Ice Off and Carbon Fluxes: Springtime on Lake Mendota

by Angela Baldocchi 
Every spring, when I spot the first buds on a tree, I always think I might actually get to see them open if I pay close enough attention. But then – as if overnight – the leaves just seem to appear.

These atmospheric instruments keep tabs on Lake Mendota’s evaporation and emissions. Photo: Desai Lab

On Lake Mendota, this springtime magic is one of disappearing ice. As the ice begins to thaw, it turns from a snow white to slushy lake-color at the edges and maybe dotted throughout.  More of the sun’s waves can then get through the ice, warming the water underneath and speeding the process up.  When I see that, I think, “Alright, it will just be a few days now. I’ll see it go away this year.”
Then it snows again, or the temperature drops and it is still frozen.                                                                       
Still, I keep waiting and watching and, suddenly, the ice is gone.  I missed it, again!  I never seem to catch that last bit where the ice completely disappears.
Luckily, we aren’t only keeping track of spring with our eyes.
At the end of Picnic Point, where the UW-Madison’s Lakeshore Nature Preserve curls out into Lake Mendota, sits a pole with a light that alerts boaters to inclement weather conditions. Some people call it the “boater warning pole,” but in Ankur Desai’s lab in the UW-Madison Department of Atmospheric and Oceanic Sciences, we call it the “eddy covariance tower.”
Image courtesy Angela Baldocchi

For example, we caught water going into the atmosphere with a big pulse of “water vapor flux” at the tower last year. The figure on the right shows evaporation (water flux) from January to March and then records a large spike that starts when the lake opened on March 7th of 2017.
With the ice gone, wind can blow across the lake surface and the sun warms its waters generating a lot of evaporation and also mixing the lake, stirring up microscopic life and releasing carbon dioxide. Some of this CO2 is taken up by trees as they produce their spring buds and sweet-smelling flowers.
Speaking of aromas, the lake is also releasing methane and volatile organic carbons and these gases get mixed into the atmosphere as well.
But lifting the ice “lid” is only one way that spring puts Lake Mendota “in flux.”
Springtime is marked by things that lay dormant under winter’s blanket as they begin to awaken and stir about. The lake is much the same – not long after ice out, and its initial pulse of carbon dioxide into the atmosphere, the lake quickly switches up and begins to pull carbon dioxide in as previously dormant photosynthetic organisms (like algae and phytoplankton) begin to bloom and grow.
As you can see, there’s a lot to watch for when it’s springtime on Lake Mendota and the flux tower is one way we collect data that show the story of what’s going on.
But the tower isn’t the only instrument keeping track of how Lake Mendota interacts with the atmosphere.
A map of carbon dioxide concentrations produced by FLAMe. Courtesy: Luke Loken

The Center for Limnology has the Fast Limnological Automated Measurements (FLAMe) system – a monitoring tool that is mounted on the back of a speedboat and continuously samples the surface water as it speeds around the lake.  The FLAMe data can then be used to create cool maps that illustrate near real-time measurements of everything from carbon dioxide flux to water temperature to algae growth.  
During open water season, our lab, along with the CFL, puts out a buoy that takes similar measurements as the flux tower. The buoy stays positioned in the deepest part of the lake, monitoring gas flux and air speed and even conditions at different depths of the lake until it is taken in just before the ice comes back in the fall.
Another instrument, called the flux chamber, is a particularly unique way to measure gases. We like to call them hats because of their shape. They are a part of a global synthesis project in the Global Lakes Ecological Observatory Network (GLEON). Last summer, our team participated and conducted overnight sampling campaigns to add to a global database of flux measurements on lakes across the world. The measurements were taken during a 24-hour cycle so the lake’s metabolism or daily cycle can be recorded at different times of the year. We hope to do it again this Spring while the lake is mixing.                      
But why, exactly, do we go to all the fuss measuring carbon dioxide fluxes so many different ways?  
For starters, there is world-leading work being done here at UW-Madison on lakes in general and lakes are hotspots in the global carbon cycle. For example, a new study has shown that Lake Mendota is at times a carbon source that emits CO2 to the atmosphere and at other times it is a sink and sequesters carbon from the atmosphere. Without good measurements we don’t know how important these processes are to larger climate conditions or how they are changing over time. We need to know how they influence factors like the rate of global atmospheric greenhouse gas (GHGs) emissions.
That is our overarching goal – to be able to keep global records and determine at various regional scales if GHG emissions are going down or not. Before we can do this, we need be able to prove the carbon budget of a lake with enough confidence to get to the next level, which is the addition of remote sensing to our dataset. If we can show that satellite calculations are an accurate measurement tool, we could move beyond our extensive instrumentation of a single lake to a technology that could help monitor changes at larger, regional levels.    
Spring is in the air, and, soon, the carbon dioxide and water vapor and methane that the big thaw releases will be, too. Which means that, while I may yet again miss the magic of the disappearing ice, I’ll still be able to see it in the data we’re collecting out at the end of Picnic Point.