Ash Lake vs Blackduck Lake
Water quality, depth, fish species, and recreation comparison.
Ash Lake reads lower on nutrients and algae than Blackduck Lake. Both are in St. Louis County, Minnesota.
Both Ash Lake and Blackduck Lake sit in Minnesota. A same-state comparison strips out the state-level water-quality regime as a variable: any differences in the readings here are about the lakes themselves, not the agencies sampling them. The readings are close: Ash Lake and Blackduck Lake sit within a narrow band of each other on phosphorus, chlorophyll-a and clarity. The per-parameter table below shows where the small differences actually live.
With readings this close, the choice between the two lakes turns on non-water-quality factors: depth, fish species, public access, distance from home. The per-lake pages below cover all of those.
Ash Lake
Murky, only visible to about 5.5 ft.
Blackduck Lake
Moderate clarity, visible to about 6.8 ft.
Side-by-Side Metrics
| Metric | Ash Lake | Blackduck Lake |
|---|---|---|
| Water Clarity | 5.5 ft | 6.8 ft Better |
| Phosphorus | 39 µg/L Better | 43 µg/L |
| Chlorophyll-a (Algae) | 11.1 µg/L Better | 23.3 µg/L |
| Maximum Depth | 25 ft | 30 ft Better |
| Surface Area | 690 acres | 1,250 acres |
| Public Access | Yes | Yes |
| Fish Species | 9 Better | 8 |
| Trophic State | eutrophic | eutrophic |
A green ✓ marks the better value for that metric (lower phosphorus / chlorophyll = cleaner; higher Secchi / depth / species count = better). Surface area and trophic state carry no tick — a bigger lake is not a better lake.
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Verdict
Ash Lake reads lower on nutrients and algae than Blackduck Lake. Water clarity: 5.5 ft vs 6.8 ft. For fishing diversity, Ash Lake also leads with 9 species.
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Source: EPA National Aquatic Resource Surveys, 2026.