Before a beach day, a paddle or a fishing trip, check two things: FWC's red tide status for salty coastal water and FDEP's algal bloom dashboard for blue-green algae in rivers, canals and lakes. Red tide is caused by Karenia brevis, a marine dinoflagellate; blue-green algae are cyanobacteria, such as Microcystis, that grow mostly in fresh and low-salinity water. Neither color nor smell tells you whether toxins are present, and conditions can change within a day or differ between beaches a few miles apart.
Two different blooms
Around Charlotte Harbor, location narrows down what to check. Red tide is a concern on Gulf beaches, in the passes and in the saltier parts of the harbor. Blue-green algae are a concern in the rivers, canals, lakes and the upper Caloosahatchee, and fresh water can carry them downstream into brackish water.
| Feature | Red tide | Blue-green algae |
|---|---|---|
| Organism | Karenia brevis, a marine dinoflagellate | Cyanobacteria, including Microcystis |
| Usual setting here | Gulf beaches, passes and salty harbor water | Rivers, canals, lakes, the upper Caloosahatchee |
| What you may notice | Often nothing in the water; dead fish, coughing on the beach, sometimes discolored water | Green paint-like streaks, floating mats or scum, a musty smell |
| Effects on people | Airborne toxins irritate eyes, nose, throat and lungs | Skin, eye and stomach illness from contact or swallowing water |
| Fish and shellfish | Fish kills; toxins build up in clams and oysters | Some blooms produce toxins harmful to animals and pets |
| Who tracks it | FWC sampling; NOAA beach respiratory forecasts | FDEP sampling; Florida Department of Health alerts |
There is no way to tell by looking whether a bloom is producing toxins. Read the sample result, where it was taken and when, together with any health notice for the area.
What FWC's red tide categories mean
FWC reports Karenia brevis in cells per liter of seawater. Its status key, as checked in November 2026, links each category with the effects it can cause.
| Category | Cells per liter | Possible effects listed by FWC |
|---|---|---|
| Background | 1,000 or less | None anticipated |
| Very low | More than 1,000 to 10,000 | Possible respiratory irritation |
| Low | More than 10,000 to 100,000 | Respiratory irritation, shellfish harvesting closures, possible fish kills |
| Medium | More than 100,000 to 1,000,000 | Probable fish kills; chlorophyll visible to satellites |
| High | More than 1,000,000 | All of the above, plus discolored water |
Shellfish harvesting areas close when Karenia brevis reaches about 5,000 cells per liter, because clams and oysters concentrate the toxin long before people notice anything. A chlorophyll reading alone cannot identify red tide; the water quality measurements page explains what chlorophyll does and does not tell you.
Why the harbor watches red tide closely
Red tide blooms start offshore and are carried toward the coast by wind and currents. NOAA notes that the 2017 to 2018 Southwest Florida bloom began offshore in October 2017, moved close to shore in June 2018 and at its peak stretched along about 130 miles of coastline in seven counties. Charlotte County and Lee County beaches were among those affected. Runoff after storms can feed blooms already present near the coast, a link covered on the storm surge and recovery page.
Protect your lungs, skin and pets
Red tide aerosols can cause coughing, sneezing, a scratchy throat and watery eyes. People with asthma or chronic lung disease feel it more. Onshore wind carries the irritants onto the beach and beyond, while an offshore breeze can make the same beach comfortable, so check wind direction along with the bloom report.
With blue-green algae, avoid touching scum, keep water out of your mouth and rinse off with clean water after any contact. Leave the area if your skin or eyes start to sting.
Dogs are especially at risk. Florida Department of Health guidance says they can be exposed by swimming in or drinking bloom water, by eating scum, foam or dead fish, and by licking algae off their fur, and that symptoms can appear within minutes to days. Keep dogs out of discolored water, rinse them with clean water if they get in, and call a veterinarian if they seem unwell. This page is general information, not medical or veterinary advice; for symptoms, contact a doctor, a veterinarian or poison control.
Fish and shellfish
Never eat fish found dead or dying. Do not harvest clams or oysters from an area that is closed; the Florida Department of Agriculture and Consumer Services posts the status of shellfish harvesting areas. For other questions about eating your catch, the fishing rules page points to the Department of Health's fish consumption guidance.
Before you go
- Check FWC first. The FWC red tide status page links a daily sample map, updated each afternoon with about the last eight days of samples, and a written status report every Friday afternoon. Click the sample nearest your destination and note its date.
- Check fresh water separately. FDEP's algal bloom sampling dashboard maps blue-green algae samples and results for up to 90 days. An old result does not describe today's water.
- Use the state portal. Protecting Florida Together combines water-quality status information from several state agencies in one map.
- Look for health alerts. County offices of the Florida Department of Health issue alerts and cautions for blooms; read signs at the access point too.
- Check the breathing forecast. NOAA's red tide respiratory forecasts rate the risk for individual Gulf beaches.
- Have a backup. If one beach is bad, a bay-side spot or a different stretch of coast may be fine on the same day.
Frequently Asked Questions
Can clear water still contain red tide?
Yes. Karenia brevis can reach levels that cause coughing and fish kills without visibly discoloring the water. FWC sample results and NOAA respiratory forecasts are better guides than water color.
Does green scum always mean toxins are present?
No, but you cannot tell by looking. Treat any scum or paint-like streaks as potentially toxic, keep people and pets out, and check FDEP sampling results and health alerts for the area.
What should I do if my dog swims in suspicious water?
Rinse the dog thoroughly with clean water and stop it from licking its fur. Watch for vomiting, weakness or other unusual behavior, and call a veterinarian promptly if symptoms appear.
Is it safe to eat fish caught during a red tide?
Never eat fish found dead or dying, and do not harvest shellfish from closed areas. For fish caught alive, follow current Florida Department of Health guidance and check the state's red tide information before deciding.
Does a report from one beach apply to the whole harbor?
No. Blooms are patchy and move with wind and currents, so conditions can differ between beaches and change within a day. Check the sample closest to where you plan to go.