Harmful algae blooms are painting coastal shores in southwestern Florida red and leaving swaths of dead fish – just weeks before thousands of people flock to the beach for spring break.

The phenomenon, called red tide, generally occurs in the fall, affecting the southern coasts of Texas and Florida. But this year the red wave came early.

Since the tide’s premature return, jarring images of dead fish have alarmed officials, as have reports of people complaining of respiratory irritation.

A map from the Florida Fish and Wildlife Conservation Commission shows various red-tide levels dotting the state’s southwest coast. In some areas, including near Naples, levels were “high,” described as more than 1 million cells per liter, according to the commission. South Pasadena, Sarasota Bay, Englewood Beach, Seagate and Naples Pier recorded the highest levels in recent weeks.

Here’s what you should know about red tide.

What is red tide?

The toxic algae form in waters offshore, move toward the shore and begin to fester as they find nutrients, said Richard Stumpf, an oceanographer at the National Oceanic and Atmospheric Administration. Droves of algae move toward shore as nutrients from the ocean floor move up and inward – a process called upwelling. When the Karenia brevis algae accumulate in large numbers, they form blooms that stain the water red in high concentrations.

Normally, the toxic algae linger at the bottom of the ocean. But when the nutrients move, they move.

“They just accumulate there, and then they grow on whatever nutrients they can find,” Stumpf said. “They’re really good at scavenging the limited nutrients in the water.”

How is it harmful?

The National Centers for Coastal Ocean Science issued an alert early Monday, warning of “moderate to high risk of respiratory irritation” because of red tide in some areas including Charlotte, Collier, Lee, Manatee, Pinellas and Sarasota counties.

Respiratory complications, especially for people with preexisting respiratory conditions such as asthma, can be triggered by exposure to red-tide levels beginning at 10,000 cells per liter, according to the Florida Fish and Wildlife Conservation Commission. For people without those respiratory conditions, exposure to the toxins released from the algae can cause coldlike symptoms, according to Stumpf. Exposure can also lead to eye irritation.

The blooms also cause problems for fish. The algae release toxins that paralyze their gills, according to Stumpf. Summer blooms kill organisms at the ocean bottom, such as corals and sponges. Warm water from higher summer air temperatures can lead to anoxic water – water without any oxygen in it – causing other animals to die.

The algae also kill seabirds, especially diving birds, that eat fish exposed to the toxins. Sea turtles, manatees and dolphins have also been casualties of red tide.

When does red tide form?

The algae live in the Gulf of Mexico and tend to form high-concentration clusters off the coasts in the fall, Stumpf said. By winter, high-pressure weather systems swoop over Florida with northerly winds that push the algae from the shore and back to the gulf.

During a typical year, the algae would be pushed in by winds in the fall and pushed out by the winter – and often not seen again until the following fall, Stumpf said. But that wasn’t the case this year.

“If you have a year where we don’t get those persistent northerly winds pushing it out, it can hang around,” Stumpf said. “And that’s what’s happening this year.”

How is climate change affecting the algae?

Climate change can cause harmful algae blooms to occur more often and more intensely, according to the Environmental Protection Agency.

Nutrients from fertilizers run off into waterways and become food for algae when it reaches the ocean. The changing climate means there are higher water temperatures and more extreme storms that lead to more storm-water runoff. The combination of conditions is a perfect recipe for large algal blooms.

In recent weeks, Florida has had abnormally warm air and water temperatures. Along the state’s western Gulf Coast, waters are 5.4 degrees to 10.8 degrees above average.

Stumpf predicts climate change’s impact on wind patterns will also be a significant factor in future red-tide activity. Red-tide blooms going through spring and into summer are generally a once-in-12-years event, he said. There have been two such events in the past five years.

“If we tend to get fewer persistent northerly winds in the winter because of climate change, we will tend to see longer-lasting blooms,” Stumpf said.