Mary Lake
Lincoln County, WisconsinLimited DataOligotrophic
Mary Lake earns an A — every scored parameter (clarity) rates among the best monitored waters in Lincoln County.
A TSI value of 33 puts Mary Lake in the oligotrophic range, where biological productivity stays low and the water column remains clear. A maximum depth of 42 ft puts Mary Lake in the middle of Wisconsin's depth distribution. At 25 acres, Mary Lake sits below the Wisconsin median: a small water where shoreline activity has an outsized effect on water quality. Mary Lake ranks 2 of 18 in Lincoln County, putting it in the upper tier of locally monitored lakes.
No invasive species are currently listed at Mary Lake — the lake remains off the Wisconsin infested-waters roster. Bass fishing is part of the appeal: the species list runs to 1, anchored by largemouth and/or smallmouth bass. The grade is based on limited monitoring — fewer than three independent measurement years contribute, so future updates may shift the letter.
Source: EPA Water Quality Portal sampling records, Wisconsin DNR Surface Water, last sampled 2023-07-24. Grade methodology: LakeGrade methodology.
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Swimming Safety
Excellent for swimming, crystal clear water with minimal algae
Water Quality Grade: A, Excellent
Crystal clear, you can see 21 ft down. Trophic State Index: 33.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 21 ft | A |
| Phosphorus | No data | |
| Chlorophyll-a (Algae) | No data | |
| Trophic State Index (TSI) | 33 | Oligotrophic |
Low nutrients, clear water, excellent for swimming
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 42 ft |
| Surface Area | 25 acres |
Fish Species
Click a species to see all Wisconsin and Minnesota lakes where it is found.
→ Best fishing times for Mary Lake (14-day solunar calendar)
→ Is it safe to eat fish from Mary Lake? (mercury & PFAS guide)
→ Mary Lake fishing regulations (limits, seasons, special rules)
Water Quality Trend: ↑ Improving
Based on 6 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +0.929 m/yr | 6 |
| Phosphorus | ↑ Improving | -1.67 µg/L/yr | 6 |
Location
County Ranking
Ranked #2 of 18 lakes in Lincoln County
Nearby Lakes in Lincoln County
More Wisconsin Lake Rankings
State Parks Near Mary Lake
Other Grade A Lakes in Wisconsin
Washington County · Clarity: 11.8 ft
Washington County · Clarity: 12.6 ft
Walworth County · Clarity: 11 ft
Waukesha County · Clarity: 11.3 ft
Walworth County · Clarity: 20.2 ft
Kenosha County · Clarity: 10 ft
Oligotrophic Lakes in Wisconsin
WI DNR Lake Profile
Authoritative data from the Wisconsin DNR LakePages. 1 station on file (most recent sample 2023).
Fish Species (DNR-rated)
DNR Reports & Resources
Wisconsin DNR Fisheries scientists publish multi-year survey reports for Mary Lake covering fish populations, stocking, water quality, and management. 1 report on file.
- 2025Pine Lake spring summary report, 2025Fisheries Survey · WI DNR · 2025 (PDF)
EPA Impairment Status
Mary Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 5).
Causes of impairment
Source: EPA ATTAINS assessment unit WI10026794 · Official waterbody report · Matched by proximity (1.57 km)
Data Sources
Water quality data from the EPA Water Quality Portal
Impairment status from EPA ATTAINS 303(d) database
Grading methodology based on Metropolitan Council standards
Most recent sample: 2023-07-24
Monitoring stations: 1