What is causing Florida's algae crisis?

Editor’s note: Two large-scale algae outbreaks in Florida are killing fish and threatening public health. Along the southwest coast, one of the longest-lasting red tide outbreaks in the state’s history is affecting more than 100 miles of beaches. Meanwhile, blue-green algae blooms are occurring in estuaries on both coasts. Karl Havens, a University of Florida professor and director of the Florida Sea Grant Program, explains what’s driving this two-pronged disaster.

What’s the difference between red tide and blue-green algae?

Blue-green algae are called cyanobacteria. Some species of cyanobacteria occur in the ocean, but blooms — extremely high levels that create green surface scums of algae — happen mainly in lakes and rivers, where salinity is low.

Red tides are caused by a type of algae called a dinoflagellate, which also is ubiquitous in lakes, rivers, estuaries and the oceans. But the particular species that causes red tide blooms, which can make water look blood red, occur only in saltwater.

What causes these blooms?

Blooms occur where waters have high concentrations of nutrients — in particular, nitrogen and phosphorus. In Lake Okeechobee and the St. Lucie and Caloosahatchee estuaries, very high levels of nitrogen and phosphorus are washing into the water from agricultural lands, leaky septic systems and fertilizer runoff.

Red tides form offshore. When ocean currents carry a red tide to the shore it can intensify, especially where there are abundant nutrients to fuel algae growth. This year, after heavy spring rains and because of discharges of water from Lake Okeechobee, river runoff in southwest Florida brought a large amount of nutrients into near-shore waters of the Gulf of Mexico, which fueled the large red tide.