Heavy spring rains, phosphorus runoff, warm days and calm winds have created an early mess on the Yahara River chain of lakes.
A massive and toxic blue-green algae bloom is underway on Lake Mendota. Monona Bay, an offshoot of Lake Monona, is already choked with weeds. There are conditions throughout the five-lake chain normally seen in July and August.
Only we’re on the cusp of Memorial Day weekend and already anglers, boaters, skiers and swimmers have been subjected to floating mats and globules of blue-green algae that can interrupt recreational plans and be harmful to both humans and pets if ingested. Stiff winds coupled with cooler temperatures on Wednesday helped break up the surface and floating mats, but the algae and the conditions for more visible blooms remain.
“This is sort of the classic scenario,” said Emily Stanley, interim director of the Center for Limnology at UW-Madison, who has been studying Lake Mendota for 25 years. “It’s hard for me to remember a bloom this impressive this early in the year. I don’t remember a May bloom. It’s weird, frankly.”
People are also reading…
On Wednesday, Public Health Madison and Dane County closed Lake Mendota’s Spring Harbor Beach to swimmers due to blue-green algae, a group of bacteria known as cyanobacteria. Contact with the toxins can cause upset stomach, rashes and respiratory irritation. Dogs who ingest harmful algal blooms can also get sick and sometimes die.
The closure comes as Public Health staff begin their annual water quality monitoring at local beaches from Memorial Day through Labor Day. The Clean Lakes Alliance on Thursday will launch its 10th consecutive season of monitoring the lakes with a team of more than 80 trained volunteer citizen scientists who collect water samples twice a week and upload their data and visual observations into the free “LakeForecast” app.
“They are essentially a first line of defense. They are going to spot it quicker than anybody else,” Adam Sodersten, a spokesperson for the Clean Lakes Alliance, said Wednesday. “It’s really important for people to understand how dynamic our lakes are. So while there may be a cyanobacteria bloom that means you can’t use the beaches on the south end of Lake Mendota, it could be completely different on the north side of Lake Monona.”
Conducive conditions
Like algae, weeds can also be tough to tame, thanks to the same conditions that help fuel algae blooms. The Dane County Land & Water Resources Department manages an aquatic plant harvesting program that uses 12 mechanical weed cutters to harvest Eurasian water milfoil and other invasives that are hauled by truck to remote compost sites.
Monona Bay is among one of the most challenging and visible spots for the weed cutters to operate, all in an effort allow boaters, anglers and paddlers better access and conditions. On Wednesday afternoon, two of the harvesters plowed their way across the bay’s surface, while some homeowners used pitchforks to remove weeds in shallow areas.
“It’s nutrients and warm weather,” Stanley said of the weed growth. “Monona Bay just has a very healthy population of plants, and they are running the show there instead of the blue-greens.”
The Yahara chain of lakes — which includes Mendota, Monona, Wingra, Waubesa and Kegonsa — is primarily fed by the Yahara River and the 72,093-acre Lake Mendota watershed that stretches north into Columbia County. And while Dane County bans the use of lawn fertilizers that contain phosphorus, a key fuel to weed and algae growth, the Lake Mendota watershed is largely farmland where phosphorus is allowed to be spread on fields.
“There’s a long history of farming there and a lot of phosphorous in the soil from fertilizer and manure being applied,” Stanley said. “It’s a large bank account, and when you get a lot of those rains, especially in the spring before the plants have started growing, the phosphorus comes in and it’s the phosphorous that’s fueling the blooms. Every little bit of phosphorus reduction helps, but most phosphorus is upstream in that agricultural area.”
Altered ‘rhythm’
For more than a week, floating globs of blue-green algae covered much of Lake Mendota. On Saturday evening, there was little blue-green algae visible in University Bay. But by Monday night, the conditions had dramatically changed with the west and south side of the bay covered with mats of blue-green algae as the population grew in size, needed more light and began to “shade themselves out,” Stanley said.
“What you didn’t necessarily see on Saturday is that this particular blue-green algae starts cooking at a lower depth,” Stanley said. “So they have the ability to rise up in the water column, and that’s when you start seeing them. So it seems like it’s really fast, but you don’t see what’s been going on under the water for the past couple of days.”
Stanley and her staff at Limnology have a prime perch to monitor the conditions of the 9,781-acre lake, the largest and deepest lake in the Yahara chain. The Center’s Hasler Lab is located just west of Memorial Union and provides an ideal spot for research.
Grace Wilkinson, a limnologist and assistant professor in the lab, laid on her stomach Wednesday on the pier just steps from her office to gather a sample of lake water. Moments later she was peering through a microscope explaining colonies of aphanizomenon that when magnified look like strands of grass. In reality, they can form threads and then ultimately rafts of blue-green algae as lake temperatures warm and winds calm. Seasonal changes, particularly in the spring and fall, with warmer temperatures and more frequent and intense storms, is leading to earlier and more pronounced algae blooms, Wilkinson said.
“The lake’s rhythm is being altered,” said Wilkinson, a Minnesota native who joined the lab in 2021 after a three-year stint at Iowa State University. “So that’s making it even less predictable. And that’s the biggest question for us in trying to understand how Lake Mendota is changing in response to these changes.”