The Lake Michigan “doughnut” – a huge ring-shaped bloom of phytoplankton that circles the late-winter waters of the Lake’s southern basin – is disappearing. (Read more about the doughnut here.)
The cause is quagga mussels, invasive filter-feeders that crowd the lake bed and clear the water of tiny aquatic plants and the nutrients the plants need to grow.
The doughnut has been blooming for the past 20 years, and maybe longer than that. But scientists have only known about it for around a decade because the bloom thrives during a time of year that’s dangerous for Great Lakes research boats.
The breakthrough discovery didn’t come until satellite imagery made pictures like this one from March 30, 2000 possible:
The Great Lakes winter blooms. Green means phytoplankton; yellow and red mean a lot of phytoplankton. Click to enlarge. Photo: Charles Kerfoot/Michigan Tech
The green ring at the bottom of Lake Michigan might not look as good as a gas station Krispy Kreme, but it’s an unusually defined structure for a bloom in the open waters of the Great Lakes.
The image shows the winter phytoplankton blooms of the other Great Lakes, which are especially thick in Green Bay, southern Lake Huron and western Lake Erie. But only southern Lake Michigan’s winter bloom takes the doughnut shape thanks to unique winter wind and water currents.
This 10-year-old image also shows the doughnut at its peak. A more recent image wouldn’t show much of a bloom at all.
“The doughnut’s just kind of shrinking,” said Charles Kerfoot, professor of ecology at Michigan Technical University and lead author of two studies of the bloom. “It’s almost gone.”
The researchers have been able to use other Great Lakes records to figure out that the doughnut phenomenon has occurred for at least 20 years. The next step is to determine if it goes back even earlier than that.
To do that they’ll turn to the bottom of the lake. Digging down into the lake bed is like traveling through time: The further down you go, the older the sediment is. If researchers dig deep down in the sediment and find remnants of the same phytoplankton species that make up the doughnut, they can get a better idea of just how long the doughnut has been blooming.
“Were hoping that the assemblage may give us a fingerprint,” Kerfoot said. “We can then take sediment cores and look through time and see how long this has actually been happening.”
A quadrillion mussels
Great Lakes scientists like Tom Nalepa conduct a lake-wide survey of mussels in Lake Michigan every five years. The last one was in 2005, after which Nalepa said he calculated that there were about 330 trillion invasive mussels in Lake Michigan.
More sampling in 2008 showed that mussel populations had tripled in some places. So now it’s possible that there are nearly a quadrillion mussels in the lake.
To try and fathom what a number that big actually means, Nalepa turned to Google, where he found that there are around 4 trillion fish in all the world’s oceans.
“And I estimate that there’s around 900 trillion quagga mussels in Lake Michigan alone,” he said. “That gives you a perspective of how many there are out there.”
The 2010 survey will start this summer, where scientists will collect mussels at 160 sites around the lake.
“Of course, that’s the easy part,” he said “The hard part is counting all the mussels.”
That will take well into next year, he said.