Mercer County Lake Quality
Ohio, 2 lakes, average grade F (Very Poor)
Mercer County has 2 graded lakes — a thin enough sample that one or two stressed lakes can move the county average noticeably. Use the per-lake pages below for what actually matters. The county average is F — the most-stressed bracket in our rubric. Most monitored lakes here show persistent nutrient loading and algal pressure.
Most of the county's monitored lakes are shallow — under 50 feet — which means wind keeps the water column mixed all summer and sediment-bound phosphorus releases back into the surface layer whenever the bottom is disturbed. Grand Lake St Marys (F) is the cleanest in the county.
Quick Answers for Mercer County
Planning a trip? Check special fishing regulations for Grand Lake St Marys — or check the official Ohio fishing regulations.
All Lakes in Mercer County
| Rank | Lake | Grade | Clarity | Phosphorus |
|---|---|---|---|---|
| 1 | Grand Lake St Marys | F | - | 350 µg/L |
| 2 | Grand Lake St Marys @ Intake | F | - | 395.5 µg/L |
Other Counties in Ohio
See how Mercer County stacks up in the national county water-quality rankings, or browse the full directory of graded lakes across all 36 states and 2 Canadian provinces.
Where to stay near Mercer County
Top-rated places to stay near Mercer County
Availability and prices from Stay22 partners (Booking.com, Vrbo, Expedia). We may earn a commission from bookings, at no extra cost to you.
Planning a trip to Mercer County?
Get lake water-quality report cards plus the best cabin & lodging deals near Mercer County — straight to your inbox. No spam, unsubscribe anytime.
Source: EPA National Aquatic Resource Surveys, 2026.