Cool Weather Means Clear Waters for Wisconsin Lakes

Most people think of green, algae-filled waters when talking about southern Wisconsin lakes. And, sure, these highly productive bodies are full of nutrients that run off from the landscape and feed huge algae blooms every year.

But, each Spring, we get a little break between ice-out and algae season. And, depending on certain climate conditions, these breaks can last several weeks.

It’s called the clear water phase and, according to Center for Limnology research specialist Ted Bier, the current forecast for Madison’s lakes is looking good.

Ted Bier (reflected) captures a Secchi disk 24 feet below his boat. Photo: Ted Bier

“Yesterday, on Lake Monona my Secchi reading was 7.5 meters,” he says. That means that the water so clear in Lake Monona at the moment that Ted could see a scientific tool called a Secchi disk dangling from the end of a rope nearly 24 feet below his boat while he was out doing routine fieldwork.

Well…okay, it wasn’t exactly “routine.”

Like most of the world, we here at the CFL are operating under social distancing guidelines and that means boats full of research crews aren’t an option for the time being. So Ted was out on Lake Monona by himself while his fellow researcher, Alice Ogden-Nussbaum, sat on shore with a pair of binoculars acting as a “safety spotter” monitoring Ted’s progress and ready to call for help should any need arise.

Ted leads the sampling efforts of our North Temperate Lakes Long-Term Ecological Monitoring team. Since the 1980’s the NTL-LTER, as it’s known in scientific circles, has monitored everything from water temperature to fish populations to nutrient loads in 11 Wisconsin lakes for nearly 40 years.

Under the microscope, you can see that the intestives of the daphnia Ted caught were full of phytoplankton. Photo: Ted Bier

Ted’s been doing this since 2002 and, when he started working on Madison lakes, he says, “I thought I had the greatest job in the world. Whatever the lakes seemed to need, they had and clear water phase would last several weeks, sometimes into July.”

Ted is an avid spearfisher and it wasn’t uncommon, he said, for him to watch multiple species of fishes move through their yearly spawning routines – from walleye, to crappie, to bluegill.

While last year also saw a strong clear water phase, the recent trend in the 2000’s has been weaker clear water phases that don’t get quite so clear and only last a few weeks.

The problem, mostly, has been temperature. When the lakes stay in these cooler temperatures of 55 to 65 degrees Fahrenheit or so, it favors two important organisms – a native zooplankton called Daphnia pulicaria, and the tiny aquatic plants and algae called phytoplankton that daphnia love to eat.

As long as our lakes stay in this temperature window, daphnia populations are so massive that they clear the water column by eating phytoplankton almost as fast as it can grow.

As part of his sampling routine yesterday, Ted took a couple of plankton tows, tossing a fine-mesh, cone-shaped net into the lake and pulling it through the water column. The result was a jar full of thousands of daphnia, those tiny, phytoplankton-eating powerhouses.

Not only were Ted’s nets full of daphnia, hie daphnia were full, too. “The intestines of all the daphnia are all green,” indicating that they were “full of phytoplankton,” he says. The huge numbers in his nets mean, he says, that “we can see an absolute, direct, and immediate correlation between lots of daphnia and clear water.”

How long that lasts is up to the weather, Ted says. But the fact that Lake Monona is already so clear even though it is still a few degrees shy of “optimal” clear water temperatures gives him hope. Lake Mendota, he notes, is currently only at a two meter deep Sechii reading, but it hasn’t warmed up enough for phytoplankton and daphnia to really take off just yet.

“What we’re seeing right now is that Lake Mendota has conditions and a timeline that represents the good old days,” he says. “If we keep these cold nights, phytoplankton and daphnia have the advantage.”