Unexpected Find Under the Ice for Lake Mendota Researchers

Last Wednesday, a couple of researchers ventured out onto frozen Lake Mendota to drill through the ice and take their yearly winter samples for the Long-Term Ecological Research (LTER) project.
A surprise awaited them as they broke through the ice.

LTER researcher, Kirsten Rhunde, takes samples through the ice on Lake Mendota. Photo: Ted Bier
LTER researcher, Kirsten Rhunde, takes samples through the ice on Lake Mendota. Photo: Ted Bier

Instead of the usual cold-weather conditions of clear water with not much growing in it, they found the brown-colored soup of a plankton bloom.
Unexpected "soupy" water awaited researchers after they drilled through the ice. Photo: Ted Bier
Unexpected “soupy” water awaited researchers after they drilled through the ice. Photo: Ted Bier

By the time the Secchi disk reached three meters, says Ted Bier, a research specialist with the LTER, it was disappearing from sight.
Most winters, the clear water lets Bier track the disk’s descent through the water column for six meters or more. This year, however, the water was clouded by a giant bloom of diatoms, a common kind of unicellular algae.
And whatever favorable conditions led to the algae bloom were good for other organisms as well.
This water sample taken from Mendota last week is filled with thousands of tiny copepods. Photo: A. Hinterthuer
This water sample taken from Mendota last week is clouded by thousands of tiny copepods. Photo: A. Hinterthuer

Copepods, tiny crustaceans that feed on algae, were swarming the near surface waters. Bier dropped a zooplankton tow, a conical fine-meshed net, through the hole and pulled up a sample. “It was the thickest I’ve seen in twelve years of winter sampling,” he said. “I didn’t look at the sample under a microscope, but it appeared to be 99% copepods, likely Diaptomus (a genus of copepod identified by its single eye spot).”
Bier says the find was unexpected, since snow cover on the ice had reduced the amount of sunlight entering Lake Mendota and winter conditions in general don’t add up to a whole lot of biological production in the lake.
But, countless species of zooplankton exist in Lake Mendota, most of them surviving at fairly low densities, waiting for conditions like a bountiful food source, increased light levels, or some other variable of biology or chemistry to get just right – then the quick growing critters explode in a sort of “insta” population  boom.
The LTER crew sets up shop on Lake Mendota ice. Photo: Ted Bier
The LTER crew sets up shop on Lake Mendota ice. Photo: Ted Bier

CFL director, Steve Carpenter, remembers other years with big diatom blooms under the ice and says its not unheard of, since many species of plankton are cold adapted and just waiting for a source of nutrients to trigger growth. What’s a bit unusual, he says, is that Bier didn’t find any of the zooplankton called Daphnia taking advantage of the diatom buffet. Daphnia are excellent predators of diatoms and can eat algae so fast that they clear it right out of the water column.
“Copepods aren’t great grazers of Daphnia,” Carpenter says. “You really need Daphnia to clean things up out there.”
Jake Walsh, a graduate student at the CFL, is studying the invasive zooplankton known commonly as the “spiny water flea.” They are a voracious predator of Daphnia pulicaria, which is the best zooplankton for clearing Lake Mendota’s water column. This fall, Walsh says, spiny water flea populations were at incredibly high levels in the lake.
“This is just a ‘blind guess’ hypothesis,” he clarifies, “but it’s possible that spinies knocked the Daphnia populations so low over the fall that they couldn’t really take back off this winter and take advantage of all that food (diatoms).”
That would help explain Bier’s observations over the last few years, as the act of drilling through the ice and finding mid-winter algae blooms gets more routine. But it’s a hypothesis that begs more research. What Bier’s finding really shows, is how little we truly know about lakes – even Mendota, the “most studied lake on earth.”
Maybe next year some hardy grad student will grab an auger and a plankton net and start weekly monitoring of the conditions of the UW-Madison shoreline. Until then, LTER winter sampling will keep dragging new mysteries to the surface.

1 thought on “Unexpected Find Under the Ice for Lake Mendota Researchers”

  1. Really there are so many wonderful and mysterious thing beneath the ice of the lake. I hope the scientists and students will discover more about this, the pioneer in this field.That’s great information. Thanks for sharing.

Leave a Reply

Your email address will not be published. Required fields are marked *