Big Lake vs Blackduck Lake
Water quality, depth, fish species, and recreation comparison.
Big Lake has a higher water quality grade (A, Excellent) than Blackduck Lake (C, Fair). Both are in Beltrami County, Minnesota.
Both Big Lake and Blackduck Lake sit in Minnesota. A same-state comparison strips out the state-level water-quality regime as a variable: any grade differences here are about the lakes themselves, not the agencies grading them. The grades are meaningfully apart: Big Lake grades a A while Blackduck Lake grades a C. That two-letter spread reflects real underlying differences — different watershed inputs, different depth profiles, or different monitoring rigor.
For a recreational visitor, the wider grade gap is decisive — Big Lake is the better water-quality choice. For a researcher, the gap is the interesting part: what is different about the two watersheds, and which of those differences is mutable?
Big Lake
Moderate clarity, visible to about 9.7 ft.
Blackduck Lake
Murky, only visible to about 6.2 ft.
Side-by-Side Metrics
| Metric | Big Lake | Blackduck Lake |
|---|---|---|
| Overall Grade | A (Excellent) | C (Fair) |
| Water Clarity | 9.7 ft | 6.2 ft |
| Phosphorus | 15 µg/L | 34 µg/L |
| Chlorophyll-a (Algae) | 4.8 µg/L | 12.3 µg/L |
| Maximum Depth | 35 ft | 28 ft |
| Surface Area | 3,598 acres | 2,711 acres |
| Public Access | Yes | Yes |
| Fish Species | 17 | 16 |
| Trophic State | mesotrophic | eutrophic |
Bold value = better for that metric (lower phosphorus / chlorophyll = cleaner; higher Secchi / depth / species count = better).
Verdict
Big Lake wins on overall water quality with a Grade A versus Blackduck Lake's Grade C. Water clarity: 9.7 ft vs 6.2 ft. For fishing diversity, Big Lake also leads with 17 species.
Source: EPA National Aquatic Resource Surveys, 2026.