Harmful algal blooms, otherwise known as red tides, have forced the closure of at least six public beaches in the Miami area, the Associated Press reports. Lab tests revealed the presence of moderate levels of toxic algae in waters off North Miami-Dade and parts of Broward counties, prompting fears of fish kills and harm to beachgoers.

Red tides caused by the algae species Karenia brevis occasionally occur in the Gulf of Mexico, but they’re rare on Florida’s Atlantic-facing east coast. It’s believed that the currents of the Gulf Stream carried these tides to the Atlantic. Now, the concern is that algal blooms will travel south to other beaches.

In addition to killing marine life, red tides are also harmful to humans. Last weekend, visitors to public beaches in Palm Beach County complained of physical symptoms that are commonly associated with red tides, the Miami Herald reported. That’s because the algae produces toxic chemicals that may cause eye irritation, sneezing, vomiting, disorientation, and difficulty breathing.

Not all algal blooms are red—some are blue, green, brown, or purple. According to the Florida Fish and Wildlife Conservation Commission, Florida red tides can last anywhere from a few weeks to a year or longer. The length depends on several factors, including sunlight, nutrients, salinity, wind speed, and water currents.

[h/t AP]