Is Ferguson Lake safe to swim?

As of July 20, 2026, Ferguson Lake in Penobscot County, Maine carries a swim caution. Satellite imagery measured 47,357 cells/mL of blue-green algae on July 18, 2026, elevated but below the 100,000 cells/mL advisory threshold. Rainfall over the past week totaled 0.5 in, which can wash bacteria and nutrients into the water.

How this read is built

Ferguson Lake is a lake in Penobscot County, Maine covering about 0.4 sq mi, the 325th-largest of the 331 lakes SwimZone monitors in Maine. Satellite imagery measured 47,357 cells/mL of blue-green algae on July 18, 2026, elevated but below the 100,000 cells/mL advisory threshold.

Blue-green algae (cyanobacteria) blooms are most common from midsummer into early fall, when the water is warm, calm, and rich in nutrients from runoff. They can produce toxins that make people sick and can be rapidly fatal to dogs, so skip water that looks green, scummy, or like spilled paint, and keep children and pets out of it.

Over 7 summers of satellite monitoring since 2020, Ferguson Lake has reached blue-green algae advisory levels in 4 of them, most recently in September 2025. Its highest recorded reading was 625,897 cells/mL in September 2025.

Is it safe to swim in Ferguson Lake right now?

Ferguson Lake carries a swim caution today. Satellite imagery measured 47,357 cells/mL of blue-green algae on July 18, 2026, elevated but below the 100,000 cells/mL advisory threshold. Conditions can change quickly with wind, heat, and rain, so check the posted signs and look at the water before you get in.

Does Ferguson Lake have blue-green algae (cyanobacteria)?

Low levels. Satellite monitoring measured 47,357 cells/mL at Ferguson Lake on July 18, 2026, below the 100,000 cells/mL advisory threshold. Keep an eye out for surface scum, which can concentrate quickly.

Can dogs swim in Ferguson Lake?

Use caution. Blue-green algae at Ferguson Lake are elevated (47,357 cells/mL on July 18, 2026). Dogs are far more sensitive to cyanobacteria than people, so keep them out of the water and away from surface scum.

How often does Ferguson Lake have blue-green algae blooms?

Over 7 summers of satellite monitoring since 2020, Ferguson Lake has reached blue-green algae advisory levels in 4 of them, most recently in September 2025. Its highest recorded reading was 625,897 cells/mL in September 2025.

How many Maine lakes and beaches are unsafe for swimming today?

Today, 21 Maine waters are above a safe-contact threshold and 27 more warrant caution, out of 469 monitored waters SwimZone tracks in the state.

How is Ferguson Lake's water quality measured?

SwimZone builds this read from EPA CyAN satellite imagery for blue-green algae, official beach and fecal-bacteria advisories where a program monitors Ferguson Lake, and a recent-rainfall overlay, all refreshed daily. It was last updated July 20, 2026.