Michigan scientists in 2024 dyed a lake blue and added fertilizer to test if blocking sunlight could prevent algal blooms; rain washed away the dye and the water turned green

For a brief period in the summer of 2024, Peter Lake in Michigan’s Upper Peninsula turned an unusual shade of blue. The color was not caused by pollution or a natural change in the water. Scientists had deliberately added a non-toxic blue dye to the lake as part of an experiment to answer: could reducing the amount of sunlight reaching the water help prevent an algal bloom?

The answer, as it turned out, was complicated. Researchers later added fertilizer to the lake to simulate the nutrient pollution that can fuel algal growth in natural lakes. But before the experiment could fully test whether shading would suppress the bloom, repeated rainfall washed much of the blue dye out of the water.

With more sunlight reaching the lake again, algae took advantage of the added nutrients. Peter Lake eventually shifted from blue to green as an algal bloom developed.


The unexpected outcome offered researchers something just as valuable as a successful experiment: a clearer picture of how difficult it can be to control an entire lake.

Why did scientists dye the lake blue?

The experiment was led by University of Wisconsin-Madison PhD candidate Danny Szydlowski and his research team. Peter Lake was chosen because of its unusual experimental history. The lake sits next to Paul Lake, separated by a narrow strip of land and gravel. The two lakes are similar enough that scientists have used them as a natural comparison for decades.

According to the University of Minnesota's Cascade Project, researchers have conducted whole-lake experiments on Peter and Paul lakes since the 1980s, manipulating one lake while leaving the other largely untouched as a control.

This setup allows scientists to observe what happens when an entire ecosystem is exposed to a particular environmental pressure rather than testing the same process only in a laboratory container.

For the 2024 experiment, the researchers used Aquashade, a non-toxic water-soluble mixture of blue and yellow dyes designed to reduce the amount of sunlight penetrating the water. Because algae need light to grow, limiting that light could, in theory, make it harder for an algal bloom to develop.

The researchers then added fertilizer to Peter Lake to create conditions similar to those caused by nutrient runoff.

The idea was that if excess nutrients encourage algae to grow, could reducing sunlight at the same time keep the bloom from taking hold?

The weather changed the experiment

Nature had other plans. Repeated rainfall diluted and flushed the dye from Peter Lake. As the blue pigment disappeared, more sunlight could penetrate the water.

The nutrients added by researchers, meanwhile, remained available to support algal growth.

The result was a striking transformation. The lake that had been deliberately turned blue eventually became green as algae multiplied. But researchers did not consider the experiment a failure.

Instead, the outcome demonstrated one of the central challenges of working with natural ecosystems. Scientists can control what they add to a lake, but they cannot control the weather, rainfall or countless other environmental factors that influence what happens next.Peter and Paul lakes have a long history of experiments

The 2024 experiment builds on decades of research at Peter and Paul lakes.

According to a 2011 study published in the journal Science titled Early Warnings of Regime Shifts: A Whole-Ecosystem Experiment, researchers led by the University of Wisconsin’s S. R. Carpenter previously manipulated Peter Lake's food web while leaving Paul Lake as a control.

That research found statistical changes in the ecosystem that appeared before the lake underwent a major ecological shift. Some of these changes emerged more than a year before the transition itself, suggesting that ecosystems can show warning signs before reaching a tipping point.

The bigger problem is still nutrients

The blue dye experiment also points back to a more familiar cause of algal blooms: excess nutrients. Fertilizers, sewage and other sources can introduce nutrients such as nitrogen and phosphorus into waterways. When conditions are favorable, those nutrients can fuel rapid algal growth.

Reducing sunlight may affect how quickly algae grow, but it does not remove the nutrients already in the water. That is why the experience at Peter Lake offers an important lesson. Even an intervention designed to limit one of the basic requirements for algal growth can be overwhelmed when natural conditions change. Rainfall effectively removed the shading treatment, while the nutrients remained.