Winter Sampling on Lake Mendota Yields Unusual Results

Madison skyline from the frozen surface of Lake Mendota. Photo: Adam Hinterthuer

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On February 22nd, Ted Bier, senior research specialist for the North Temperate Lakes Long Term Ecological Research study (LTER) and Dave Harring, the Center for Limnology’s facility manager, walked a few hundred meters off shore onto the Lake Mendota ice to take a battery of samples for the LTER. The LTER has monitored Madison-area and Northern Wisconsin lakes for the last 30 years in an attempt to document trends and changes in our lakes with a long-running record of hard data.

Ted Bier takes a plankton tow on Lake Mendota. Photo: Adam Hinterthuer

Bier has been at the CFL for ten years but had never seen anything like the readings he was getting last Wednesday. For starters, Lake Mendota’s measured secchi depth was 2.3 meters. That means that, at that depth, the black and white disk used to measure water clarity was no longer visible. Over the last decade, Bier says, the average secchi depth this time of year has been around 10 meters. The previous low was 5.3.
So why the murky water? Well, when Bier towed a zooplankton net up through a hole in the ice, it was filled with all sorts of algae and critters. Normally winter lake production is so low there’s nothing but water to stare down into. This year, though, biota was booming under Lake Mendota’s ice.
Bier holds up a vial of water full of plankton, an unusual sight in winter water samples.

Bier says the high productivity is likely the result of a number of factors: above average water temps (if you’re scoring at home, there’s a BIG difference between .3 Celsius and 3 Celsius!), relatively thin ice with little snow cover (which allows more sunlight to get to the photosynthetic organisms below) and lots of above-freezing daytime temps (during which the ice melts back into the lake, providing a small dose of nutrients and mixing).
While a quick glance at the contents of the plankton net was enough to confirm high productivity, Bier also noted that, as he ran water through a peristaltic pump and then through a filter to gauge total particulate matter, the filter clogged much sooner than usual. Where it normally takes between 5 to 8 milliliters of water before the filter fills up with suspended solids during the winter, this time out it only took 2.
It’s safe to assume that this winter’s data in the LTER’s decades-long database will stand out. Of course, if current forecasts for our changing climate are correct, winters like 2012 may become much more than mere outliers.
Confirming that this winter hasn’t been “business as usual” in Wisconsin, over the weekend, 36 cars crashed through the ice of Lake Winnebago during a sturgeon spearing tournament. To many people, creating a parking lot on a frozen lake is just asking for trouble. But, says Bier, Winnebago had 12 inches of ice. “12 inches of ice in below freezing conditions is actually really strong,” Bier says. But, winter 2012 has seen above average temperatures. The result? A “soft, dark, bubbly” ice that, even at 12 inches, is far from parking lot material.