Blue-Green Algae in Florida Fresh Water — When Not to Swim, Why It Happens, and How to Read the Bloom

You pull up to a boat ramp on a canal off the Caloosahatchee in July. The launch looks fine from the truck — until you walk to the edge and the water has a sheen on it like someone tipped a can of latex paint into the river. Bright green, swirled, a little foamy where it collects against the dock pilings. There’s a smell, too: not low-tide funk, something sharper and more chemical. A heron stands in the shallows looking unimpressed. You were going to put the kayak in here.

Don’t.

That green isn’t pollen, and it isn’t pond scum in the harmless sense. It’s a cyanobacteria bloom — what everyone calls blue-green algae — and depending on the day and the strain, that water can give you a rash, wreck your stomach if you swallow it, and kill a dog that drinks it. Knowing how to read it before you launch is the difference between a good Florida summer and a bad one.

Blue-green algae aren’t algae and they aren’t always blue or green. They’re bacteria old enough to have invented oxygen, and in a warm Florida canal with enough fertilizer in it, they will absolutely ruin your afternoon.

What blue-green algae actually is

The first useful fact is a technicality that matters: cyanobacteria are bacteria, not true algae. They photosynthesize like plants and they’ve been doing it for billions of years — they’re some of the oldest life on Earth, the organisms that first pumped oxygen into the atmosphere. In small amounts they live in basically every freshwater body in Florida, harmlessly, all the time.

The problem is the bloom. When the water is warm, slow-moving, and loaded with nutrients — nitrogen and phosphorus — cyanobacteria multiply fast enough to take over the surface. That’s what you’re looking at when a lake or canal goes the color of split-pea soup. The bloom can look like spilled green or blue-green paint, like thick floating mats, like a scummy film, sometimes with a swirly marbled surface or foam along the shoreline. And it usually smells bad — musty, earthy, or septic.

This is fresh water only: lakes, rivers, canals, retention ponds, the slow nutrient-rich water inland. That distinction is the one most people get wrong, so let’s nail it down.

Why it’s not red tide

Florida has two notorious summer-into-fall blooms and they get conflated constantly. They are not the same thing.

• Red tide is Karenia brevis, a marine organism. It blooms in the salt water of the Gulf, kills fish by the wrack-line, and aerosolizes a toxin that makes beachgoers cough. That’s an ocean problem.
• Blue-green algae are cyanobacteria, and they bloom in fresh water inland — the lakes and rivers and canals. That’s a freshwater problem.

Same season, similar “stay out of the water” advice, completely different organisms and chemistry. If you’re standing on a Gulf beach with burning eyes, that’s red tide. If you’re standing on a lake shore or a canal bank looking at green paint, that’s cyanobacteria. Don’t let anyone tell you the lake is “having a red tide” — it isn’t, and the confusion gets people checking the wrong dashboard.

Why it’s dangerous

Most of the time a bloom is ugly and smelly and that’s the end of it. The trap is that some cyanobacteria produce toxins, and you can’t tell by looking which bloom is the toxic one.

The most talked-about are microcystins, but there are several cyanotoxin families. What they can do to people:

• Skin and eye irritation — rashes, itching, burning from contact in the water.
• Stomach illness — nausea, vomiting, diarrhea if you swallow contaminated water.
• Respiratory irritation — coughing, throat irritation from spray and aerosols near a heavy bloom.

Healthy adults usually shake the mild stuff. The serious risk runs to kids, who swallow more water and weigh less, and to dogs. Dogs are the quiet body count here: a dog that swims through a bloom, drinks the water, or licks the dried scum off its own fur can ingest a lethal dose, and dogs have died from cyanotoxin poisoning around the country. They don’t read warning signs. Never let a pet in or near scummy fresh water, and if one gets in by accident, rinse it with clean water immediately and watch it closely.

Where and when it happens in Florida

Cyanobacteria can bloom anywhere the conditions line up, but Florida has a recurring main event, and it has a name: Lake Okeechobee.

Lake O is large, shallow, warm, and chronically over-fertilized — a near-perfect cyanobacteria factory in summer. When the lake blooms and water managers discharge it to relieve pressure on the aging dike, that green water travels down the rivers the lake feeds: the Caloosahatchee west to Fort Myers, and the St. Lucie east to Stuart. Those two estuaries take the brunt of it, and a bad discharge year can paint miles of waterfront the color of antifreeze.

The fuel is nutrient pollution — phosphorus and nitrogen from farm runoff, urban stormwater, and leaking or undersized septic systems — plus Florida’s warm water and long summer. Put those together and summer is peak season, roughly the warm-water stretch when the lake and its rivers are most likely to go off.

How to stay safe (the actual checklist)

You don’t need a chemistry degree. You need a routine.

1. Check the dashboard before you go. The Florida Department of Environmental Protection (DEP) and FWC run an algal bloom dashboard with sample results and locations, and counties post health advisories during active blooms. Look it up the night before and the morning of a paddle, swim, or launch.
2. Trust your eyes and nose. If the water is bright green, scummy, mat-like, foamy, or smells bad — stay out. This overrides any “no advisory” status (more on that below).
3. Keep kids and dogs out. They’re the high-risk group. No “just a quick dip.”
4. Rinse off if you’re exposed. Splashed, waded, fell in — get clean water on your skin (and your dog) as soon as you can.
5. Don’t eat fish from an active bloom. Toxins concentrate in the food chain. Skip fish caught in visibly blooming water.
6. Report it. Florida runs a bloom reporting hotline — flagging a fresh bloom helps the state’s sampling catch up to reality and warns the next person at that ramp.

What most guides won’t tell you

Here’s the honest part the tidy safety lists skip.

“No advisory posted” does not mean safe. Advisories follow sampling, and sampling lags the bloom. Someone has to collect the water, send it to a lab, and post the result — that’s days, and a canal can go from clear to pea soup in a fast warm spell. By the time the map turns red, you may already have launched into it. The dashboard is a tool, not a guarantee. Your eyes and nose are the real-time sensor. Bright green and bad-smelling wins the argument every time, dashboard or no dashboard.

It’s not just swimmers. The standard warning is aimed at people getting in to swim, but boaters and anglers are exposed too. A bloom throws spray and aerosols you can breathe; scum coats your hull, your paddle, your fishing line, your hands, and your cooler; your dog rides along and licks it off the deck. You can do a whole day on the water without “swimming” and still go home with a rash and a sick dog. If the launch looks like the canal I described up top, the right move isn’t “I’ll just stay in the boat” — it’s “I’ll go somewhere else.”

The water can look fine in patches. Blooms drift and concentrate on the downwind shore. A clear-looking middle doesn’t mean the cove you’re paddling into isn’t packed with scum. Wind moves it around hour to hour.

The conservation throughline

Cyanobacteria are natural. Florida’s lakes have always bloomed a little. What’s not natural is the scale and frequency — and that part is on us, because the root cause isn’t mysterious. It’s excess nutrients. Too much phosphorus and nitrogen running off farms, lawns, and failing septic systems into warm, slow water is the fertilizer that turns a normal summer into a toxic one.

That also means it’s fixable, which is the optimistic note. The long game is unglamorous and entirely doable: reduce fertilizer runoff, convert septic to sewer in the watersheds that need it, and restore water flow south of Lake Okeechobee so the lake isn’t forced to dump its overload east and west into the Caloosahatchee and St. Lucie. None of that is a quick fix. All of it is the difference between a Florida where summer means a green-water lottery and one where it doesn’t.

You can’t personally drain Lake O. You can keep fertilizer off your own lawn before a rainstorm, support the watershed restoration that’s already funded and fought over, and — smallest and most immediate — report the blooms you find so the next person gets a real warning.

Key takeaways

• Blue-green algae = cyanobacteria = fresh water. Red tide = Karenia brevis = salt water. Different problems, don’t confuse them.
• Bright green, scummy, foamy, bad smell = stay out. Believe your eyes over the dashboard.
• Kids and dogs are the high-risk group. Dogs have died. Keep pets away from scummy water, rinse immediately if exposed.
• Boaters and anglers are exposed too, not just swimmers — aerosols and hull scum count.
• Check the DEP/FWC bloom dashboard, report blooms to the state hotline, don’t eat fish from a bloom.
• The root cause is nutrient pollution. The fix is less runoff, septic-to-sewer, and flow restoration south of Lake O.

Check the water before you back the trailer down the ramp. And when in doubt, the green water gets the benefit of the doubt — you find another spot.