Dye study and monitoring buoys help study Great Lakes currents

By on July 29, 2013

A new study being conducted by researchers at Purdue University is seeking to gain a better understanding of the role that Great Lakes currents play in dispersing material like invasive species.

On July 14, a five-member team led by Cary Troy, an assistant professor in Purdue’s School of Civil Engineering, boarded a research vessel for a week-long study aimed at tracking currents in the middle of Lake Michigan.

Through funding from the National Science Foundation, researchers gained access to the Blue Heron, a research vessel from the University of Minnesota Duluth, to conduct their experiment on the lake’s currents.

“The general idea is to get a better understanding of where stuff is going in Lake Michigan. That is the broadest definition of what we’re doing,” Troy said. “You could be thinking about if you introduce an invasive species and you have the larvae of the invasive species floating around the lake, where are they likely to be transported? You could be thinking about phytoplankton, or anything that is a passive particle that essentially goes with the flow. Our experiment is designed to get that kind of data to improve the models that predict where these substances go.”

The university’s study is one of the largest-scale examinations of Great Lakes’ dispersion in the last 30 years. Tracking current patterns can prove complicated, because unlike oceans, the Great Lakes flow in unpredictable ways due to variables such as water temperature and wind velocity.

To track water dispersion in Lake Michigan, the researchers injected a non-toxic fluorescent dye called Rhodamine WT into the surface water behind the Blue Heron. The researchers then used a WETStar fluorometer to map the concentrations of the dye as it diffused into the lake.

The researchers also used an instrument known as a TRIAXUS. The TRIAXUS is a towable instrument package that was pulled behind the vessel to measure dye concentration, water temperature and turbidity, in addition to other parameters.

By employing a technique known as tow-yoing, researchers moved the TRIAXUS up and down through the water to attain two- and three-dimensional maps of dye concentrations in the water.

While the researchers don’t have specific numbers to quantify their expedition just yet, the experiment provided some surprising initial results.

“The unexpected thing for us was how quickly that dye patch grew into this massive blob of dye. Within 5 or 10 minutes, it was already 100 meters to a side and growing rapidly,” Troy said. “The reason we think that the dye patch exploded so quickly is that you have sort of a conveyer belt mechanism where the water at the top is moving very quickly away from the slower moving fluid down below. And once that dye gets mixed down through those layers, you have an effective dispersion mechanism where the difference in the velocity from the water at the surface and the water a little bit below the surface really serves to spread out the water and the dye to a very big patch.”

Researchers launched a zodiac from the research vessel to better survey the rhodamine dye plume (Credit: Cary Troy)

That water disperses so easily in Lake Michigan could be either beneficial or detrimental to the Great Lakes depending on the circumstance. Having water disperse easily could mitigate some of the damage done in the event of an oil spill or other contamination incident. However, the free-flowing nature of the lake could be a disadvantage in trying to corral an invasive species.

The research conducted aboard the Blue Heron is just one facet of the study, however.

University researchers also placed monitoring buoys in the middle of Lake Michigan to help measure the currents’ movement throughout the water column over longer periods of time. They plan on retrieving these monitors at the end of the summer to download the gathered data to study over the winter.

Facilitated by funding from the National Science Foundation’s Physical Oceanography Program, the monitoring of Lake Michigan’s currents will continue to be a point of emphasis for Purdue University over the course of the next two years.

Top image: Graduate student Jun Choi and the large dye plume just after release (Credit: Cary Troy)

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