Limnology in Action: Like Baking a Cake in a Lake

Besides the rowboat being pulled across a grid of buoys, the surface of Peter Lake is calm. But, just 5 meters below the surface, a full experiment is under way.
“Annndddd…. There is C-13 spilling out down there!” shouts Grace Wilkinson. Looking into the water doesn’t do any good, the carbon-isotope tracer that Wilkinson and her assistant, Carol Yang, have just released is sitting well down in the middle strata of the lake.

Grace and Carol put the isotope label in the WILCs before they are deployed.

“Basically, we are putting really expensive baking soda in the lake,” explains Wilkinson. Yang laughs, “Like baking a cake in the lake!”

While the experiment is far from making a cake, Wilkinson and Yang are doing a very effective job of layering. Like most lakes in this area of Northern Wisconsin, Peter Lake stratifies into three layers during the summer months. The top layer, called the epilimnion, is warm and less dense. At the bottom sits a layer of cold, dense water called the hypolimnion. In between these two is the metalimnion, and this is exactly where Wilkinson is putting her tracer down.
Wilkinson is performing an experiment to trace food web subsidies in the lake using a Carbon-13 isotope tracer to “label” the metalimion. “What we are doing is investigating the importance of metalimnetic algal production as a pelagic food web subsidy using a stable isotope tracer,” she says. In other words, she will be looking at how important organisms living in between the cool bottom and warm surface waters are to the lake food web. Is the metalimnion an aquatic buffet for fish, or a food desert?
Wilkinson and research technician Jason Kurtzweil load WILCS in the boat.

The way Wilkinson and Yang will study this is by looking at zooplankton. Since they know that zooplankton migrate daily in the water column to feed in the epilimnion at night, they want to be able to see if they also feed on the algae and phytoplankton in the metalimnion too. However, this is very difficult to see in the lab. Each layer of the lake has a specific carbon “signature” that can be seen in the lab by measuring carbon isotopes. However, “the isotopic signature between the metalimnion and epilimion are extremely similar,” explains Wilkinson, “so I added Carbon-13 to the metalimnion to see the difference.”
In order to successfully label the metalimnic layer, she has even designed her own device: the Wilkinsin Isotope Labeling Contraption (WILC).
WILC- Wilkinson Isotope Labeling Contraption

There are 19 “wilkies” (as the researchers call them) set up in a grid cross the lake. Together, Wilkinson and Wang row along the grid, stopping at each buoy. They use an empty syringe to pump air into a small plastic tube, which releases the vacuum seal in the WILC that allows the Carbon-13 tracer to be released.
Wilkinson’s project is very similar to the experiment Ryan Batt is conducting on Ward Lake. In the pursuit of further understanding aquatic food webs under the *CASCADE project at **UNDERC, Wilkinson is determining where zooplankton and fish are eating in the lake.
“Well, really, the ultimate question we are asking is: what is feeding the food web?” While Batt is changing the source of fish food in Ward Lake, Wilkinson is simply labeling part of her fish food in order to track it.
As the summer goes on, Wilkinson and Yang will be taking zooplankton samples from various depths  of the lake. If the zooplankton are feeding in the epilimnion, they will show no traces of the Carbon-13 Wilkinson and Wang have placed below. If they demonstrate high levels of the label, then it will suggest that they do in fact depend on primary production in the deeper layers.
“What we are doing is trying to answer a part of a large fundamental question,” says Wilkinson, “which is, what is it that supports the food web?” And what supports the food web inevitably supports the ecosystem. That is why all environmental and ecological scientists are so interested in discovering the answer to this and other ecosystems around the world. Once food webs and ecosystems are fully understood, more effective management strategies can be developed to protect the natural places humans hold dear.

All summer long, Trout Lake Station outreach assistant, Ali Branscombe will be bringing you stories from the field. Join Ali as she follows researchers slogging through wetlands, boarding boats, and wrangling fish – all to bring you Limnology in Action.

*CASCADE has been an ongoing whole-ecosystem experimental program that studies food web recovery rates and regime shifts, along with many other theories. Principal investigators include CFL’s Director Steve Carpenter and faculty Jim Kitchell
**The University of Notre Dame Environmental Research Center (UNDERC) is a reserve located on the Wisconsin/Michigan (Upper Peninsula) border. UW-Madison and Notre Dame have had strong research connections since the early 1900’s, when UW-Madison limnologists sampled many of the lakes on the reserve. Today, CFL’s Trout Lake Station and UNDERC continue their strong research ties and collaborate on many projects.