Lake Health
Live Lake Data
A monitoring buoy run with the University of Wisconsin-Milwaukee records water temperature and oxygen at depth through the open-water season. It runs on solar power and sends new readings every 15 minutes over a cellular connection. Past readings can be downloaded.
Trained resident volunteers measure water clarity and take samples as part of the state's Citizen Lake Monitoring Network. Their records are kept by the Wisconsin Department of Natural Resources in its Water Explorer.
Blue-Green Algae and Your Safety
Blue-green algae are not true algae. They are bacteria, called cyanobacteria, that grow like algae. They occur naturally in Wisconsin lakes. Warm water and phosphorus can feed heavy growths called blooms. Some blooms make microcystin, a toxin that can harm the liver.
If You See a Bloom
Toxin levels change from place to place and from day to day. Scum along the shore carries the highest readings.
- Stay out of water that looks like pea soup or spilled paint.
- Keep children and pets away from shoreline scum.
- Rinse off with clean water after contact with lake water.
A safe reading at the buoy says little about the water at your dock.
In Wisconsin, only the state Department of Health Services or the local public health agency can close a beach. Local health departments may use the U.S. Environmental Protection Agency's swimming advisory level for microcystin, 8 micrograms per liter, to decide when to post an advisory.
What the Measurements Show
Every figure below was measured on Tug Lake and names the study it comes from.
Phosphorus. Phosphorus is the nutrient that feeds algae in Tug Lake. Summer phosphorus averaged 33 micrograms per liter from 2015 through 2023, with no significant summer trend across those years (Oldenburg, 2023).
Deep-water oxygen. Each summer, water deeper than about 10 to 12 feet loses nearly all its oxygen. In 2025 the buoy recorded that condition from early July to mid-October (Miller, 2025). Low-oxygen water near the bottom lets the sediment release phosphorus. On one day in 2016, phosphorus measured 36 micrograms per liter at the surface and 490 near the bottom (Cason, 2016).
Toxins. The District has tested for microcystin with the University of Wisconsin-Milwaukee in 2017, 2018, 2022, and 2025. Open-water readings have usually been low. The 2018 season averaged 0.7 micrograms per liter, and the highest deep-water reading in 2025 was 1.07. Shoreline samples taken during blooms have run far higher. Readings reached 130 micrograms per liter in October 2018, and 531 and 698 at a dock in August and September 2025. Open water measured below 2 on those same 2025 dates (Miller, 2018 and 2025). The U.S. Environmental Protection Agency's recommended swimming advisory level is 8 micrograms per liter.
Clarity. Clarity is measured by lowering a black-and-white disk until it disappears. Volunteer readings in 2026 averaged about 2 feet through July 20. Because the water is naturally tea-colored, clarity runs low here even without algae.
In Plain Language
The lake's deep water loses its oxygen every summer. When that happens, the lake bottom gives up phosphorus, and phosphorus feeds algae. Most of the time the open water tests low for toxins. During a bloom, the water near shore can test far above safe swimming levels.
Studies
Reports on Tug Lake, newest first.
- Tug Lake Sediment Assessment Report (PDF), Cason & Associates, 2020. Tests of the lake-bottom sediment and the phosphorus it holds.
- Tug Lake Internal Phosphorus Loading Summary Report (PDF), Dr. Todd Miller, University of Wisconsin-Milwaukee, 2018. How much phosphorus the lake bottom releases in summer.
- Tug Lake Water Quality Monitoring (PDF), Dr. Todd Miller, University of Wisconsin-Milwaukee, 2017. The first season of the university's monitoring program.
- Tug Lake Water Testing (PDF), Cason & Associates, 2016. An early water quality assessment made for the Task Force.
For a summary of every study, who did it, and what it found, see Studies of Tug Lake.
Page last updated: October 30, 2026.