used for the experiments, fluorescein and Rhodamine WT, each with advantages as an ocean tracer. The fluorescence signature of fluorescein is more easily distinguished from the background. Fluorescein is also less than one tenth as expensive as Rhodamine WT. However, because fluorescein decomposes in light, it cannot be used near the surface. Exponential decay rates on the order 0.1 h−1 in water in direct sunlight were found in a number of studies summarized by Smart and Laidlaw [1977]. Consideration of the optical depth typical of the shelf led us to conclude that fluorescein could only safely be considered conservative over 5 days at depths greater than 40 m. The photo decay rate of Rhodamine WT is negligible for 5-day experiments in shallow water.

[7]   The minimum detectable level for Rhodamine WT and fluorescein in clean water with a commercial fluorometer is approximately 0.01 μg/L. Neither dye has been found by us to be adsorbed appreciably on mud or organic particulates in lab experiments lasting over a year, despite claims to the contrary in the literature [e.g., Smart and Laidlaw, 1977]. Neither is toxic to those handling it, and both are considered to be safe tracers to use in nearshore ecosystems, lakes, and reservoirs.

2.2. Release Technique

[8]   Dye was released by pumping from 200-L drums on deck through a garden hose to a simple dispersing system mounted on a frame, which also held a Sea-Bird 9plus CTD. The frame was made neutrally buoyant by attaching floats, and it was towed during the injection behind a weight lowered from a sheave at the waist of the R/V Oceanus. The dye had been mixed with fresh water and isopropyl alcohol to bring its density to within 0.001 kg/m3 of the water to be tagged, account being taken of the thermal expansion coefficient of the mixture. The flow rate was approximately 0.2 L/s, with 15 min required to empty each of four drums, so an hour is normally required for an injection. Trouble with clogged lines caused interruptions in the injection for Experiment 3, the first one described below. The ship maintained a speed through the water of approximately 0.5 m/s during the injection.

[9]   Data from the CTD were used to keep the release system on a target density surface, within a narrow tolerance. The root mean square density excursion at the injector was equivalent to less than 1 m when translated to depth through the mean vertical density gradient. The winch operator controlled the depth of the package while watching a display of the density error on a video screen. The pumps were automatically shut off when this density error exceeded a value equivalent to 2 m in depth in the mean gradient.