In 2013, scientists released 11 kg of red dye into Lake Michigan to measure offshore mixing; the study recorded dispersion rising from 1.5 to 4.2 m²/s over about 21 hours

Stretching more than 500 kilometres and bordered by Wisconsin, Michigan, Illinois and Indiana, Lake Michigan is the only one of the five Great Lakes entirely contained within the United States. The lake frequently appears leisurely to anyone standing at its edge, rippling softly in the breeze with minimal drama beneath. Researchers knew that currents inside large lakes could transport heat, nutrients and pollutants, but offshore dispersion in lakes this large had rarely been measured directly.But to prove it, one had to figure out how to see water move in a way that was invisible to the human eye. And that’s precisely what a team of Purdue University engineers, working with NOAA’s Great Lakes Environmental Research Laboratory, set out to do in the summer of 2013. They took eleven kilogrammes of Rhodamine WT, a fluorescent tracer that turns water a detectable pink-red, out to the heart of the lake’s southern basin on the research vessel Blue Heron. Their work was later published in the journal Limnology and Oceanography, where the researchers explained that they wanted to measure something notoriously difficult to pin down, namely how quickly and how far a patch of water spreads once it enters the open lake well away from any shore.A cloud takes shape offshoreThe dye was diluted in ethanol and lake water to the density of the surrounding surface layer and pumped through a floating diffuser over several minutes so that it would diffuse uniformly rather than sink or coalesce. Within hours, the pink patch had expanded from a tight cloud to a large, uneven smear on the water, visible from the boat and later mapped in detail with fluorometers towed behind survey vessels. The crew tracked the plume for roughly a day, seeing its edges stretch and fold as it travelled with the surface currents of the lake.But what they actually wanted was a figure called the dispersion coefficient, which tells you how rapidly a blob of fluid spreads out over time. Friction from the lake bottom and the shoreline tends to stretch and shear a dye patch rapidly in coastal waters. Scientists had typically expected mixing to be gentler and slower far offshore, in deep open water, because there is no nearby boundary to perturb the flow. That assumption was part of why the experiment was important: testing it required measuring dispersion in the open heart of a large lake rather than near its boundaries.What the spreading rate revealedThe team was surprised by the result. Over roughly 21 hours, the team observed the dye patch's dispersion rate rise from about 1.5 to 4.2 square metres per second. That is a rapid spreading rate for water so far from any shoreline, and it pointed to a process the researchers had not expected to dominate the open lake. The researchers found that near-surface shear induced by near-inertial Poincaré waves partially explained the elevated dye dispersion rates observed during the experiment, which ripple through the boundary between warm surface water and the colder layer beneath it during summer stratification.The Poincaré waves themselves are internal motions associated with the lake's stratified water column, but the researchers linked them to vertical shear that extends through the mixed layer. That shear can cause different layers of the water to move at different speeds, helping stretch and spread the dye patch. Dye included. As a second, longer experiment, the scientists also launched floating drifters that lasted twenty-four days and exhibited more placid, more progressive dispersal in still conditions before turning to a speedier, patchier pattern when the winds kicked up.Why hidden mixing mattersKnowing how quickly water is stirred up in the open lake has implications far beyond scientific interest. It matters for predicting how toxins will move after a spill, what happens to nutrients from agricultural runoff in deep water, and how heat from a warming climate is distributed through the lake over a season. Earlier studies of nearshore waters had already shown that mixing can be brisk near the coast because of wave action and bottom friction. The 2013 dye release showed that the lake's seemingly tranquil interior can, under the right conditions, mix almost as vigorously, but for different physical reasons. That finding challenges a core assumption about the vastness and widespread use of Lake Michigan.Catch the latest World News and Live updates. Download the TOI app.