Simple Tides / Guides / What Is Red Tide?

What Is Red Tide? Causes, Safety, and Where It Happens

Despite the name, red tide has nothing to do with the tide. It's the common name for a harmful algal bloom — a rapid overgrowth of certain algae that can discolor water and, in some cases, make the air and water genuinely unsafe.

What's actually happening

"Red tide" most often refers to blooms of the algae species Karenia brevis in the Gulf of Mexico, though the term gets used loosely for other harmful algal blooms elsewhere, some of which aren't even red — water can look rust-colored, brown, or barely discolored at all depending on the species and concentration.

What causes it

Blooms tend to develop where warm water, sunlight, and nutrient-rich water (from runoff, upwelling, or river discharge) combine to let algae multiply rapidly. Wind and currents then concentrate and move the bloom along the coast, which is part of why it can seem to appear or clear out from a beach within days.

Why it matters for your trip

A red tide bloom can kill fish in large numbers, and the toxins involved can become airborne in sea spray, causing coughing and eye irritation for people on the beach even without going in the water. Filter-feeding shellfish like clams and oysters can concentrate the toxins to levels that are unsafe to eat — this is a real health risk, not just a nuisance, which is why states issue shellfish harvesting closures during active blooms.

Where it happens most

Florida's Gulf Coast sees the most consistent red tide activity and the most public tracking of it, but algal blooms happen seasonally in New England, the Pacific Northwest, and other coastal regions under the right conditions.

Before you go

Check your state's current marine or wildlife agency shellfish and beach advisory page before harvesting shellfish or spending extended time on a beach where a bloom has been reported recently. If the water looks unusually discolored or you notice dead fish washed up, that's a reasonable signal to avoid the water and check for an active advisory.