Ten years ago, a group of Great Lakes scientists discovered Lake Michigan’s “doughnut in the desert,” a huge ring-shaped bloom of tiny aquatic plants circling southern Lake Michigan’s frigid offshore winter waters.
The phytoplankton bloom that circles southern in March and April, illustrated here, is disappearing thanks to invasive quagga mussels.
The bloom appears in the southern 100 miles of Lake Michigan in March and April. Winter winds and storms whip up a counter-clockwise current loaded with nutrients from the lake bottom and tributary rivers.
It likely explains how some animal species survive through the winter in the Great Lakes – a time when biologists previously thought there wasn’t much food to go around.
But the sweet discovery has since turned sour: They found the doughnut just in time to watch it disappear.
“Isn’t that frustrating?” said Charles Kerfoot, professor of ecology at Michigan Technical University. “You discover something and then all of a sudden five years later it gets eaten up.”
Literally.
Quaggas to blame, fish could suffer
The guilty gluttons are invasive quagga mussels, which hitched a ride in ship ballast tanks and colonized the bottom of Lake Michigan in the late 1990s. They developed a steady doughnut diet throughout the 2000s.
The doughnut bloom’s vanishing act isn’t just a problem for the scientists who study it. It will probably send shock waves up the food chain, according to research published recently in the Journal of Great Lakes Research.
The bloom sends a big dose of food to microscopic animals that wait out the winter in Lake Michigan’s open water, Kerfoot said.
“So rather than starving, they’re actually getting a pulse,” he said. “It will become a starvation period now.”
And if the tiny doughnut-eating animals starve, so could fish that depend on them for a big meal in spring.
Meanwhile, quagga mussels are sitting fat and sassy on the lakebed.
One of the thumbnail-sized critters can filter a quart of water in a day, and as many as 10,000 to 20,000 of them can cram into a single square yard, said Tom Nalepa, biologist with the Great Lakes Environmental Research Laboratory.
The mussels are filter feeders, constantly sucking in lake water and clearing it of everything worth eating.
Kerfoot said the outside edges of the doughnut used to be the most turbid, or clouded with tiny plants, animals and suspended dirt.
“Now they’re the least turbid,” he said “And they’re the ones that overlie the highest concentration of quagga mussels.”
The quagga mussels sitting right under the doughnut aren’t the only problem.
Mussel colonies settled near river mouths can clear up that nutrient-rich water before it can flow out and add fuel to the deep-water bloom.
“The impacts of high numbers of mussels in shallower waters can affect what’s going on in deeper waters,” Nalepa said.
Blooms of tiny aquatic plants, or phytoplankton, are an essential part of the Great Lakes food web. But they’re more commonly known to show up in spring and summer and closer to shore.
Doughnut discovery
The doughnut bloom went unnoticed for years because it shows up in the winter, which is a dangerous time for Great Lakes research boats, Kerfoot said. But a 1998 breakthrough in Great Lakes satellite imagery opened scientists’ eyes.
“All of a sudden, we saw these things that had never been seen before in the open lakes,” he said.
The big doughnut of phytoplankton, complete with a hole in the middle, was so strange that some scientists thought it was a mistake – the result of using ocean-based satellite tools on a freshwater lake.
But biologists managed to take a boat out into Lake Michigan in the winters of 2001 and 2002, when they found plankton samples that showed that this was no technical error, Kerfoot said.
“It’s real and extends all the way to the bottom of the lake,” he said.
But now it’s nearly gone.
Measurements of the doughnut in 2008 showed that around 30 percent more light shines through its water than in 2001, Kerfoot said. Levels of chlorophyll, the green pigment holed up in phytoplankton that drives photosynthesis, have dropped 70 percent.
“It’s actually gotten to the point right now where the increase in transparency is starting to rival Lake Superior,” he said. “That’s really remarkable clarity for Lake Michigan.”
The doughnut isn’t the only bloom disappearing. Quaggas are sucking up the springtime nearshore bloom, too.
Those two blooms account for around 70 percent of the phytoplankton production in Lake Michigan, Kerfoot said.
“We’re saying, ‘My God, what’s left?'” he said.
And all this ecosystem upheaval is driven by an invasive species that nobody saw coming 20 years ago.
Kerfoot was part of a project in the mid-1980s in which he and other researchers were charged with listing the major problems the Great Lakes would face over the 1990s and 2000s.
None of their wildest guesses included what’s playing out in the Lake Michigan and the other Great Lakes now.
“I’ll tell you, we never had this – the quaggas or the zebra mussels – as a part of that,” he said.
“The fact that it’s causing the collapse of the third largest freshwater lake in the world right now is just absolutely amazing.”