Red Tide Explained

"Red tide" sounds like a single, simple phenomenon, but it covers a harmful algal bloom with causes, effects, and risks that work differently depending on whether you're breathing the air near it or thinking about eating something from the water it's in. Those are two separate hazards with two separate explanations, and mixing them up is where most red tide confusion starts.

What red tide actually is

Along the U.S. Gulf Coast, red tide is almost always caused by Karenia brevis, a single-celled marine algae that's present in Gulf waters year-round at low, harmless background levels. A "bloom" happens when conditions let that population explode — sometimes enough to visibly discolor the water reddish-brown, though blooms don't always change the water's color noticeably, and a bloom can be present and dangerous without any visible tint at all. The name describes what it can look like, not a reliable way to detect it.

Karenia brevis produces a family of natural toxins called brevetoxins. Those toxins are the source of essentially every red tide health effect, by two completely different routes: they become airborne in sea spray and wave action (the respiratory hazard), and they accumulate in filter-feeding shellfish that eat the algae (the seafood hazard). A healthy respect for red tide means understanding both routes, not just one.

What causes a bloom

Red tide blooms are driven by a combination of factors that researchers are still refining: warm sea surface temperatures, high salinity, calm seas, and the availability of nutrients that the algae can use to grow — both naturally occurring nutrients upwelled from deeper water and, in some cases, nutrient runoff from land (fertilizer, stormwater, and wastewater). Blooms typically start offshore and get pushed toward the coast by wind and currents, which is also what usually ends them — a shift in wind direction or a cold front can disperse a bloom fairly quickly, while persistent onshore winds can hold one in place against the coast for weeks or, in some historically severe cases, months.

The respiratory hazard: airborne toxins

Breaking waves and surf aerosolize brevetoxins into the air right along the shoreline, and onshore wind carries that aerosol further inland. For most people, breathing it causes temporary coughing, sneezing, watery eyes, and throat irritation that clears up once you move away from the affected air, typically within hours. For people with asthma, COPD, or other chronic respiratory conditions, the reaction can be more severe, which is why health guidance for this group is more cautious: avoiding the immediate beach area entirely during an active bloom with onshore winds, not just leaving if symptoms start.

The seafood hazard: neurotoxic shellfish poisoning

The other route is eating shellfish that have filtered and accumulated brevetoxins from the water. Finfish aren't affected the same way — brevetoxin exposure is actually what kills large numbers of fish during a severe bloom (washed-up dead fish are one of the more visible signs of an active red tide), but a healthy, normally-prepared finfish isn't a shellfish-style accumulation risk the way a filter-feeding clam, oyster, or mussel is. Eating contaminated shellfish can cause neurotoxic shellfish poisoning: symptoms include tingling in the lips, tongue, and throat, nausea, and reversal of hot/cold temperature sensation, usually resolving within a few days.

This is also why the standard safety guidance draws a sharp line between recreationally harvested shellfish from a potentially affected area (never safe to assume is fine during a bloom) and commercially sold shellfish in restaurants and grocery stores, which comes from waters that state health agencies actively monitor and close to harvest the moment unsafe toxin levels are detected — the commercial supply chain is specifically built around testing for exactly this risk.

How red tide is tracked

In the United States, NOAA's Harmful Algal Bloom Observing System (HABSOS) compiles water-sample results collected by state and federal monitoring programs along the Gulf Coast, classifying concentrations from background/not present up through high. RadarBug's optional red tide layer plots these HABSOS sample results directly on the map, so you can see which stretches of coastline have recent confirmed samples and at what concentration, rather than relying on word of mouth or a single news report.

Because it's built from physical water samples rather than a continuous sensor network, coverage is only as current and as dense as the most recent sampling in a given area — a gap in plotted samples doesn't necessarily mean a bloom isn't present nearby, just that a sample hasn't been reported there recently.