Limnology in Action: How to Turn a Lake Blue

Aquashade in the water will decrease the growth of primary productivity in Ward lake.

Ryan Batt dips a bucket into the lake, and pulls up a quarter-gallon of… Gatorade?
At the surface, Ward Lake is like any small bog: tall trees and sedges are reflected on the dark surface and beaver-gnawed branches stick out at odd angles.  Looking down into the water, the lake doesn’t seem that remarkable. But inside the white gallon bucket, the water is sapphire blue, like jell-o or a sports drink. This summer, Batt is turning a lake blue.
The process that Batt, a UW-Madison graduate in CFL director Steve Carpenter’s lab, is using in his experiment is actually very common. In fact, if you have ever seen a golf course pond, chances are you have seen the effects of adding Aquashade to the water. It is a concentrated liquid formula that is similar to food coloring, or blue dye. It is used in lakes and ponds to block light rays from penetrating deep into the water, therefore reducing photosynthesis. In other words it keeps algae from growing.
 And that is exactly what Batt is hoping to do.
The Experiment: Lake Manipulation
In a resource manipulation experiment, Batt is using Aquashade to dramatically reduce primary productivity in the lake, and then observing how the lake fauna react. Because the fish and zooplankton in Ward Lake mainly rely on primary productivity (algae) within the lake to get their food, Batt wants to see if they will switch their diets to terrestrial resources like the food provided by stream inflow, leaves and fallen logs.
Batt and Tomamichel row out to collect traps.

“What we are doing is taking away a high quality resource to force them to eat a low quality resource,” says Batt. “It is like eating imported, pre-processed food instead of fresh, delicious food.”
Batt is using isotope sampling for a lot of his measurements in order to trace the source of these dietary inputs and quantify the terrestrial carbon the fish and zooplankton are taking in. Batt’s undergraduate research assistant, Megan Tomamichel is tracking the physical conditions of Ward Lake’s fish over time, to see if they are remaining healthy and growing normally. So today they are sorting fish.
The Method: Fish Conditions
Tomamichel demonstrates a minnow trap.

The fish in Ward are very small, minnows in fact. But there is an enormous variety. Tomamichel and Batt start by rowing out and collecting fish from pre-set minnow traps. Batt points out the sedges and Alder surrounding the lake that provide direct sources of carbon, along with a nearby marsh that also provides nutrients. “The surrounding wetlands act like a giant teabag seeping out dissolved carbon,” he says. Clearly, Ward Lake has plenty of terrestrial input.
A small clip on the tail marks that a fish has been “captured” so scientists can get an accurate capture/recapture estimate.

Back on shore, research technicians join the crew and set up a miniature sorting station. First, two researchers sort and measure the fish by species, then the fish are weighed (despite all the flopping) by another technician. At the end of the line, another technician is stationed for clipping. The tail fins on the  minnows are gently trimmed to mark all captured minnows. As the summer goes on, Tomamichel will be able to see how many recaptured minnows she finds after each sampling and be able to compile an accurate population estimate, without double (or triple) counting already-caught minnows.
Despite the efficient system, the sorting takes hours. The variety of fauna is incredible: Black Nose Shiner, Fathead minnow and Northern Redbelly Dace are just a few. A groan of pain and frustration is uttered whenever someone gets pricked by a Bullhead catfish and enormous horse flies buzz and bite at the researchers, but they keep working. “I think you have to be a glutton for punishment if you are a scientist,” laughs one of the technicians as she rubs her sunburn. Finally, all the minnows have been recorded and released back into the lake.
The group tries to keep fish from flopping out of their coolers.

The Question
The seemingly ironic question of the experiment is: are fish terrestrial feeders, or do they prefer the aquatic buffet? If it turns out that fish, zooplankton, and plants are in fact dependent on terrestrial nutrients (carbon), then this could have an impact on land management strategies surrounding aquatic ecosystems.
Reading the Landscape
Today, scientists are starting to realize the importance that the landscape holds in providing resources to lake food webs. The difficulty is finding out just how much the terrestrial landscape contributes. The *CASCADE project at the University of Notre Dame Ecological Research Center (**UNDERC) is working to understand these and other food-web questions.
Ward Lake

*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.

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, bringing you – Limnology in Action.