What a Difference a Day Makes: Lake Mendota Goes From Blue-Green to Crystal Clear

Two women on a pontoon boat look into a jar containing a sample taken from the water.

by Anna Mueller – I showed up at the Center for Limnology’s Hasler Lab a few hours after the sun had risen and it was already getting hot. I was sweaty from biking to work and could feel the sun burning against my hair. Like any other day, I walked around the squat CFL building and looked over the railing at the wooden dock, the same one the CFL has used since 1963. It was encased in thick, blue-green scum swirling along the rocks and into the boat slip. I went downstairs and was immediately surrounded by hot, putrid air. Eyes watering, I tried to breathe in but the smell of rotting cyanobacteria felt like cotton in my head and stomach. One woman who walked along the lakeshore path that day told me it reminded her of walking beside a long row of porta-potties. 

Blue-green algae and a boat motor.
The blue-green bloom made it into our boat slip, turning the basement air noxious – and potentially toxic. Photo: A. Hinterthuer

It was the 23rd of May, and the lakeshore was nearly unbearable. Aaron, our boat manager, lasted until lunch in his office downstairs before he began feeling ill from exposure to the potentially toxic air and relocated upstairs. Toxic algal blooms thrive in warm environments with an excess of nutrients, shallow waters, and little wind. They can occur quickly and intensely. In this case, Lake Mendota was suffering from an Aphanizomenon bloom. Aphanizomenon is a type of phytoplankton called cyanobacteria (or blue-green algae). Under a microscope, it looks like tiny clumps of grass. Although it is almost always in the water, when present in these large quantities it can cause consequences worse than an unpleasant smell. Aphanizomenon produces liver and kidney toxins, as well as neurotoxins that can affect the nervous system. An adult exposed to a bloom like this may become sick, but small children and dogs are at risk for serious health problems. 

On May 24th, the day after this unpleasant bloom, I returned to the lab to find a strong northeast wind. It was churning the water into white caps and much of the algae was gone from the shore. The waves were a dark, milky green and visibility in the water remained low.

We had come in to take members of the Clean Lakes Alliance’s citizen water monitors out on the lake so that they could be trained to take measurements like dissolved oxygen and water clarity, which they submit each week to the Wisconsin Department of Natural Resources for their Surface Water Integrated Monitoring System, or “SWIMS.” SWIMS is a database that holds state-wide information for the Clean Water Act. This citizen data helps inform water quality policy and maintains a record of limnological trends. 

A man on a pontoon boat talks to a group about water quality.
CFL graduate student, Bennett McAfee talks to Clean Lakes Alliance volunteers about the importance of long-term data. Photo: A. Hinterthuer

Near the northeast shore, we used a Secchi disk to measure water clarity. Secchi disks are black and white circular disks that are lowered into the water from a pier or boat. Researchers take note of the depth where they can no longer see the disk and use that measurement to calculate transparency. Secchi disks have been used since 1865, and the longevity of their use has allowed researchers to compare water clarity over time and between different bodies of water. 

On the northeast side of Lake Mendota, we measured 20 feet of clear water. We might have measured more, but the Secchi disk hit the bottom while it was still visible. Secchi disk measurements that deep usually occur only during this time of year, a period researchers call the ‘clear water phase.’ The average reading during the rest of the summer is about three feet. 

A woman holds up a black and white Secchi disk on the side of a boat.
Caitlin McAleavy from the Clean Lakes Alliance holds a Secchi disk that measured good water clarity in Lake Mendota. Photo: A. Hinterthuer

After the Secchi disk training, I used a conical-shaped net to take a sample of zooplankton in the water. Among other species, we observed a large number of daphnia mendotae. Daphnia mendotae are named for Lake Mendota, but thrive in many other lakes and aquatic environments. They are characterized by their relatively large size and ability to improve water quality and clarity by eating tons of the algae that makes the water green. 

In the 1980s, UW scientists worked with the Wisconsin DNR on a “lake manipulation experiment” in Lake Mendota. The DNR began stocking species like pike and walleye and relaxed bag limits on panfish. The idea was to reduce the populations of panfish, a major predator of daphnia. This successful experiment resulted in huge populations of daphnia and an extra meter of clear water in Mendota. Unfortunately, since the introduction of the invasive spiny water flea, which also eats daphnia, that meter has been lost. 

What struck me most about these clear water measurements was the stark contrast between the water quality on one side of the lake and the other, as well as the difference day to day. I would have felt comfortable jumping in for a swim on the northeast side of the lake, yet you couldn’t have paid me to swim along the south shore, where the water disappeared under mats of toxic scum. 

Already as I write this, the bloom has receded. Inconsistency in weather and lake conditions may not pose a threat to the earth as a biosphere, or even to the continuation of aquatic biomes, but it affects the systems that make our environments habitable for humanity. Tracking water quality is one way of better understanding aquatic ecosystems, and can inform policy and behavior. If you are interested in active participation, consider contacting your representative, joining a local “friends of the lake group,” or volunteering for the Lake Water Quality Monitoring project organized by the Clean Lakes Alliance.