Lake Mendota’s waters may have seemed clearer last summer, with fewer reported blue-green algal blooms than in 2015 and data showing higher near-shore clarity. But the long-term records on water quality reveal that 2016 looked no different than summers qualified as “bad years.”
Like the weather, the Yahara lakes experience year-to-year fluctuations. Factors like the amount of rain and the populations of certain aquatic critters will make the waters clearer or murkier.
Then there is the long-term “climate” of water quality, and those trends tell us the lakes are not getting any better. In fact, despite a history of efforts to improve the lakes beginning in the 1950s, there has been no progress.
“The biggest challenge is reducing phosphorus input, and so far there has been no reduction in phosphorus input to Lake Mendota,” says Stephen Carpenter, director of the University of Wisconsin–Madison Center for Limnology, adding that improving Mendota, the top of the Yahara chain, is the key to improving all of the lakes.
Phosphorus is what Carpenter calls the “lynchpin” of our water quality problems. We simply have too much of the nutrient in our soils and lakes, a surplus that comes predominantly from a history of excessive agricultural fertilizer and manure. Excess phosphorus affects water quality conditions for decades.
Other problems include more big rainstorms flushing more phosphorus runoff into the lakes, and a changing landscape, with urban growth and bigger livestock farms on fewer acres making the land more susceptible to erosion and runoff.
And then there’s the game changer: the spiny water flea, an invasive species discovered in Lake Mendota in 2009 that threatens to be a death knell in the pursuit of clean lakes. It eats Daphnia, a species of zooplankton that eats algae and, thus, serves a critical janitorial role in the lake.
Daphnia are actually the stars of the most successful water-cleaning effort to date. In 1987, the Center for Limnology, Wisconsin Department of Natural Resources and local fishing clubs teamed up to tweak fishing regulations to help these algae vacuums flourish in the lakes, and flourish they did.
But the fleas’ arrival may have put an end to Daphnia’s glory—except, for reasons still unknown, the fleas didn’t do so well in 2016 and Daphnia made a comeback, which is one cause of the seemingly clearer water.
“2016 looked great if you look at just post-spiny water flea years, but if you look at … 1995 through 2008, it’s closer to a bad year. So our baseline has shifted,” says Jake Walsh, a postdoctoral researcher at the Center for Limnology.
The bright side is the lakes have not gotten worse, which, to Carpenter, means our efforts are making progress. He thinks getting to where we want to go will come down to solutions for manure, despite its political sensitivities.
Paul Dearlove is watershed program director for Madison-based nonprofit Clean Lakes Alliance, which collected the data showing last year’s improved near-shore water clarity. He finds promise in the increasing public participation in support of the lakes, but he says a challenge will be keeping people motivated over the long haul.
“Lakes are slow to respond sometimes. It’s not like turning a switch and all of a sudden you go from green to blue. It’s a process that unfolds over many years,” he says.