Blue-green algae: how to tell if a lake is safe
Blue-green algae, also called cyanobacteria, can turn parts of a lake into a health hazard for people and especially for dogs. You don't need a lab test to make a reasonable call on the shore: learn what a bloom looks like, and if anything looks off, stay out of the water.
What a harmful bloom looks like
- Water that looks like spilled green or blue-green paint, often thickest near shore or in a calm cove.
- A scummy, foamy or paint-like film sitting on the surface rather than mixed into clear water.
- Streaks or clumps of green, blue-green, brown or even reddish color, sometimes looking like thick pea soup.
- A noticeable, unpleasant odor coming from the water, though some blooms have little to no smell.
- Dead fish or dead aquatic life along the shoreline near discolored water.
- Unusual stillness or a thick, almost oily sheen on an otherwise calm surface.
What normal, healthy-looking water looks like
Most lakes have some natural algae and plant life, and a greenish tinge in water isn't automatically dangerous. Ordinary pond scum or filamentous green algae usually looks stringy or mat-like and stays more localized, while a cyanobacteria bloom tends to look more like a spill of paint or a thick, uniform surface layer. If you're not sure which you're looking at, treat it as a bloom until you know otherwise.
A lake can look perfectly clear in one cove and show a visible bloom in another just a short paddle away, since calm, warm, shallow water is where blooms tend to form first. Judging the whole lake from a single glance at one access point isn't reliable.
Why dogs are at the highest risk
Dogs are especially vulnerable to harmful algae because they're more likely to swim through, drink, or lick water directly from the surface or off their fur afterward, and some toxins can affect them quickly at low doses. A dog that swims in a bloom and then grooms itself can ingest a meaningful amount of toxin in a short time. If you see any sign of a bloom, keep dogs leashed and away from the shoreline entirely, not just out of the water.
Watch for symptoms in pets after any lake swim during algae season, including vomiting, stumbling, drooling, seizures or sudden lethargy, and get to a veterinarian immediately if you see any of these signs.
Rinsing a dog off with clean water right after any lake swim, even when the water looked fine, is a reasonable habit during warm months, since it reduces how much of anything the dog might later lick off its coat.
Risks for kids and adults
- Swallowing even small amounts of water during a bloom can cause stomach upset, vomiting or diarrhea.
- Skin contact can cause rashes, itching or irritation, especially after prolonged contact.
- Breathing in spray or mist near a bloom can irritate the throat and lungs.
- Kids are more likely to swallow water while playing and tend to spend more time directly in shallow, bloom-prone areas near shore.
- Symptoms can show up hours after exposure rather than immediately, so don't assume everything is fine just because no one felt sick right away.
The simple rule: when in doubt, stay out
You can't reliably tell how toxic a bloom is just by looking at it, and toxin levels can change quickly with weather and water movement. The safest approach is also the simplest: if the water looks unusual in any way described above, don't swim in it, don't let pets near it, and don't let kids wade or play in it, even if a different part of the same lake looks completely clear.
This rule matters more than memorizing every visual detail of what a bloom looks like. If something about the water gives you pause, that hesitation is reason enough to choose a different spot or a different day.
When blooms are most common
Harmful algae blooms tend to show up in warm weather, often from mid-summer into early fall, and are more likely in calm, shallow, nutrient-rich water such as coves and the backs of bays. Wind can concentrate a bloom against one shoreline one day and disperse it the next, so conditions can change within the same lake over just a few days.
A stretch of hot, still weather after a period of rain, which washes nutrients into the water, is a common setup for a bloom to develop, so it's worth being a little more cautious in those conditions even before you see any visible sign.
Check your state's current advisories
State environmental and health agencies track and post advisories for lakes with known or suspected algae blooms, and these are far more reliable than a single visual check from shore. Before heading to a lake in warm-weather months, look up whether an advisory is active for that specific lake, and follow any posted signage at the access point or beach. LakeScope can show you today's water temperature and conditions to help you plan a trip, but it isn't a substitute for checking an active state advisory, and no app or website can tell you a lake 'is safe' from algae on a given day.
Questions
What does a harmful algae bloom look like?
Water that resembles spilled green or blue-green paint, a scummy surface film, or streaks of unusual color, sometimes with a bad smell or dead fish nearby.
Why are dogs more at risk from algae blooms than people?
Dogs are more likely to drink lake water directly or lick it off their fur after swimming, which can mean ingesting more toxin, and some toxins act on dogs quickly at low doses.
Can you swim in a lake with a little bit of algae?
If any part of the water looks like a bloom (paint-like color, surface scum, unusual odor), the safest choice is to avoid swimming there, even if other parts of the lake look clear.
When are algae blooms most likely?
Warm weather, typically mid-summer into early fall, in calm, shallow, nutrient-rich water like coves and the backs of bays.
What should I do if my dog swims in a bloom?
Rinse your dog off promptly, watch closely for vomiting, drooling, stumbling, seizures or sudden lethargy, and get to a veterinarian immediately if any of these appear.
Where can I check if a lake currently has an algae advisory?
Your state's environmental or health agency typically posts current advisories for affected lakes, and these should be checked before any trip during warm-weather months.
Your lake every morning
Water temperature, level, wind and bite times on your phone.
