It seems like every time you look at Lake Michigan, it’s a different color. Pearly white, steel gray, tropical postcard blue. Sometimes, it’s several colors at once.
That led one Bubbler Talk listener to ask:
“How does Lake Michigan often have several shades of blue: lighter near shore, almost violet farther out, and deep blue at the horizon?”
To find out, I climbed aboard the Neeskay, the UWM School of Freshwater Sciences research vessel. Starting in the harbor, we chugged up the Milwaukee River towards town. Later, the ship would wind back south past the three-river junction, before heading out into the lake.
Scientists are also interested in the many, ever-changing colors of Lake Michigan. They’ve been collecting data along the rivers and in the lake for more than two decades. This extensive data set helps them track changes over time, especially as climate change mixes things up.
This summer, the UWM researchers also collected data with the ongoing dredging in the Milwaukee River in mind; their observations will help them understand how that activity impacts the flow of nutrients and sediment throughout the rivers.
At the first stop, a bell rang and someone yelled, “On station!” At the command, students scattered to their assigned posts. It was science chaos. They filled coolers with samples of water and smelly sediment. They took temperature, conductivity, and fluorescence measurements.
“It’s kind of like, ‘Ah!’” said UWM freshwater scientist Carmen Aguilar-Diaz, making a frenetic, jazz hands gesture. All together, the data offered a portrait from the surface of the water to the sandy bottom.
“That rounds up a vignette from the one day of the different rivers and the harbor and the lake,” Aguilar-Diaz said. “You get a little bit of an idea of what’s going on.”
It was a bright June day. Sunlight leapt into the water. It’s this interaction — between light and water — that makes water blue.
“Pure water absorbs light,” said Russell Cuhel, a UWM marine biologist and Aguilar-Diaz's research partner.
Sunlight is made up of all the colors. Colors with longer wavelengths, like green and red, are absorbed first. Blue light is shorter.
“In open water, the blue light is the least absorbed,” Cuhel explained. “That means that it goes the deepest. As you dive down into it, you see the light changing from nearly white, which is the sum of all colors, to mostly blue because that’s the only color that gets down there.”
That’s why the lake sometimes sports stripes. Where the lake is shallow, it’s light blue. As it gets deeper, so does the shade of blue.
Color is complicated. Sediment, dissolved plant bits, tiny photosynthesizing plankton — they all influence the dance between light and water. So does the angle at which sunlight hits the water, the presence of clouds overhead or little, teardrop-shaped hills on the lake floor.
“There are what we call mini drumlins, which are 10-foot-high ridges that are like the Kettle Moraine, except tiny,” Cuhel said. “They are left over from the glacial retreat when the glaciers melted away.”
That’s high enough to change the color, he said.
There’s also the matter of how clear the water is. In a 2017 study, scientists used 14 years of satellite data — between 1998 and 2012 — to track how clarity in the Upper Great Lakes changed. What they found surprised them.
“Lake Superior was no longer the clearest Great Lake,” said Mike Sayers, a remote-sensing limnologist at Michigan Tech Research Institute who participated in the study. “That was always the prevailing narrative, that Lake Superior has the most pristine, clear water out of all the Great Lakes.”
Lake Michigan and Huron had become dramatically clearer in those years, with the greatest changes occurring off-shore, where the lake is more than around 295 feet deep. The trend coincides with the invasion of filter-feeding mussels. Around three decades ago, zebra and quagga mussels, which are native to eastern Europe, stowed away in ballast water along the shipping corridor that connects the Atlantic Ocean to the Great Lakes.
Numbering in the trillions, they’ve since colonized the lakes, gorging on the phytoplankton, or algae, that form the base of the food web. Efficient filter-feeders, the mussels can filter the entirety of Lake Michigan in less than two weeks. They have transformed the lake, and with it, the availability of food for the zooplankton that eat the phytoplankton, the small fish that eat the zooplankton, the bigger fish that eat the small fish and on through the food web.
Without nutrients and plankton to muddy the water, clear water also wreaks havoc with the interactions between different species by making it harder for small prey fish to hide from larger predators.
“Some people might say, ‘Well, clear water is great, right? This is great for boating, great for swimming,’” Sayers said. “Until we just talk about the food web. Because that’s where the biggest implications for the clearing water really play.”
Murky water is a sign of life. Clear water is more like an ecological desert, Sayers said.
With all these things influencing Lake Michigan, it’s no surprise that its color has changed over time. Decades ago, it would have been less blue — and more green, a result of thriving phytoplankton and abundant nutrients.
If Lake Michigan looked different in the past, what might it look like in the future?
Depending on climate change, there are a few possibilities, said Ashley Elgin, a research ecologist with NOAA’s Great Lakes Environmental Research Laboratory. Shifts in water temperatures could make life difficult for the invasive mussels.
“If things change in a way that is not favorable for the mussels, we could see more phytoplankton in the water and productivity go up,” Elgin said. “That could change the appearance. Maybe we won’t have that deep blue. You’d see a little more green in the water.”
Current mussel populations are down from when they first peaked, but their numbers are still strong enough to keep up with the phytoplankton.
Or, Elgin said, as climate change drives extreme rainfall, more sediment could make its way into the lake.
“If you have increased weather events and you have more runoff, then bringing sediments into the water; if you have more storm events that are mixing up and re-suspending sediments from the bottom,” Elgin said, “All of that could come into play and could make the water look more murky, more brown.”
Nutrients in the runoff could also boost algae, giving the water a tinge of green.
Lake Michigan is, and always will be, a mosaic of colors. Colors that reflect light, water, mussels, plankton — and, it turns out, us.
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Editor’s note: WUWM is a service of UW-Milwaukee.
